slab insulation - is this enough?
Last Post 23 Oct 2015 06:21 PM by walleygirl. 15 Replies.
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walleygirlUser is Offline
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04 Oct 2015 07:40 PM
We are building a house in climate Zone 4c (marine). The foundation is slab on grade and the plans call for 2" of rigid foam insulation under the entire slab, with 3/4" around the inside perimeter of the foundation wall. Originally the plans called for 2" around the inside perimeter, as per this diagram: http://buildingscience.com.678elmp02.blackmesh.com/sites/default/files/migrate/jpg/BSI059_Figure_01_web.jpg However, as you can see in that diagram, it shows the wall sitting on top of the foam. That is apparently not allowed by our building code. So the stud wall must sit on the outside of that wall of foam and then we have 2"around the interior perimeter of the floor that is exposed foam. Drywall is not that thick. I'd end up having to have very thick baseboards. We are doing polished concrete floors so I can't just pour concrete over that as it would defeat the purpose of the thermal break. So the builders suggested going with 3/4" foam around the inside perimeter edge of the slab. What do you think of this solution? I'm concerned that this might not be enough insulation, as I've read that more heat loss occurs around the perimeter. But by "perimeter" I'm not sure whether they mean the stem/foundation wall or the inside slab edge. Which leads me to my next question: the diagram above shows insulation on the interior of the foundation wall, but other diagrams show it on the outside (some even have a 'skirt' but that is not necessary in our milder climate). Which is correct? My builder says here people insulate on the outside edge of the foundation wall. Seems to me that is where the cold is coming in, not from the ground underneath the slab, which should be warmer by virtue of having a house on top of it.


arkie6User is Offline
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05 Oct 2015 10:30 AM
If you put the rigid insulation on the inside perimeter of the foundation, you can cut the top of the 2" foam at a 45 or 60 deg taper on a table saw so that the top of the concrete slab goes all the way out to the wall, or cut the taper so that there is 1/2" to 3/4" remaining at the top next to the wall. This will then be covered by the drywall and baseboard.

What size studs are you using for the walls? 2x4 or 2x6?

Is your foundation wall poured concrete or filled CMU / concrete block?

What type of exterior will be on your home? Brick, siding, or what?


walleygirlUser is Offline
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05 Oct 2015 12:23 PM
Wow, what a great idea!! Thank you very much!!

To answer your other questions, the wall studs are 1 x 6, the foundation wall is poured concrete and the exterior is board and batten cedar siding.



walleygirlUser is Offline
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07 Oct 2015 12:09 PM
My builders are concerned that the angle on the vertical insulation will affect the polished concrete flooring. They are concerned that the thinner bit of concrete at the edge might crack because it isn't very thick. They are off to check with the concrete flooring guy. Any thoughts?


arkie6User is Offline
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09 Oct 2015 06:16 AM
It sounds like they are looking for an excuse to not have to taper cut the top of that foam.


walleygirlUser is Offline
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09 Oct 2015 12:36 PM
Arkie6: They are concerned that the polished concrete floor will crack around the edges where the slab thins out as it goes over the tapered edge of the foam.

The working solution right now is to just use a foot underneath the baseboard trim that will cover the exposed bit, which I'm perfectly happy with.


HP HomeUser is Offline
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10 Oct 2015 12:07 AM
I was going to do the same thing with the 45* angle on the perimeter insulation. I also was worried about the cracking and at the last minute decided to leave the insulation square on top and fur out the walls with 2x2 instead. Added bonus is R30 cavity insulation in those walls instead of R23.


