Below grade foundation plan… Have I reached the point of diminishing returns?
MikefromtheMountainsofUtah
| Posted in General Questions on
I’m in the process of detailing the exterior of my 9′ tall sub grade wall in zone 5.
It is damp-proofed and I’ll be installing the dimplemat today in preparation for backfill. The question is, from the inside out it will be R-19 batts, 1-2″ of closed cell. If I spend 1200 on 2″ xps, that brings me to an R35-40.
Should I cut back on the exterior foam detail? 1″ rather than 2″, or not do the full height of 9′?
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Replies
UtahMike,
Your description is a bit confusing when it comes to the mention of exterior foam. IMHO all exterior foam makes the most sense although it does complicate the detailing of both above and below the sill plate line.
If you are asking if should you apply the foam after the dimple mat, I would say no. If anything the dimple mat should go on the outside of the foam and both foam and dimple mat should extend the full height from sill plate down to footing. With this lay-up any water will drain down the face of the foam and not be able to build up enough to saturate the foam. I assume proper footing drainage lines are part of the plan. The dimple mat will also protect the foam from damage during backfill. I think the foam needs a minimum of R-15 to meet code and remove the need for interior insulation.
If it is too late to alter the overall placement of insulation, then exterior foam outside of the dimple mat is relatively pointless. Most codes require a minimum of 6" of elevation drop below foundation top which leaves the concrete completely exposed to ambient temperatures. The thermal conductivity of the 6" of exposed concrete will negate any R value provided by exterior foam pretty far down the wall height. This exposure will also cause your sill plate and joist pockets to become very cold during winter and create the condensation situation much discussed here on GBA
You have not indicated how the above grade house walls are insulated, so it may your choices are limited. As I noted the addition of exterior insulation on a foundation forces many complications. I can address them to you if there is need or time to do them.
Onslow, Thank you very much for the quick response. The foundation wall goes from 7-8' of retained soil in the back to daylight around the front. My plan was to place 2" xps from footer to the top of the wall all the way around and I certainly understand the detail complications but it's all exposed and I'd rather stay on the side of properly addressing the efficiency of the addition and deal with the extra need for flashing and metal work later. It's on the back side and not seen much so the finishing can go a lot of directions and be ok.
I was surprised to not find a detail from the dimple mat company on installing over foam, and it made me realize that my ramset's 3" nails are going to be ineffective I believe. I might try it but with only .5" - 1" of grab it made me think my installation plan is flawed. Any idea if that will hold enough to get dirt in?
Also, I've seen it mentioned on gba that foam is most critical above grade on the exterior and not very helpful once below the frost line. Do you agree with that?
How does this look? Complimented by a space behind the interior stud wall for closed cell and R-19.
Mike,
The purpose of the exterior foam is to both reduce energy loss, and keep the inside face of the concrete warm enough that the warm interior air does not condense on it. If that foam is not continuous, it will do the first, but perhaps not the second. Like Onslow I would suggest moving that foam to the inside. https://www.greenbuildingadvisor.com/article/three-ways-to-insulate-a-basement-wall
Hey Malcom! So to be clear, are you saying that even if my local building code allows foam to cover the entire foundation wall I should not? I guess I don't understand the logic, in areas where termites are irrelevant, considering the greatest fight against the cold would be from the frost line up to where the roof begins.
Mike,
No I'm saying the opposite. If you only cover the top, the part below will have no exterior foam to keep it warm enough to stop moist interior air condensing on it.
Every part of those walls needs foam on either the inside or outside.
I see. Would you support just scrapping the exterior rigid board entirely (just hot tar and dimple mat then), and using closed cell inside + batt? I've created a little bit of a moisture sandwich between the tar and closed cell in this case, but I know it's common practice.
Mike,
My preference is to have the foam board inside as it doesn't need protection, the foam can be made continuous with that under your slab, and it removes doubt as to whether the inside face of the concrete will become a condensing surface. However if the walls are going to remain unfinished I agree with Onslow that exterior foam may be a good choice.
(Reply to #9)
A "moisture sandwich" of the concrete isn't a problem. Moisture doesn't harm concrete.
