Retrofit addition of footing at foundation base; capillary break?
We’re finally addressing our wet gravel-floored utilities basement – we’re putting in a slab floor. In digging down around the inside of the foundation wall to lower the floor and make room for the slab etc., we discovered that there is no footing under the formed concrete foundation wall! (This wood-frame house was built in 1936, has 2 stories above the basement, and is in Maine, zone 4-5). Argh. The wall sits on hard dense clay. And guess what? Water trickles in underneath. Contractor suggests forming a footing to the inside of the existing wall. This footing would sit above and below the bottom edge of the wall, and have a small (2″ or so) lip under it. Imagine a fat lower-case “h” shape. We’re drilling into that wall 6″ deep, along a line about 3″ above the bottom, every 2′, to insert 6″ of a 12″ long, 1/2″ rebar, the other end of which will be incorporated into the new concrete footing.
If this is a bad idea for a solution for the missing footing, please tell me!!
Now, about the capillary break. I know there’s supposed to be a capillary break between the footing and the concrete foundation wall. That would be, say, a layer of 6mil plastic, or TenoArm, or Tu-Tuf#4, right? However, the usual scenario is that the foundation wall sits square on top of that nice slippery layer. But how about in this scenario, where the footing is kinda on the side? The capillary break layer would surely cancel any holding power of the concrete. But maybe the puny 1/2″ rebar is enough, and the perceived adhesion between the new and old concrete wouldn’t deal with the shear anyway, so just count on the rebar and the 2″ lip, and install the plastic capillary break? Man, I’m not very confident about all this…
Help!!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Anita,
If the house has not settled or had the walls crack from heaving since 1936, then there's no need for an additional footing. Most 2-storey homes on reasonably solid subsoil can support the gravity loads on 8" of foundation.
Unless the building inspector is requiring a retrofit footing, this appears to be a waste of money. What's more important is an adequate perimeter drain, a sub-slab radon vent, and a good air/vapor barrier under the slab.
Placing a capillary break between an offset footing and the existing concrete wall (I assume it's concrete) will do nothing to isolate the wall from the soil. You should, instead, isolate the slab from both the wall and any footing with either slab-edge insulation or an expansion joint.
Sounds like your guy is describing a simple "bench" beam running parallel to the existing with a slight lip and "hot joints" with the rebar dowels. A couple of thoughts:
1) You really should think about bringing in a structural PE on this. They can evaluate your soil conditions, load paths, and existing compaction. They can also verify whether the rebar is sized and spaced correctly.
2) I don't think the 2" lip can be counted on to hold any weight as you asked. I suspect this is intended simply to provide a bond point that would prohibit the new beam from moving independently of the existing wall. In theory the rebar may also provide this, but I wouldn't trust it by itself, thus the lip
3) Wherever the new concrete will be intended to join the existing concrete, you want to be certain to use a bonding primer on the existing to increase adhesion. You also want to be certain that the new mix being spec'ed is designed for a bonding application.
4) The only way to actually get a capillary break into a retrofit situation like yours would be to expose the entire bottom of the existing wall. Clearly you can't do this because it would remove all existing support.
5) If you're already having water issues, you need to design an interior drain tile system into this solution which will go to a sump pump and be able to evacuate the water from the footing.
6) I suspect a PE may also suggest underpinning your existing wall at select points. This will provide more substantial support than a lip and some rebar. This process also does not require you to remove substantial amounts of the existing clay soil thus risking compromise.
Thank you, Robert and Andy!
Robert, we are indeed intending to install drain tiles (perforated pipes) on the inside perimeter as well as across the room (room is 23' x 29' and water is significant), and connect them to a daylight drain, as well as an air/vapor barrier.
Re. radon - would a daylight drain connected to the sub-slab drain tiles, as planned, provide adequate ventilation? Must a vertical stack be added? The daylight drain does connect the sub-slab gases to the outside atmosphere, but there isn't the vertical warm-house-air effect (or heat loss effect).
Hmm, Robert suggests a footing is a waste of money, and Andy suggests an engineer's opinion of the proposed strategy would be wise. Wow. How to decide? Engineers are expensive too. Guess I'll make some calls.