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10 Oct 2015 06:19 AM
Posted By walleygirl on 09 Oct 2015 12:36 PM
They are concerned that the polished concrete floor will crack around the edges where the slab thins out as it goes over the tapered edge of the foam.
I can understand the concern to some degree.  But if you cut the foam so that the top edge is 1/2" thick and then taper it down at 45 deg, the edge of the foam would be under your drywall.  You may get a little bit of edge chipping on the slab, but I wouldn't expect it to be more than ~1/4" or so.  This part of the slab would be under your baseboards.  Assuming 5/8" to 3/4" thick baseboards, any edge chipping of the slab edge would be hidden by the baseboards.  If you did get an occasional chip during construction because somebody dropped a hammer or board on the edge of the slab, and it extended further inward than the edge of the baseboards, just mix up a little portland cement and water paste to fill and smooth the crack prior to applying the stain and sealer and you will likely never see it.  Once construction is complete and baseboards are installed, I would not expect any further potential for edge chipping.


arkie6User is Offline
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19 Oct 2015 05:10 PM
Here is an example of what I was describing; although, this was cut in place with a handsaw. Edit: For some reason I can't get the image to upload.


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23 Oct 2015 12:44 PM
walleygirl - did you get a reason that foam is not allowed under the footings? It is in most jurisdictions but may require an engineer's stamp.


walleygirlUser is Offline
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23 Oct 2015 01:49 PM
The reason given was that we can not have any part of a load bearing wall resting on the foam, it must be on the footing completely. Apparently that is code here. We are in an earthquake area, so perhaps that has something to do with it?

Nevertheless it is not going to be a big problem. We will find some way to cover whatever foam peeks out from under the baseboards, and right now I'm leaning towards a little wooden "foot" under the baseboard trim.

My builder thinks we should go with 1.5" of foam instead of 2", but I'm feeling like I need to stick to 2" so the R = 10. Anything below that is below the recommendations I've read for my area (though not below code), and since I've read that the most heat loss occurs through the perimeter of the slab (the other side of that wall is air, not ground), I am worried that even going 1/2" thinner will compromise my R too much. I think my builder thinks I'm being too picky, but he's too smart to push me on it, lol.

What do you guys think?


Bob IUser is Offline
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23 Oct 2015 02:17 PM
I think you need to watch your builder. If he thinks you're being too picky with the thickness of the basement foam, pay attention to how picky he is with air sealing. Definitely do a blower door test BEFORE insulating. Let him know you're doing this and let him know what your goal is.


Bob Irving<br>RH Irving Homebuilders<br>Certified Passive House Consultant
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23 Oct 2015 04:45 PM
Posted By walleygirl on 23 Oct 2015 01:49 PM
The reason given was that we cannot have any part of a load bearing wall resting on the foam, it must be on the footing completely. Apparently that is code here. We are in an earthquake area, so perhaps that has something to do with it?



What do you guys think?


Yes, it can't be under the wall and on top of the footing. Not enough surface for bearing, but it can be under the footing and then you put foam up the sides and over the exposed top of the footing. Encapsulate all concrete in foam.


Bob IUser is Offline
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23 Oct 2015 05:00 PM
if your codes or inspectors are disallowing foam under the footings, ask if they would allow a stronger foam designed for structural loads. This is available from EPS manufacturers.


Bob Irving<br>RH Irving Homebuilders<br>Certified Passive House Consultant
walleygirlUser is Offline
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23 Oct 2015 06:09 PM
I think I might be using the word "footing" incorrectly. The foundation walls will be poured such that they are a bit thicker than usual, and a "shelf" will be created along the inside edge, upon which will sit the concrete slab, thermally isolated by virtue of being wrapped in 2" of foam (except on top of course).
It looks like the diagram below, EXCEPT in that diagram, the wall extends over the foam. We were not allowed to do that, so instead we are making the foundation walls thicker so that the entire wall sits on the top of the foundation wall, and the 2" of vertical foam will sit just inside the wall with nothing resting on top of it (except drywall and baseboards).

Attachment: slab_insulation.jpg

walleygirlUser is Offline
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23 Oct 2015 06:21 PM
Sorry, I can't seem to figure out how to get this image to show other than an attachment...

Attachment: slab_insulation.jpg

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