Is a capillary break between the footing and foundation wall really necessary?
stearel
| Posted in Green Building Techniques on
We are meeting a lot of resistance from local concrete contractors & structural engineers when we tell them that we want a capillary break between the footing and the foundation wall. Are there any documented cases of this capillary action being a source of moisture/ mold / finish material problems? Are there any scientific studies that compare the moisture levels in foundations walls with and without a capillary break? The capillary break makes sense to me intuitively, but nobody wants to pay for it or change their ways unless we can show them proof that it is necessary.
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The proof is everywhere, appearing most commonly as efflorescence on both near the bottom of the interior side & near grade on the above-grade exterior sides of the foundation. This isn't a new problem, and not rocket science (even the Romans had well evolved methods for managing it without capillary breaks.) In the UK they call it "rising damp". Do a web images search on "rising damp" and you'll find literally thousands of not-so-pretty pictures.
http://www.buildingscience.com/documents/insights/bsi-011-capillarity-small-sacrifices
With well-drained footings the problem may not always show up as mold, but it is measurable as higher interior air humidity.
Eric,
If you are more comfortable not including a break, you can stop the capillary action with concrete admixtures like this one:
http://www.xypex.com/products/additive-products
Dana, I appreciate your reply. However I'm afraid if I show your evidence to the contractors in my area they are going to tell me this is only a problem in the UK (where it is considerably wetter than it is in the Midwest, correct?) A "rising damp" image search returns a lot if photos of brick masonry foundations in old buildings that probably have no waterproofing on the exterior face of the wall, and maybe no footing drainage system. Does anybody know of a well documented case of "rising damp" damage on a house in the US with cast-in-place footings and foundation walls, waterproofing on the exterior face, a good footing drainage system, and a proper under slab vapor barrier? I also acknowledge your comment that the problem may show itself as higher interior humidity levels. Has anybody ever done a study that supports this assumption?
"I'm afraid if I show your evidence to the contractors in my area they are going to tell me this is only a problem in the UK (where it is considerably wetter than it is in the Midwest, correct?)"
Annual rainfall in Cardiff Wales is about 44".
Annual rainfall in Manchester England is 32".
Annual rainfall in Rome Italy is about 33"
Annual rainfall in Peoria IL s 36".
Annual rainfall in Chicago is 39"
Annual rainfall in Duluth MN is 32".
There is no special dispensation that gives the midwest the ability to defy the laws of physics regarding the movement of moisture. The midwest even has higher humidity air to deal with than most of Europe.
There is no mystery about why basement air humidity is higher than above grade rooms. The proof is in what happens when you DO put down ground vapor retarders and capillary breaks. I can't point to some specific study, but this stuff has been fairly well understood for centuries (millenia?), but perhaps better quantified in the pass century than prior. If you want references to scientific authority (rather than a formal controlled peer-reviewed study with the full data & write-up), try the Oak Ridge National Laboratory prescriptives:
http://web.ornl.gov/sci/buildingsfoundations/handbook/section2-1.shtml
Or figure 2-3 and 2-4 from the EPA's moisture control handbook:
http://usgbc-centraltexas.org/wp-content/uploads/2014/02/moisture-control_EPA_Dec_2013.pdf
Or the Minnesota Sustainable Housing Initiative's prescriptives:
http://www.mnshi.umn.edu/kb/scale/basement.html (between 1/2-2/3 of the way down, under the "Durability" section.)
See the section on moisture transport mechanisms beginning on p13, and the comments on p23 of this document:
http://www.buildingscience.com/documents/reports/rr-0906-field-monitoring-hygrothermal-modeling-basement-insulation
Putting the capillary break under/around the footings is pretty straightforward prior to pouring, and arguably better that putting it between the footing & wall.
Dana, Not to downplay your point about the importance of some form of capillary break, but I'm not sure how useful annual precipitation levels are in determining the effect of climate on buildings.
What the statistics don't tell you is how that precipitation occurs. The cities you cited are all in the same range as Victoria B.C. and Seattle, but as a past resident of the PNW you will no doubt remember the climate where, even though there may be no measurable precipitation, there are weeks of damp overcast or foggy conditions where moss grows on metal and a cloth left outside doesn't dry. The ground is saturated from October until May and foundations subject to those conditions are, as you say, prone to dampness. A city like Montreal, with a higher annual precip. rate, with much of its precipitation coming in the form of snow and the rest as infrequent but intense deluges, doesn't experience anything like the same problems and I doubt buildings there would see much benefit from capillary breaks.
