Slab on grade with stem wall above grade in a cold snowy climate
This is something I have seen done in garages, but I can’t find much on doing it for a house. Where we live, we have a 4 foot frost depth, and a 50 lb snow load. Most houses around here have full basements because by the time you get 16″ above grade, you already have 5+ feet of block or concrete.
I would like to build on a slab with radiant floor heat, but I really can’t get behind the idea of melting snow sitting up against the stick frame and gradually rotting the walls. For everything except a FPSF, code says 16″ above grade. Ideally I’d like to have the stem walls be 2 feet above grade. With a platform framed house, now we’re talking about the floor being close to 3 feet off the ground. From a design standpoint, I really don’t like being way up off the ground. I’d rather one step down and I’m walking on the grass.
Anyone think there is anything structurally wrong here? Has anyone seen this done before? The engineer that is designing our septic will probably do the final foundation design for us too. He has already really pushed for a full basement (because he likes having one), but said he will design anything we want.
Here’s a basic diagram of what I’d like to do (http://i.imgur.com/XQmZMpA.png). Many details are omitted – the main idea here is that I would stack block from frost depth to final stem wall height, (with all the proper details) pour the slab at ~6″ above grade. Doors would be about one step off the ground.
Not sure if this image link will work…
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Replies
Nathan,
I don't want to discourage you if that's what you want to do but a couple of points come up.
In all the codes I'm familiar with, the stipulation of 16" separation between grade and untreated lumber applies to structural members like deck beams outside the structure, not the exterior wall framing. What governs there is a separation of 8" from grade to the top of the concrete foundation.
Seeing as this is the case, and almost all houses in cold climates are built with wood framing which is covered by snow during the winter, is there really enough evidence of widespread rot to make you need to deviate from how everyone else builds?
Nathan,
Q. "Anyone think there is anything wrong here?"
A. At least three problems: (1) the rigid foam on the interior of the CMU wall looks thin -- so you don't get much R-value. (2) There is a thermal bridge at the top of the CMU wall (at the interior corner of the top of the wall). (3) It's going to be hard to make the drywall on your wall co-planar.
Malcolm, I have seen sill plates rotting and water damage all over the place where I live. I would say it is a widespread problem on houses that are less than 1 foot above grade. I think you're right that the code doesn't say 16". The old houses that are in good shape around here all have at least an 18" stem wall.
Martin, you deleted 'structurally' when you quoted me, and then took what I said out of context.
I suspect the rot that Nathan is seeing is more because of drainage issues than distance above grade. In a snowy climate, you can easily have snow piled against the house much higher than 16 inches above the ground. In Maine last winter we had at least six feet of snow piled against the north side of our house for three months. So long as the melted snow or rain drains away from the house, the sills are going to be OK.
Nathan,
You're right, of course, that I deleted the word "structurally" from your question. I hoped that my comments would help you think about thermal issues in addition to structural issues. (This is a green building web site, so thermal performance issues are often discussed here.)
I don't see any structural problems to your suggestion.
Nathan, to get back to your question:
There is nothing structurally wrong with what you are suggesting. Shorter walls similar to those have shown a greater tendency to collapse in seismic events, but by the description of your climate I'm assuming that isn't a problem.
It does complicate things like exterior doors, but it also offers opportunities to increase the insulation on the whole walls when you fur them out to match the foundation.
NATHAN - We don't know where you live, but I agree with Malcolm and Stephen. 8" above grade ( 4" slab with 4" of insulation) is plenty for a cold snowy climate like Maine where I live. If very concerned, you could use pressure treated wood framing on the first floor and even pressure treated plywood the first 4', but it's not necessary. Use a rain screen and detail your air and water barriers well.If you do go with your plan, ICF's might be a good choice for the walls. These should probably go below grade as we'll, as there is nothing in your drawing preventing frost from getting under your slab.Or you could extend foam down the inside of the stem walls to protect the soil under the slab. Martin is right that getting enough insulation will be tricky, and detailing that interior shelf will be tricky and very time consuming around doors, kitchens,etc. With more insulation you wouldn't need heat in the floor. Keep reading and asking questions. Best of luck to you with this building project.