Thanks for the suggestion too of the bonding primer and bonding mix.
Question - Is slab-edge insulation the same as sill gasket insulation, the roll of foam that's kind of corrugated and about 1/8" thick and about 6" wide? Or is it something else? Like XPS stiff board? I'm using 1" XPS boards under the slab. Should a strip run along the edge of the slab as well, between it and the footing?
Thanks again, you guys are amazing - just great!
Anita
I have to agree with Robert - if it ain't broke don't fix it. If the foundation wall is showing no signs of distress the offset footing is of no value. It could actually be detrimental in disturbing stable soil adjacent to the existing bearing point of the slab and depending on the geometry could act as a dam to push water up and over the slab, defeating the point of the drain tile. You want the easiest path for any water coming under the wall to be right into the perimeter drain.
Radon - probably above my pay grade to comment on this as I don't operate in a radon area but if you have a radon issue (has it been tested?) then I'll bet you need the vertical stack. If you don't have a radon problem then the drain to daylight will likely be sufficient.
Anita,
Since clay has high compressive strength you may be fine without the compromised footing. I'd still give a PE a call to get a sense of how they felt about the idea, don't necessarily have to hire them to do calcs.
Sill gaskets are not insulation. They are only intended to stop air flow between the junction of two materials meeting. I'd consider using at least 2" of XPS under the slab if not more. Since your foundation walls are not thermally isolated from the soil insulation around the edge of the slab will not contribute much.
Concerning your radon strategy I have heard when it is difficult to vent through a vertical stack a technique similar to the one you described is used. However I think there is a "back flow preventer" fitting necessary on the daylit pipe. We did something similar on a new project. Our intention is that it is a passive system but if our radon readings are not low enough we will have to add an active fan to the system which is an energy consumer. The "back flow preventer" may only be necessary if the fan is used I can't remember and I can't find the information at this moment.
This is just a thought here, and I am neither an engineer nor a highly experienced builder. My gut says "Why change?", as Robert suggested. However, if one feels the need, or a house inspector does, would it be possible/advisable to do this instead of disturbing the soil around the bottom of the wall? Why not pour a short wall, about 6" thick and one foot high, inside the existing foundation wall, monolythically with the slab? Rebar could be run through this short wall into the existing wall and slab, and you'd at least gain some bearing area for the wall by transferring load to the slab. The slab may need to be thicker for a foot or two into the room. This may be a poor use of a slab, but it seems like we are not looking for a huge boost to a wall that has been working a long time. Just a thought.
Normally I agree with the concepts of "less is more" and "if it's not broken..."
However, in this situation, it sounds from your initial description like you've dug the entire basement down to the point that you've now exposed the bottom of the foundation wall for the full perimeter. If that's accurate, then you've completely changes the existing dynamics which have allowed that assembly to perform satisfactorily since it's original construction.
That's why I recommend getting a PE's opinion. In my experience, once you tinker with the dynamics of an existing situation (and in particular structural ones) then it often calls for an engineered solution to ensure that nothing's been compromised to the point of failure. If nothing else, you might spend a few dollars just for peace-of-mind. The PE may well look at it and say every thing's peachy as-is and save you a bunch of money and hassle. But, he may also be able to catch a big mistake in the offing and save you a lot of grief (and possibly worse) in the long run.
I agree a PE should be consulted.
Also, this may seem silly but just in case..... are you sure you've already done all you can do at the surface around the exterior, to make bulk water go away? Working gutters, runouts, proper grading, all that good stuff. If water flows on a slope toward the house, you may be able to divert a great deal of it with a shallow trench drain system. Of course, the water in your floor may be coming from other sources, but you have't done these other cheaper basic things, IMO its important to go back and do them now even though you're already into the archeological dig in the basement
Yes, do get a PE to check it, shouldn't cost more than a couple hundred bucks and could save you heartache and mucho $$$. Make sure s/he's a STRUCTURAL engineer with extensive knowledge/experience of soil mechanics. There are many flavors of PE and some of them are quite happy to give advice well outside their area of expertise. Don't hire your buddy's buddy who works for the local telcom and thinks their seal means they know everything about everything.