Eric,
Here's my take on capillary breaks between the footing and the foundation wall:
1. Rising damp is one factor affecting the rate of moisture transfer from the soil to the basement. It may be hard to quantify, but it is real.
2. Installing the capillary break at the time of construction is incredibly cheap and easy. Retrofitting a capillary break later is either incredibly expensive or close to impossible. So including a capillary break during new construction is what we in the building science community call a no-brainer.
The type and frequency of precipitation is irrelevant, what counts is, is the soil damp/wet can the walls/foundations absorb water. If they can then water will rise up the wall by capillary attraction to a height of four feet above the water source. Having a capillary break more than nine inches above ground level, will ensure the wall above stays dry.
Roger,
Even more than four feet.
Here's what I wrote about capillarity in a ruminative piece published in the January 2004 issue of Energy Design Update.
"Historians report that the first person to describe capillarity was Leonardo da Vinci, who recorded his observations of capillary action in 1409. Many centuries later, in 1901, Albert Einstein found the phenomenon of capillarity intriguing enough to make it the topic of his first published scientific paper, Folgerungen aus den Capillaritätserscheinungen. The paper examines capillarity from the perspective of the laws of thermodynamics and Einstein’s theory of molecular forces. Some Einstein biographers consider it likely that Einstein’s brilliant Serbian girlfriend, Mileva Maric, provided important contributions to the paper.
"Capillary rise occurs when the forces of adhesion (the attraction between the molecules of a liquid and those of a solid) are stronger than the forces of cohesion (the attraction of the molecules of the liquid for each other). In some circumstances, equilibrium between these two forces is not achieved until the liquid has risen many feet.
"Contrary to popular belief, capillarity is not responsible for the rise of maple sap from underground roots into sap buckets. In many houses, however, it is responsible for the rise of water from damp soil beneath concrete footings into foundation walls, contributing up to 15 gallons of water a day to a house’s interior moisture load. Capillarity can also cause water to rise up behind courses of lap siding.
"Capillary action in soils and masonry—called “rising damp” in Britain—is a function of pore size. Small-particle clay soils allow water to rise as much as 20 feet by capillarity, while crushed stone permits water to rise only a few inches. That’s why a 4-inch layer of crushed stone under a concrete slab is an effective capillary break."
Here you go:
http://foundationhandbook.ornl.gov/Kohta%20Ueno-Capillarity%20Presentation.pdf
See page 11, Figures 9 and 10:
http://www.buildingscience.com/documents/guides-and-manuals/gm-read-this-before-you-design-build-renovate
Ask them why they would waterproof the outside of a foundation wall while "leaving the door open" to moisture being sucked up through the footer? As Joe says "concrete sucks" (moisture). Then be quiet while they try to explain their "logic." Hint: "we've always done it this way" is not logic.
Billy
From my experience studying solid concrete walls – the problem is due to the amount of air entrained into the concrete mix. Where you use good quality concrete and a vibrator the air can be shaken out of the mix and water may not rise at all.
With block and brick walls, the mortar is the problem, with water rising through the air bubbles added to the mortar when mixing.
From a building perspective I have never know water to rise above four feet in a traditional wall, unless the wall has a waterproof render, probably meaning the rise is to an extent subject to the speed of evaporation from both surfaces.
Interesting point, a hundred years ago and more, before the onset of modern water proof materials, it was normal for expensive homes to be built with the living area more than four feet above the ground, to avoid rising damp, others used two of three rows of staggered slates to provide a capillary break.
We just finished pouring the concrete for our footers and basement walls. Before the walls were poured, we used Super Thoroseal to serve as a capillary break. Our footers were 18" wide and totaled 204 linear feet. The basement walls were 10" wide so we didn't need to coat the entire 18" footer with the thoroseal, but we did anyways because it wasn't the much extra work. Using 2 - 35 pound buckets of the product, we were able to do 2 coats with some leftover to do a third coat over the keyway. Each 35 pound bucket cost just $25.00. On another GAB forum about capillary breaks, a product that was mentioned for use as a capillary break was Tremproof 250GC. I called around to check on the price of that product and it was around $250 for a 5 gallon bucket. I probably would have needed 2 buckets to cover my footers and so the cost would have been expensive. Also, I kind of liked the fact that the thoroseal is cement-based and might be tougher than some of the thinner waterproofing paints, especially considering the amount of stomping and scraping it will have to withstand when the workers form the basement walls. Most importantly, I called the makers of the thoroseal and they said I could apply it after 24 hours of the footers being poured. Being cautious, I waited two days to allow the concrete to cure a little bit more before I applied it. After the first coat, you have to wait 12 hours to apply the second coat. The application is done with a thick block brush. The product is supposed to be applied with a pancake batter consistency so it's not exactly like you are painting with thin latex and it will take some more time to brush on. Wetting the footer will help.