Thanks for all the responses. I am surprised that everyone is saying 8" is plenty of clearance off the ground.
I just created my account, but have dedicated myself to learning building science. Joe Lstiburek's "Builder's Guide to Cold Climates" has been my bible for the last year (same with buildingscience.com)
I am in Central NY, average snowfall is 7 foot per winter in the valleys, and I'm on the mountain.
I first read about extending stem walls higher than usual in Ben Falk's book about building for resiliency. "Let's start the discussion with a simple fact: The most common and serious problems in cold-humid-climate buildings have occurred at the interface of the foundation and frame. ... go into the basement right off the bat, and look at the corners of the building and tops of foundation walls. Rot in this location is all too common even on modern buildings."
Before committing to buying land (which I've now done) we saw several houses. Many of them were water damaged at the foundation/sill interface... I even saw one farmhouse with a spring running through basement, but that's another story. Another standout was a 1970s timberframe with ~8" above grade on one corner. The PT sill was hanging in there, but they also put down a timber sill that was completed rotted away to the point that the house was sagging and there were gaps around the windows. I want my house to last longer than 40 years.
Snow piled up against a house... it's going to melt on the house side first, similar to what happens with ice damming. The capillary break that the rain screen provides should help, but that's not really what it was designed for.
I guess you guys are saying there is a way to detail your wooden wall so that slush drains away from it... it just seems easier/safer/more resilient to lay an extra two courses of block and be done with it. Just about every well built old house in this area has at least 16" of clearance above grade, and I don't think those guys were stacking rubble for fun.
Nathan,
I happen to agree with you about raising the lowest wooden components of a building more than 8 inches above grade. I also agree with your observations: one often sees rot at this interface. I tend to blame splashback more than snow -- but in any case, increasing the distance from grade to the lowest wooden components of the house certainly helps.
There are no structural problems with your solution, as I said before, but I would proceed differently. I would go for a full basement (with poured concrete walls, not CMUs), and my first floor would be framed with joists rather than using a slab.
Thanks Martin. I have been trying to think of ways to balance design with resiliency and energy efficiency. I will likely use block because I am doing the entire build myself, and don't have access to concrete forms (and often no second set of hands). The functionality of a full basement does win, no argument there.
This picture actually shows what I was thinking of - the stem wall extends above the door, which is almost at ground level. It is a little difficult to see with all the vegetation.
http://static1.1.sqspcdn.com/static/p/547988/4990475/1270693995970/635_wsd_studio_shop_oct_10_2009_south_side.jpg?token=R60C6yUxz9CHrGEOzqNIKvy4fCw%3D
I think semi-DIY one-person building with ICFs is pretty feasible, maybe one of the easier solo building approaches if you don't mind calling a concrete company to come fill them. You could even do the whole first first floor with ICF. But my knowledge of the process comes only from reading and watching, not doing.
The practice of including a capillary break between the foundation and the wood components on top of it has greatly increased resilience with 8" of foundation showing compared to some of what you have seen, but I agree that more exposed foundation is better. Generous overhangs are also helpful.
Having the top of the foundation wall located halfway between the slab and the floor joists above does make it less structurally robust than if that joint is close to one or the other. For example, if someone were to drive a car into your wall, the wall could be more easily pushed in without that junction supported. That structural issue is sometimes an problem with a 4' high foundation wall above the slab, if wet soil is against the foundation wall, putting pressure on it. But you won't have soil piled against it, so you won't be subject to that problem, and if you have cars driving into your wall there are probably other changes you should make rather than designing the wall to survive that.
Thank you Charlie. My main concern was/is, with platform framing, anywhere you have a top plate junction with bottom plate you need a floor in between to hold everything together.
Intuitively, I would think a concrete and rebar wall stuck 4 feet below the ground, and 2 feet above could hold the 6 foot stud wall in place above it. I have intentionally selected a simple house design to avoid needing engineers or very specialized tradesman to do everything.
I will likely further detail my foundation design and present it to my soil engineer, and he can modify it as necessary.