Anita,
Most heat loss from a slab is at the edges, not the bottom. This is particularly true with a slab-on-grade but also where a below-grade slab is poured against an uninsulated (on the outside) concrete or masonry foundation wall, which will become a thermal bridge to the outside.
If the space is to be heated and used for more than simply utilities (as finished space), then both subslab and slab-edge insulation are important (I don't know why J Chestnut said otherwise - the fact that the basement walls are contiguous with the soil makes it more important to insulate the slab-edge). Sill seal or expansion joint are not sufficient. A minimum of 1" XPS is required. You can bevel the top at 45° if necessary to hide it at the surface, and then pour the slab to the high point of the bevel against the wall.
A daylight drain is better than nothing for relief of soil gas pressure and potential radon infiltration (will the drain be bedded in a layer of stone to allow gas permeation?), but the system that almost invariably works is a passive stack through the roof. This cannot, however, be connected to a daylight drain as that will only draw outside air up the stack rather than depressurize the soil. The radon vent should be independent of the subslab drain.
Also,
If the only excavation you've done is on the inside of the foundation walls, and you'll be pouring a slab up against those walls (even with 1" of foam between them), then any lateral restraining benefit from the soil will be better achieved with the slab and there will be no change of dynamics.
Unless your building inspector is requiring the footing work, then not only is there no need for an additional footing, there is no need for an engineer. What you need is a competent builder or house inspector who knows how to do a forensic evaluation of the existing foundation. If there's no evidence of vertical or lateral movement or fatigue, there's no need for any reinforcement. You can concentrate on getting the basement dry and usable.
Anita excuse me for directing a question to Robert Riversong.
Robert,
I'm a little surprised that you are saying most the heat loss is through the slab edges. If I do a quick calculation of square footages of a 20x20 slab for instance the area of the bottom of the slab is 400 sqft and the area of the slab edges (assuming a 4" slab) is only 27 sqft. My understanding is that area is a primary factor in determining heat loss. Can you explain or give reference to how more heat is leaving the slab edges?
Also I'm assuming the basement walls are uninsulated. If this assumption is correct wouldn't the heat retention of a 1" by 4" piece of foam be negligible?
Thanks
J Chestnut,
The heat loss from an assembly as thermally conductive as concrete (like with a steel stud) is dependent on both the size of the heat path and the surface area in contact with a different thermal environment.
A basement slab in contact with a concrete basement wall doesn't lose heat just from the 4" edge surface, but through the enormous thermal bridge (or conduction path) of the entire concrete wall that it is now thermally connected to. The wall loses heat to the earth, which in winter is colder closer to the surface, and to the air in the above-grade portion. Heat conduction is a 3D process.
The 1" of foam does "retain heat" but slows the conduction of heat at that critical 4" edge before it can move in a 3-dimensional vector space to the earth and the outside air. That 1" thermal break has the same value as a 1" foam board on the outside of a stud wall, except it limits a dimensionally broader heat flow through a much larger surface area.
Also, the uninsulated basement wall will always be colder in winter than the interior cellar environment, while the insulated slab will be much closer to interior air temperature and so will have a constant delta-T at the edge where it contacts (or not) the cellar wall.
John Siegenthaler developed an equation for calc'ing slab heat loss. The variables in it are the linear exposed edge of the slab, the delta T, and the R value at the edge of the slab. No area. You can find the equation on page 24 of his book Modern Hydronic Heating, 2nd Edition. He told me in an email that he developed the equation from data in some ASHRAE handbook dealing with colder climates.
Thanks Robert and John.
I still wonder if the basements walls are loosing so much heat that any difference would be noticed in terms of lower heating bills or sensible comfort if the slab edge is insulated or not.
I did not consider the phenomena that if the slab is thermally isolated from the walls the slab will maintain its temperature closer to the air temperature than the walls could, and so the effect of "pulling" radiant heat from one's body would be overall reduced.
John I'll bet that the equation you are talking about extrapolates an area from the linear variable assuming a standard 4" deep slab. The standard heat loss formula is: Heat loss equal the U-value (inverse of R) multiplied by delta T multiplied by a surface area. Because the R-value (U-value) unit contains a unit for area (hr*sqft*T/Btu) you need an area to cancel out that unit (sqft) to leave a result of heat transfer (BTU/hr).