For homeowners actively involved in the building process, I think this is a good product because if your foundation contractor gives you a hard time about capillary breaks, you can just go to your local Big Box store and buy this stuff and apply it yourself. Just tell your foundation contractor to hold off pouring the basement walls for a few days, while you put this stuff on. Maybe you can do it over a weekend.
While I had the footer all to myself for a couple of days, I decided to apply Earth Shield Type 20 waterstop to prevent water from running into my basement through the footer/wall joint. I understand that the spray-on exterior basement wall waterproofing that will be applied is going to down onto my footer to cover that joint, but I wanted extra protection. I have lived in a house that had a basement that flooded and most of the water came through that footer/wall joint. The waterstop product came in 16'-8" strips and cost $21 each. So to cover 204 linear feet, it cost me almost $300. I applied the waterstop on the side of the keyway closest to the exterior and not directly in the keyway like the website shows because I didn't want to have to go around the rebar that was placed right in the middle of the keyway. I think it will still do its job. I didn't use the primer that the website recommends because I talked to the company’s technical support person and he said the clay-based waterstop strips are already sticky enough and if you have a relatively clean footer, it will adhere to it with some foot pressure. And this was true. As an alternative to the primer, you could use the 3M adhesive spray that come in cans. It might save you some money.
I have attached some pics for your viewing pleasure. Notice the paper backing still on the waterstop strip. You are supposed to remove it before the walls are poured. I kept it on because there was a slight drizzle the morning of the pour and I didn't want the clay-based waterstop to expand. That's another thing, do not apply this stuff when it is raining or it will compromise its effectiveness.
I'll be documenting my build on my website: http://www.mustbuildhouse.com. Check me out.
Martin, perhaps my situation is an example of your comment “retrofitting a capillary break later is either incredibly expensive or close to impossible.” But hopefully, it's neither? We are underpinning our foundation in order to lower our basement floor. We've specified the new slab assembly eg 4” wire reinforced concrete, 6 mil. poly vapor barrier, 2" EPS R10 on 6” gravel, and subsequently, we'll excavate the exterior walls and damp proof, rain screen, and insulate them too. Yes, expensive – but we’re not retrofitting to install the capillary break, we’re doing it to increase the square footage of our two-story house by 50% without expanding the footprint. So far so good. Per William Geary’s point re: “leaving the door open”, would laying down a barrier before pouring the underpin be the equivalent of installing a capillary break inches or feet above the footing? If so then I assume that the best practice would be to form a tub of some kind so that the interior edge can meet the vapor barrier to be installed for the slab, and the outside edge is high enough so that the subsequent excavation would expose it and allow us to form a continuous seal up to grade. If this sounds reasonable given our situation does anyone have ideas on materials and techniques that we should be considering?
Jim,
I think a cross section diagram that shows how much you are dropping the interior slab relative to the existing footing would be helpful. You need to show where the exterior perimeter drain depth falls relative to the new slab inside. I am seeing a foot worth of material depth for the slab profile by which I am inferring a substantial excavation. Is this work proceeding in sections around the perimeter?
thanks, Roger! You got it. The only thing is there is no existing footing. The foundation sits on the soil and the existing slab was poured onto the dirt floor years later. I marked up our proposed elevation to show where the "rubber bucket" might go, as well as the existing foundation wall. Yes, the underpin will be done in ~3' sections around the perimeter, and the interior lally columns and chimney will get new footings as well.
Eric, You've got some good responses here on the technical part, but let me address something you wrote: "but nobody wants to pay for it or change their ways". That's fine. Contractors need to get paid (like the rest of us) and if capillary breaks are not part of their standard work, you will need to insist on it and pay them to do it. You might even have to pay a premium because they haven't become good at this because it's not part of their standard work. Or DIY it, like Jimmy. You always end up paying for all of their work, whether is shows up on a line item or not. If you have a concrete contractor that can't work with that, then you have a different problem on your hands.
Jim,
Wow, you have set yourself a project. The diagram helps greatly, especially in being able to better describe what I think are relevant points.