UtahMike,
I have attached two views of my own resolution to exterior insulation on my foundation. Not to cross swords with Malcolm, but I guess I did not make clear my support for insulating ONLY on the outside of foundations despite the nuisance detailing involved. Thermally speaking, it makes the most sense in CZ 5-7. Termites could be a deciding factor for some. My own home, above grade, is also clad in almost 6" of exterior foam (in total), so the problem of siding plane to foundation plane differences resolved in my favor with about a 3" offset that makes the "belt line" look normal. If you are not doing CEI on the framed portion of the house then you will likely have a reverse offset in the siding to foundation surface planes. Awkward looking and even harder to detail.
You indicate that the grade falls from 8' to zero, so the frost line is moot for a couple of reasons. If I am envisioning correctly, at least 50% of your foundation surface area will be above grade. The portion that is more fully "earth moderated" will only net small reductions in net losses compared to the exposed parts. Even adding the 2" layer on the outside will be heavily defeated by ground conduction losses. It may help to think of the foundation thickness as being equivalent to a large air gap between interior insulation and exterior. I will admit to being too lazy to breakout my calculator and prove this.
First photo: Instead of adhesives we also used ramset anchors to secure the 3" reclaimed XPS to the foundation. The failure rate of the fasteners seemed excessive. But for the damp coat on your concrete I would have suggested Fuzeit or similar foam compatible adhesive. Not sure what to say will stick now. The Grace Bituthene skin was my choice to seal the foam and its gaps against ground water intrusion. Backfilling was a bit of a nightmare given how tender it was. I ended up using very thick polyfilm to protect it though dimple mat would have worked too IF it would stay in place without fasteners. No sense sealing everything only to shoot holes in it. As I noted, lots of PITA details.
The second photo is a view of my chosen skirting solution to protect the exposed portions of the Bituthene. Much of my exposure was the required 6", but one corner is about 3 feet. You indicate going to daylight on the downhill side, which makes my choice of Hardiboard rather tricky. 3'x5' is the largest size I am aware of. There are aggregate covered foam panels that are bigger, but pretty much only sensible for commercial scale use. There was a sort of recent article about one version in Fine Homebuilding or GBA. The Hardiboard I used is suspended from an unseen element that supports my exterior insulation. I aligned the panel seams with aluminum stock and washer head screws.
The basement foam's deep offset below the sill plate allowed me to start the much thicker exterior insulation below that level and grant me both a wide sealing area to prevent air intrusion at the sill line and to more effectively insulate the rimboard- joist pockets. The whole house is intended to dry to the inside and to keep all framing elements above 50F even at the coldest of temperatures. The (to some imaginary) climate warming has limited my real temperature lows to about -10F for only a few days a winter. Local weather records showed a much chillier past. The net positive effects of these choices mean I do not have any interior insulation on my foundation, no condensation worries in my joist/rimboard intersects. Upstairs, no fussing with membranes or electrical box seals and no worries that my plumbing or electrical work will be providing moist air leaks to cause future problems on cold sheathing.
Monetarily, I think it is pretty much a wash having chosen exterior insulation over interior for the basement. I have raw walls which are fine for a shop space and I did not have the expense of framing and drywall over interior insulation to meet code. I have found my wall temperatures to be lowest near the slab level which make sense as the footing is closest. There is only a few degrees difference between below grade and above grade wall areas. I will admit that the median air temperature hovers around 66F in the basement. Not quite suitable for living space, though I tend to wear flannel shirts most of the time.
If your below grade space is intended to be fully finished and occupied as living space then skipping the exterior insulation might make sense for multiple reasons. It is true that you will have detailing issues at the sill/joist/rimboard to take care of, but on balance likely not more than I did. Insulating the inside at this point may well prove to be the wiser path for comfort. I do hope the slab is insulated though.
Utah Mike,
Helps to attach the pictures :(
Mike, consider spray foam is best avoided as much as possible as it is the most expensive least green and riskiest way to get an R of insulation. It may be a good choice for the rim joist because it is fast and easy.
Walta