Re: ICF; In the beginning, I had researched this... I thought ICF's still need to be braced before being poured? When I saw a fellow DIY home builder bracing his ICF walls with metal forms he rented, I kind of abandoned that thought process. My other concern with ICF is that I don't want exterior EPS in the ground, running up to my exterior (likely 4" EPS) foam. I got turned off from that after reading "BSI-056: Leiningen versus The Ants Redux." Additionally, there are old slate-ish/shale stone farm walls on the property that I would like to use to veneer the stem wall, which I'm not sure I could... glue those to EPS...
Anyhow, I don't want to go too off topic here. I mainly just wanted some generic advice on whether or not it is a generally acceptable building practice to float a slab between a stem wall like that.
Using an EPDM sill gasket on top of the CMU does a LOT for limiting moisture migration into the framing, independently of how high it is above grade.
Deep roof overhangs does a lot for limiting the amount of snow drift against the house even on windy storm. On serious Nor'easters even 4' of above grade exposure and 2' over hangs isn't enough to keep snow off the siding, but with a very good capillary break the CMU can be completely saturated-soaked without affecting the moisture content of the sills.
My house received more than 8' of snow in just 6 weeks last winter, some of which piled up against the house higher than the WINDOW sills during the heaviest dump. I had to dig up the foundation more than once, despite 2' overhangs, but could have left much of it alone if the house had been built with good capillary breaks between the poured concrete and untreated sill 90+ years earlier.
ICFs are a good approach, but you'll still need a sill gasket with good capillary break characteristics. You can fix the ant & termite issues by using copper flashing as that fill-depth capillary break, extending over the EPS on both side. That creates a thermal bridge, and there are copper-clad plastic versions as well if that thermal bridge is too heavy a performance hit.
This is all way more complicated than it need be. There's a simpler way to get your finish floor close to grade. Here's how it's done:
Frame your exterior walls straight from the slab having first built your stem wall out with a brick ledge which terminates at least 8" below framing level. Protect your sheathing with 2' or so of peel and stick flashing down to the top of the brick ledge. Dress your WRB over the outside of the peel and stick. Install a masonry veneer (stone, brick, CMU w/ stucco) on the outside of the flashing with a metal flashing to the siding above and weep holes below. Back fill against the masonry to your heart's content. Protect exterior door cills with at least 4' of gable porch overhang. Exterior walls are insulated right down to the slab and you will have no jamb issues with inward opening doors.
You can also use this technique with framed floor systems, and even bring finish grade higher than finish floor in extreme cases if needed. Just be sure to run the peel and stick at least 16" or so above final grade and flash everything properly. Here's a sample configuration showing a framed floor, you can modify this to your slab, if that's what you want, with your preferred edge insulation detail. Peel and stick is shown in red, WRB in blue. We've used variations of this kind of detail reliably for many years when at-grade entries are either preferred or required e.g. for wheelchair accessibility.
I might be paranoid, but I don't like the peel-and-stick going up a foot or two on the stud wall. It becomes a wrong-side vapor barrier, and the wood there could get wetted by condensation even if it can't get wetted by snow melt or wicking from the ground. I don't think it would be a short term disaster (and your experience verifies that) but if the goal is to maximize robustness, I don't think it's the best choice.
Hey Charlie - you're right that it could become a problem if you have a wall profile that doesn't permit drying to the inside. All the usual rules of insulation layering and vapor management in accordance with your local climate condition will apply. Having paid proper attention to those details there's no reason this arrangement should be less robust than any other wood framed structure.
James Morgan,
There are several possible pitfalls with your detail. Charlie mentioned one of them: you don't want to use most peel-and-stick products as a WRB on the exterior side of your wall sheathing. If you insist on using peel-and-stick here, it has to be one of the expensive vapor-permeable peel-and-sticks, not ordinary Ice & Water Shield. Ordinary peel-and-stick will create a wrong-side vapor barrier.
The best approach would be to install an adequate thickness of rigid foam (not peel-and-stick) on the exterior side of the wall sheathing.
The second problem is that the brick veneer will draw soil moisture upward by capillary action. This shouldn't be a problem if the mason remembers to install through-wall flashing a few inches above grade, along with an adequate number of weep holes to allow drainage and to provide ventilation inlets. Your sketch fails to show the flashing and weep holes, but these details are essential. They are also tricky, because the mason may not know how the excavation contractor plans to backfill and grade the site.