First, I rather hope that water has not been an issue with your current existing basement. You have not mentioned cracks in the walls, water issues or sump pumps. If none of those problems exist now you are off to a good start.
Do you know whether an existing perimeter drain is in place? It would be good to know where it exits relative to the house and neighbors for code reasons, but more important is knowing the drain exit elevation relative to the new footing's excavation depth. If you have no firm knowledge of a drain exit, does the grade around the house ultimately fall to an obviously lower elevation than your intended footing excavation level? I think it would be wise to consider checking out the soil and moisture conditions under the current slab before committing to the project.
I would suggest making at least one test hole through the existing slab sufficiently large enough to use a post hole digger or auger bit to go down to the intended footing depth. Clean the hole out and then wait a week. If you are lucky, perhaps nature will provide you with a test storm to check single event conditions driven by faulty downspouts or a plugged perimeter drain. If any water gathers in the hole, you have to stop and consider what conditions have led to it being there. Has the contractor already considered this?
If you have already done all this, apologies for asking a too obvious question. You haven't mentioned any soil engineering or demands from building inspectors, so again, apologies if this also seems too obvious to raise.
The idea of a capillary break between footing and wall is great, but only as long as one controls the water presenting itself to both footing and wall. Exterior waterproofing on a wall is just one part of the water control process. You have a very unusual footing height to deal with, which may work somewhat to your advantage. It will be easy to establish a perimeter drain well below your slab top, an important condition if you are to have a successful and dry basement.
The need to cast the new footing in sections limits what you can achieve with membranes of any sort. Water is patient and persistent, so anything short of perfect sealing section to section would render the barrier moot. The footing will get wet if water is present and it will be subject to "damp rise", which is after all what you are trying to prevent.
One possible answer that can minimize the upward moisture transfer is to use Xypex additive in the mix rather than as a surface application. I elected to use it in my own footings and the entire wall casting. It is not cheap and to be honest I don't think the manufacturer agrees with my reason for using it. It might be over kill for your situation. I have seasonal runoff and a layered sandstone bedrock to deal with. I can say that my basement is dry enough that I do not hesitate to put my lumber and sheet goods up against the walls, nor do I fear putting cardboard boxes directly on the floor. The Xypex acts as my capillary break albeit by a different mechanism.
I do have other things to suggest, but given the tome I have already written, I will let you digest this much and go further after you advise what soil and water situation you have.
WOW! Thanks Roger, really appreciate the time you have taken to help me understand the potential options here. Very insightful. I’m glad you asked about the context as I’m sure there is more to learn about the choices we've made so far, so here goes.
- There is no sump today, it’s one of the reasons we bought this place two years ago. It’s rare to find an old, dry basement in our county. btw, the footprint is ~25’ square, two stories. Roughly 1,500 square feet.
- There are no interior drains
- There is an old drain pit/well in the front yard that is filled with rock. I think there was one on each side of the house that received both the footing drainage and the gutters. As of the 1980s all the gutters go to daylight at the street.
- There are likely orginal red clay drain tiles on the exterior of the walls but the assumption is that they’re probably not functioning any longer.
- No major cracks on the interior side of the foundation. It’s not parged but there is some spalling over ~1% of the total surface at about 18” higher than the base of the wall – after a wire brush nothing more than ½” deep at this point.
- There are some signs that prior owners have repaired the foundation using red brick to replace a few of the 100-year-old cinder block in a few spots. Based on some timelines I’ve pieced together the brick has been in place for ~75 years.
- The existing slab was poured directly on the dirt floor. It’s ~3” thick and has been repaired several times over the course of the decades. A hammer tap demonstrates that some spots are hollow which I attribute to water movement over time
- I chipped out two 1’ x 1’ x 1’ test pits on either end of the basement right up against the foundation. The purpose of the tests was to measure the footings which I quickly learned do not exist. Unfortunately, I did not leave the pits open to determine if water would collect. The soil was very compact and driving rebar an additional 2’ down was a bit of work, but hardly conclusive.
- I didn’t get a soil test done but I had an engineer friend use the worst case soil scenario and loads to calculate the specs for the current underpin design – we’re confident with what we have but perhaps before we get too much further we should check the conditions at the proposed foundation depth as you described.
- The engineer's plan that I posted doesn’t show the drainage. As a prophylactic measure, we’re installing interior and exterior drains that will all lead to a sump.