J Chest: His model looks nothing like the normal (if there is such a thing) heat loss equation. It has the length and delta T in the numerator, and a quadratic using the R value in the denominator. I don't see how the units could possibly work to give btu/hr, but apparently the number it generates is accurate. I believe I compared the results using his equation and Q = U A (Delta T) on my house, and they were a fair amount different. However, his model was closer to what actually happens (looking at oil usage, and adjusting a tad for heating domestic water). That is just a "FWIW" deal. I just found it interesting that "the answer" could be had w/out regard to the area, per se. To me, that indicates that the larger variable is likely the edge.
I'm sure Siegenthaler's formula is for a slab-on-grade, with which almost all the heat loss is at the edge to the exterior air rather than at the bottom to the ground. Deep ground temperatures are the average annual air temperature for any climate, and shallow ground temperatures under a heated slab soon equilibrate to something between localized ground temperatures and interior temperature - so that there is very little steady-state heat loss downward.
Measured ground isotherms in Saskatoon Canada under the slab of a full uninsulated basement were just a degree or two under indoor cellar floor temperature and at an adjacent slab-on-grade were actually a degree or two higher than interior slab temperature.
Here in cold north-central VT (8500 HDD), where average winter air temperatures are about 27° (with some days well below zero), the deep ground temperature is 42°-44° year 'round. I base my heat loss calculations on the conservative assumption that the ground under a slab-on-grade will equilibrate to 50° (delta-T = 15° rather than avg delta-T outside of 38°) while the slab edge must cope with exterior air temperatures.
Thanks so much for all of your thoughtful comments initiated by my queries about "retrofit footing," capillary break, and slab-edge insulation. Your comments were a big help as we moved forward on the project.
I just want to follow up on the project progress and a few questions that came up in the above discussion:
Yes, I wrote the initial question after already digging down about 13" straight down beyond the base of the existing (footing-free) foundation wall, exposing the hard clay underpinning of the foundation. At that point I had a PE come in and look it over. She suggested not excavating any further; in answer to my question, she said that originally, the clay base was not necessarily a bad choice for the original footing.
We discussed the plans for an interior footing (much like that described by John Klingle), moving forward with the plan of a short vertical wall, in this case ultimately 24" high by 9" wide, tied into the old foundation wall with rebar epoxied 6" into that, then coming out and bending down to the earth base, with horizontal rebar along the bend, and another course running closer to the base; rebar ties inserted into old concrete foundation at 2' o.c., ends epoxied into concrete, all new concrete at least 2 1/2 to 3" beyond any rebar. No "lip" under existing foundation wall (no further excavation, she cautioned!).
She seconded Andy Ault's suggestion of bonding primer (thank you!), and strongly recommended a "low slump" mix. In order to work with that, we rented a vibrator to help distribute the concrete into the form.
Under the slab (not yet in place) we are placing a network of perforated drain tile tied to a daylight drain, set on a stone bed, with filter cloth wrapped around a "sausage sandwich" of the perforated pipe surrounded by 3-4" of 3/4" stone. Should take care of the water, which even after pouring the new short wall continues to filter in underneath. Hopefully, the drain tile network, 1" blueboard layer with edges taped, and moisture/vapor barrier with seams taped, all under the slab, will adequately "reverse bathtub" the water and keep it at bay.
We are moving forward with insulating the EXTERIOR of the foundation with 2" of foam board up to the siding. The house foundation has two sections / two rooms: the original (footing-free) 1936 section (discussed above), and an addition that was added probably in the late 80's. The 2" foam board will go down 24" below grade in the old section, which is almost entirely surrounded by porches and decks on posts (some fun hand-digging). The 2" foam board will be carried down to the footing around the newer room (hoping that the addition put on 20-some years ago DOES have a footing!). The latter, because there we can access that section with an excavator, and must go down to the footing at the seam where the two foundations meet, which leaks a lot of water. So, from there we can put in some drain tiles at depth, and daylight-drain those out back.
Of course I've got more questions, but those are for a different thread.
Thanks again for your wonderful contributions, all.
I did hire a PE who