Response to Martin #17
This may be one of those occasions where the wide geographic and topographic range of GBA can lead to contradictory viewpoints. The OP says nothing about location, and I should say that my experience with this detail has been entirely confined to the very southern edge of climate zone 4A, one county over from zone 3A. In this location problems with sheathing rot are invariably a function of improper grading, splash back from roof drip, plumbing leaks and other bulk water issues rather than interior condensation. In this environment we have experience using this detail without adverse consequences for nearly two decades. I'd add that we have invariably used it in connection with dense pack cellulose wall insulation, allowing for for hydric redistribution, and that the vapor impermeable layer occupies only a small area at the bottom of the wall. If a more demanding climate recommends special attention to the vapor permeability of the flashing then your point is well taken, but it does not seem to negate the basic premise of the configuration.
To your second point, I admit I am confused. The concerns are no different than those for any other common masonry veneer construction, concerns which masons are very used to addressing. I already noted the need for weep holes. Where exactly would the through-wall flashing go? Maybe this is a snow-melt issue, with which I am thankfully unfamiliar.
I did not go into into the complementary details which I assumed would be taken for granted in this forum: proper roof water management strategies, good overhangs, proper surface grading etc. etc. But bottom line: I see a clear advantage in a clean and consistent wall insulation and service run condition all the way to the interior floor level (maybe I missed it but nobody seems to have mentioned the problems that the electrician and the plumber would face in the originally proposed configuration). Stepping out to grade seems to be Nathan's core performance requirement. I stand by a construction strategy which with appropriate local detailing offers a way to achieve both objectives without making a dog's breakfast of the exterior wall construction.
I live in zone 4/5 in Central NY. Lots of rain and snow, consistently all year. Lots of humidity. The advise that a lot of water damage is from poor grading, splash back, and not snow, has been noted. I understand that a 2 foot stem wall is no excuse to not have proper flashing, capillary breaks, grading, overhangs, and the other 50 things I'm forgetting.
I actually do appreciate all of the alternative suggestions here. It is true, the real question is - how can the doors be 6" off the ground, without subjecting the framing to conditions that occur within 24" of the ground.
I spent many years in the mid-atlantic, and the amount of rot I see in upstate NY, I don't recall being such a widespread problem in Maryland.
Design, engineering, thermal efficiency, it's all big balancing act. If a net zero house has sill or wall rot after 40 years, and it needs to be jacked up and fixed, that doesn't seem so 'green' to me. When I'm balancing my options, I would sacrifice *some* thermal efficiency at the stem wall to make the house more 'resilient.' Balance that with needing to heat (or cool) 50% less space because I don't have a giant underground storage space, and I think I still come out ahead. How well do those peel and stick membranes hold up after being saturated for 50 or 100 years? I know that a rock wall will be doing just fine, and in all likelihood so will the interior rigid foam board.
James,
I'm sure that your details work. I was clarifying the pitfalls for GBA readers who were considering this approach -- and I didn't mean to imply that you had fallen victim to the pitfalls. I'm sure that you successfully avoided them.
Your solution makes sense.
Nathan:
"the real question is - how can the doors be 6" off the ground, without subjecting the framing to conditions that occur within 24" of the ground"
I mentioned in my earlier post on this topic, a door cill that close to grade needs deep cover. That's what entry porches are for. There are door/jamb assemblies available in the marketplace (fiberglass, metal, specially treated wood) that are not subject to rot, and properly installed with good flashings to associated framing these potentially can be trouble-free in an exposed environment, but there are so many other advantages to proper overhead doorway protection that I'd not count on that alone unless there were really no alternative.
Your comments on resilience are well taken, it's what we all strive for. I would not propose any strategy that I do not believe after thirty years of practice with both new build and renovation work in two very different climate zones can be made fully resilient in the long term, and thank you Martin for your caveats in this respect. As to this: "How well do those peel and stick membranes hold up after being saturated for 50 or 100 years?" No part of any exterior wall assembly should be exposed to saturation conditions for any but the briefest period of time and that only in the most extreme circumstances. You cannot expect any wall assembly to compensate for improper detailing of exterior bulk water management as required by most building codes.