Xypex sounds like it could help – may I ask, which product did you end up using and at what ratios?
Yeah, I can see that sealing the barrier sections together is the trick. I would think that the force of the concrete down upon the barrier would be enough to seal it. But I suppose that as far as vapor goes even a microscopic gap is still a pathway and the concrete is still a sponge. For the outside of the wall, we’re going to excavate down to the underpin - then damp proof, rain-screen, and insulate it to grade. Looking forward to hearing more of your thoughts and thanks again for your help here.
Roger - Wow. That's a _great_ response.
Jim,
Your new input helps a lot. It would be very helpful to have an elevation plan, or maybe a pic of the house's surroundings if you don't mind being public with that. You describe the gutters as being connected to red tile going toward the street. I infer that the grade rises to the back of the house? What is up above you? The spalling and brick patch are a bit concerning, but I can explain why CMU walls do that later. Your cistern and other details are quite familiar to me. My 80 year old house had both. Am I safe in thinking that your side lot distances will allow for mini excavators to get in and move? Excavating the basement down 24" or more will be a lot of exercise. Pray that you don't have an inside well.
I will try to find the exact amount of Xypex used. We did place over 60 yds total in three billings, so not sure what I have kept in the papers. I am racing around trying to get my last outdoor work done with our last four good days before winter gets serious here. If I don't get back tonight, be patient.
Jim,
Some added notes for you to mull over. You haven't mentioned your climate zone or rainfall average, which would be good to know. The fact that you had possibly two cisterns in the front yard makes me think your rainfall is north of 25" as a WAG. I am guessing their location means your house is on a slope to the street and that possibly you still have an elevation drop from current slab to street/curb. Also, a CMA disclaimer, I am offering insights to prompt your getting solid answers from the people you are hiring, not to be a final opinion on what to do.
Think of it as getting all your ducks in a row, so the inspector doesn't show up and ask "did you contact..."
Does your jurisdiction allow you to put drain tile or sump water out to the street drains if any exist. I ask because the area I first lived in had to enforce new rules about foundation drains connecting to the sewers. In the 50's density was low, but in the 90's every lot was built out and the sewers couldn't handle the storm flow. Some localities also have very clear rules about where sump or drain tile water goes - like not onto your neighbor. The new part of town I moved to had level areas that forced new construction to meet water run-off and discharge rules that in some cases left the homes surrounded by deep swales that looked like moats. Not much fun to mow either.
The second house I lived in near the moat homes was the 80+ year old one that was cut into a hillside that sloped down from the street grade. Fortunately the street also went down hill past my driveway, which provided some relief from the often torrential rains. The foundation thus cut into a two pitch slope such that one corner was fully below grade and the diagonal opposite corner was completely above grade. The relevance being the pattern of water damage I had to deal with while I restored the CMUs and mortar in preparation of setting up a very basic shop. The two deepest set walls had the most damage with the other two showing a line that echoed the soil depth outside.
The mortar and block were set in the early 30's during the depression, so many short cuts and salvaged materials went into the house. The roof was planked with barn floor boards that were almost 2" thick and soaked in cow pee. The roof rafters were plainsawn and varied from 2 1/2" wide to almost 4" wide. Many other framing wonders revealed themselves during my 25 year process of keeping it standing while upgrading the electrical and plumbing bits.
The foundation excavation was possibly done by hand, but in any case, the "footing" was the shovel trench type. As minimal as it sounds, a shallow curved trench was cut by hand with a shovel to provide just enough room for the (to them) expensive concrete. The concrete was leveled and trued to make a good start base for the blocks that followed. The very dense clay soil in the area made such a nominal footing work pretty well for 80 years. It may be that a similar shallow bed of concrete was used for a starter bed on your home. You might have thought the lowest blocks just had lots of mortar on the bottom. It would be easy to miss what was going on as you chipped down beside the block expecting to see a "real" footing.
Ironically, the three corners that cracked and went willy-wobble were all the ones nominally set below frost depth. The one corner that floated almost free of the soil was perfect. That corner is also how I got to see just what the blocks were sitting on. Not very much.
The floor, like yours was cast over the dirt floor in stages as money could be found, or maybe the need for a flat place to put the washer and dryer. It was quite the mess and water pressure from the uphill side would make the seams weep and grow quite amazing crystal farms. The hollow sound may simply be the result of the dirt settling under the slab because they didn't compact it before setting the concrete. An inside well may not be a worry where you are.
