Insulating Both Sides of a Slab
Basement Floor Insulation Strategy – Above and Beneath?
Designing a new home in CZ5. Consultant recommended insulating both below and above basement foundation for comfort and minimizing potential moisture issues.
I know it’s code where I am to insulate either on the interior or the exterior of the basement floor, and I was able to find a BSC article that talked briefly about insulating both sides, but does anyone here have experience doing so? Is it one of things that’s overkill in most situations?
The majority (75%) of our basement will be unfinished, which is another factor – insulating above the floor means the ‘unfinished’ floors will also need subflooring and probably some sort of flooring so there isn’t exposed foam, which means the unfinished space isn’t exactly unfinished anymore. Thanks!
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Replies
david_solar,
If you insulate underneath, the slab will track the room temperature within a degree or two, so I don't see how insulating above adds to comfort, and I'm struggling to see how it influences moisture. What insulating both below and above does add is complexity.
Do the below slab insulation now while you can, and if at some point in the future you feel the need to increase insulation you can insulate above the slab at that point. I know people like Jake Bruton hate underground insulation because he claims critters destroy it but you can always insulate the top later.
I would agree with this sentiment, I have seen a lot of ants nests in foam, and not just carpenter ants. I would only use rigid mineral wool in contact with the ground. It’s not just the insulation properties but the structural issues with this. I think only time will tell but I think it is not far fetched to think we may see issues crop up in the future with this approach.
feyr_design,
I generally agree, although I think there is a distinction between how vulnerable foam is under stabs inside stem-walls, and when used on the exterior, or under slabs with thickened edges.
My biggest recent battles with carpenter ants have been in roofs.
https://www.greenbuildingadvisor.com/article/if-ants-like-rigid-foam-should-we-stop-using-it
Ya that may be true, although I have seen big odorous house ant nests in foam. It’s almost like they seek it out. These ants also create multiple nests to be closer to food sources, which makes me feel like it may not be that far fetched to think they would move in under slab.
Those projects mentioned in the link sounds bad, any resolution? I am very surprised it’s not a matter of course that they add some sort of deterrent to foams ( as you mentioned, I have never actually seen any with added insecticides. )
Freyr_design,
After a lot of time, money, and chemicals only one of the projects I wrote about still has problems. The flat roofed cabin was built on piers close to the ground, and is situated in dense PNW forest. They still fight fresh infestations there a decade later.
David Solar,
I find it very unusual that the local code would suggest insulation above a slab. Malcolm is right that with sufficient insulation under the slab you will find the slab to be within 2 degrees of room settings. I would worry more about properly insulating the walls of the foundation, especially as your current design calls for a major portion to be above ground. If the concern is "cold feet" then wood or cork flooring will make the perceived temperature more tolerable. Of course the slab needs to be dry and free from contact with ground water AND the potential transfer from the walls and footings. If you set the foam above the footing top and add edge foam you should have an isolated slab that will not transpire much if any moisture from ground sources.
Moisture should also not be an issue if the drainage is done right and the underslab foam is sitting on a bed of washed rock to prevent the potential for capillary rise present in ordinary fill. The significant slope you have on the lot should make it easy to establish footing drains to daylight or at least a below slab level drain pit. The bed of washed rock also makes it easy to install pre-need radon piping. My local radon person advised me that attempting to test the soil before building is fraught with problems and likely inaccurate. A few hundred dollars of perforated pipe now could save you thousands later.
Regarding ants, it generally would not be a natural behavior for them to bother digging under a footing and then occupy the stone on their way up to the foam under your slab. Leftover wood from form work or scraps thrown into the basement prior to placing the foam and slab could give them motivation if present. Wet wood is like the fallen logs they are meant to reduce in forests. Termites as well are here to clear wood. If you wish to avoid both, be adamant about keeping the site clean of all wood scraps and don't seed your perimeter with wood chip mulch. The roots of close plantings will also encourage both to take up residence and explore your house once their nests get large. Damp sill plates, door sills, soggy window sills provide them with good nesting options, so keeping the house dry and using termite flashing is a good idea. Use small hole perforations when flashing for rain screen gap under siding.
Exterior foam is indeed vulnerable to ant nesting. Not leaving raw foam exposed or encapsulating it with a barrier has proven effective in my case. All my roof foam is thoroughly encapsulated and so far only the bees have taken up residence in the eaves where framing pockets provide them safe haven. They are carrion eaters and don't bother the wood.
On a separate note: I followed your earlier posting about the house plan in general. I have further thoughts and suggested plan revisions that you might find helpful. If the GBA masters allow, I could send them directly to you or if you don't mind having the older thread revived I could post there. I will wait for a reply before butting in more than might be desired.
David Solar,
Sure you can insulate on the top face of concrete. If you need only like 2" then this is "ok", if you need more then the walls go higher and the stud longer - you want the floor insulation and the wall insulation to overlap. It will add in cost, also the transition to outside might be more finicky.
If you want "thermal mass" then this is also gone that way.
You need to encapsulate any moisture in the concrete, so a heavy VB on top of the concrete first (maybe a liquid flash).
regards
edit: actually two BSI come to my mind
https://buildingscience.com/documents/insights/bsi082-walking-the-plank
more important: https://buildingscience.com/documents/building-science-insights-newsletters/bsi-118-concrete-solutions with an example