The same water pressure from uphill also filled up the block wall with water during big rains and generally kept the block very damp in between. The mortar was basically turning to mush between the blocks, so I had to dig out and re-point a couple of hundred feet of mortar to salvage the wall.
I also had fired clay tile pipes that picked up the gutter drops and carried it to a big cistern next to the garage. Unfortunately, before we got the house, the county had decreed that all cisterns needed to be filled in lest some doofus fall in. The original purpose was to hold water for unknown purposes. The house was then well outside of town limits and no services were available. The fact that yours have rocks in them could mean they are dry wells meant for run off rather than a water source.
They also deliberately broke the tile connections, so not only did the area in front of the garage get soft, it forced the water to stack up in the tile pipe until it broke one winter. After that it was dumping the water into my block wall. I discovered this the day started clearing out the degraded mortar and was greeted with a strong stream of really disgusting water that probably filled the blocks up three or four courses.
In your case the erosion of the surface has been driven either from the bottom being damp and that is how far up the "damp rise" gets before running out of steam, or the outside drain tile bedding is about that deep and over the last hundred years has infiltrated with soil. The soil infiltration will prevent it from freely passing the water on to the tile, but still be open enough to be the easiest point for water to settle. Plus it is unlikely that any asphaltum coating they put on is doing much this end of a century.
If you really have lost a full half inch off the blocks at the bottom of the wall, you might want to consider making the new footing one CMU block taller and remove the compromised block course. Drill a 3/8" hole in the side of the block at 1/4 of the block width (which should put you in the thin part of the wall cross section). Then insert a piece of wire with a short el bend on the end into the hole and pull back til the bend catches on the inside of the block. Mark the wire with whatever and unhook the wire, retrieve and see what the wall thickness truly is. For
a check on original thickness, do the same on one of your pretty blocks up high.
It would be very unusual to find a block wall of your vintage that had either rebar or filled cores. It may still be that way in some areas, but a better built block wall will have at least some of the cores filled. Certain CMUs are made with a saddle dip that runs the length of the block top and bottom to allow for horizontal rebar placement. In combination with filling the cores, even if only four or five courses deep, makes a CMU foundation wall way stronger than unfilled. It is also a lot of work, but for some circumstances the only way a strong wall can be done when form work is untenable.
For your intended project, filling the cores while you cast the footing section might be overboard, but that's what engineers are for. I could echo my favorite carpenter who would answer my framing freakout questions with "It's still standing isn't it."
I will torture you with questions about any columns you have later. That and whether or not you have fully considered how you will need to step and repeat the interior excavation to avoid collapsing the wall. As a serious question, have you considered getting a quote to simply jack the house and cast an entirely new concrete wall. There may well be advantages and cost savings given the extensive hand labor and multiple concrete drops this project is incurring.
Roger. Some great ideas - thanks a bunch. I got caught up with work. Back to the fun stuff... I'm including the existing and the proposed elevations, and some photos of the exterior grade. https://www.icloud.com/sharedalbum/#B0y5O3DjH8AcFa I think this should answer a lot of your questions. In short, yes, we're pitching everything away from the building and paving 75% of the perimeter grade so bulk water will be draining away from the structure. We’re in 4A with 52 inches of rain, and 30 inches of snow outside of NYC. I think the net is that even though the house never had a sump or any water to speak of, we can't see doing the underpinning without taking reasonable precautions against bulk water or the potential for mold spilling into the basement from the CMU cells, etc. But we do get some hydrostatic pressure through cracked concrete in the floor after a 3-day soaking rain. That'll be gone after the vapor barrier and drain piping is installed. No springs that we know of. Per the town, any new permits require water to be managed on-site and as of now, the plan shows the new sump feeding an underground stormwater management system. When I dug the test holes I dug out underneath the CMU. There is literally no footing though there was some very dark material - like ash. I think it may have been crushed cinderblock that has all but eroded to sand and the dark material is literally cinder. The mortar joints are strong, but probing the walls to see if I have any bulk water has been a thought. Jacking the house up is a nonstarter as the ballon frame, plaster walls, and my marriage wouldn't stand a chance. At this point, I'm still thinking there must be some sort of technique that can be used in each section of the underpinning (yes, we're doing A/B in 3-foot sections). Perhaps the next best option to inhibit damp rise is to use an additive and/or consider going with the flow by providing a moisture pathway from the CMU to the interior and down a rain screen into the drainage and sump - which seems to be a common approach to addressing this (if YouTube is any measure of consensus). Thanks again for spending time!
Jim,
Still digesting new info. Looks like you have planned for most things. Good to have a proper storm sewer to handle the sump. Random question, you appear to have room to add to the back, is bumping out both floors off the table?
unfortunately, yes, it is.
Jim, Please don't take this as a snotty question. Is there any chance you can move? I had first envisioned a square basement with wood steps up for grade difference. I now see at least one bump in the pictures on the driveway side and a framed and insulated porch bump out on the back entry, which has cement block stairs. I now see a covered basement entry near to the drive side. The front porch appears to be ledgered to the sill plate and rim board if it exists.
Your side lot offset dimensions on the drive side reduce down to 9' while the other side is 14' so a mini excavator should be able to move about safely without getting heat from the neighbors. Just the same you have a front and back porch structure that will make for some very interesting maneuvering. Additionally, the width of the outside perimeter excavation will be putting the posts for the back porch at risk for slumping. Not much risk as the total loading seems light, but then I have no idea if the post footings are full depth.
You clearly have put a huge amount of effort into documentation of what is planned. I do wonder if you have attempted to get bids or if you plan to do this on your own. I am seeing a very large dollar amount and some very tricky scheduling to pull this off. Only you can tell if the dollars invested will be a good investment for the neighborhood and schools.
At this point I can offer some tactical notes for heading off potential snag points if you are sure this plan is supportable.
The back basement stairs will die in the excavation. If you were to plan for new stairs that meet the new slab, you will need to header the door opening lower and allow for a longer run. Alternately, you could redo the stairs to the same depth and original door opening and have step down to the new slab height inside the cast footing.
When you dig down to the existing bottom level of the block wall, you will have moat around the house. Inside you will need to bust out the old slab and discover whatever surprises and then first re-set the center column with a new footing below the intended new slab height. I am guessing that a beam lands on the exterior wall just before the street side corner of the dining room bump out. After the center column is set then you could plan on up to four 3 foot bites if you are brave. Two of your foundation walls are only see load from wall, while two are seeing wall and roof loads along with one side of floor loads from both floors. Unless the framers switched the joist runs for the second floor in order to make the stair opening work better. If that is true, then the second floor is loading out onto the walls parallel to the drive and side lot line.
Your sill plate is quite large and will help keep the loads above distributed to the sides of the undercuts you will make. An engineer might be able to calculate if you should only do 2' bites on the street and back walls and maybe 3.5' bites on the drive and side lot walls. Your walls look to be the exact same block I had in my last house, only in much better shape. They will be a bugger to coat if they used split face block all the way down. Your basement windows indicate the grade is raised on the drive side so you may need to plan for surprises.
I hope you aware that you cannot excavate either side of the foundation wall deeper than the bottom block, except for the new footing process. Even in heavy clays I would not cut any closer than three feet without specific advice from a soils engineer. You might save some hauling and manpower costs by working out from the center column to an engineered limit inside, but you will be stuck with seven or eight cycles (best case scenario) to step your way around the perimeter.
That's seven or eight minimum cement form cycles, stripping, digging, setting, casting, stripping and so on with many truck charges and so on. Meantime, as you get slightly more total rainfall than we did in Chicago, you can expect to be pumping up to four pits just after you dig them and get forms in place. And the bump corners will be special even if the second floor doesn't extend out over it.
In any case, plan for having a moat at tow levels around the house for an extended period. Pets and people problems will be a big issue. But mostly a lot of chopped up work load that is going to drive costs up sharply.
Not trying to be a Cassandra, just realist. I would recommend that as a thought experiment you start a written log of steps as you perceive them and then make a time estimate to go with each step.
You seem to be on to all this anyway, but I just want to put it out there.
Roger, thanks. you’ve captured a lot of it and proven to be exceptionally persceptive. I’ve really appreciated hearing about your experiences and suppositions about our project. You clearly know your stuff and have provided valuable insights and guidance. If you have any specific guidance on the particularly thorny issue of inserting a capillary break during the underpinning process it would be great to hear more about that.
@anyone else that’s actually worked out a method to mitigate damp risk on an underpin, if such experience or citations exists on the forum, would be very much appreciated. thanks!
Jim,
Sorry to have pushed past the original question. I do think that the only way you will be able to mitigate the upward transfer of ground moisture is to use the Xypex and not try to provide an additional capillary break between the new footing and the block. I believe it will be helpfull to the wall strength if you let the new concrete fill into the cells as you pour up the 3-4" you have indicated on the footing profile.
If your cement contractor will go along with it, I suppose you could lay down a heavy plastic sheeting under each section as you cast. Much like the plastic that should be on top of your foam and under your new slab. It will reduce the direct contact area with soil even if it does not ultimately prevent water intrusion perfectly. The Xypex additive will largely prevent the upward transport of any water that you aren't catching inside or out.
Under the "this modern world" heading, you can find Xypex on Amazon. The web link for the Xypex 500 and other formulations is - https://www.xypex.com/products/details/xypex-admix-c-500-c-1000-c-2000
Be sure to download the mix ratio guide as the 2-3% rate is fixed to the cementitous content which may mean fly ash and other extenders may alter the pounds per cu. yd. Dealing with a good concrete supplier is paramount to ensure the additive is both in the mix and in the correct ratio. The red version might be worth a thought.
For belt and suspenders dampness control you could use a foundation sealer on both sides of the footing and then on up the outside of the block. The old wall will need a good scrubbing to ensure adhesion of whatever form you use. I don't really think the above slab interior face of the new footing would be an issue if you use the Xypex. I have no fear of putting my lumber and sheet goods directly against my walls. But then, I do not have interior insulation. I went with all exterior insulation for foundation and house.
Also, go over the rebar schedule and how you will handle the form set up. You will need to have at least 10 diameters of rebar length to tie to for each following segment. The excavation size will be over the cast size by form plus rebar extensions. With care and planning it may be possible to set two sections per wall side and bring down the batch trips.
Last notes, have you checked for radon? You have the rock break scheduled, pipe is easy now. And consider a sealed sump pit if it is not already code.
Good luck with the project. I may be projecting my own age onto to it. I envy your energy.
Hey, thanks, Roger. It's really helpful and I appreciate the time and energy you put into this. Given the number of homes across the country in similar circumstances, you would think that the remedy to address capillary wicking when converting a basement into a code-compliant living space would be much easier to come by! Just a sampling from my own county can be found here https://www.towncharts.com/New-York/Housing/Westchester-County-NY-Housing-data.html
There are 374k homes in Westchester, NY and 31% built before 1939. You would think that with 116k homes there would be plenty of demand for a well-considered prescription. Once I have the complete assembly together I'll create a new post with a detailed drawing and all the specs to test the final plan with the forum + @Martin, and the rest of the GBA crew. Thanks for helping to get me closer to realizing the final design.
Roger - quick question for you. Did you look at Hycrete before deciding on Xypex?
Jim,
Nope. Xypex was suggested to me because building code in another county supposedly allowed its use to qualify for not using foundation sealer. My brief check into the product showed it to be used for subterranean tunnels, water tanks and near lakes. I managed to find a local representative who was able to get the product here on time and with the proper usage rates, so I didn't look further. It all was happening a bit quickly by the time I heard about being able to waterproof the cement. I just thought back on my sad, soggy basement in Illinois and said go for it. At least I have no regrets.
I did pull some Hycrete sheets and it seems to be very similar and boasts it can seal cracks even larger that Xypex claims. The one reference I found on ratios suggests it may take a bit more per cubic yard, but Xypex's ratio being based on the cement content of a mix puzzles me. That is why it would be best to know how fly ash content will affect the performance of either. Both seem to be manufactured in the same area of Canada. I kinda suspect that the chemistry involved is pretty similar though I could not find a statement of contents for either.
My best recollection was that we poured 75-80 yds for footings and walls total and the Xypex clocked in at around 22-2500 extra cost. I skipped coating the foundation, but probably would have anyway, as the outside is clad with reclaimed 3" XPS that is covered with Grace Bituthene. The bituthene was to protect the foam long term as well as act as drain shield for ground water. The Xypex was used to keep the footing and walls from soaking up the annual spring water that travels along the rock I am sitting on.
I am anchored to what is called bedrock here, though I am more use to the solid granite I knew when growing up north of you. The sandstone bedrock is multilayered and fractured, so water that gets into to it up slope tends to erupt out of the fissures. I put in 100% washed stone to level the very sloping interior prior to casting the slab. This was to be sure water made it to my low interior drain point. Bonus, the rock bed was deep enough that I was able to put in a radon collection system for future need if ever.
You mentioned some upward intrusion on your existing slab, so if you can plan for a bit more stone to buffer seasonal upwelling, it might be a good idea. Make the soil pitch toward your sump before adding the rock.