Feedback on foundation/slab insulation 1

[SteveMatt]

Hi,

Background: I am in northern Vermont, constructing an accessory building. First floor is a slab and will be for small equipment storage, and tool storage. First floor will be heated. Second floor will be a finished guest loft (heated, water, etc.).

Foundation will be 6ft wall (to frost level), on an appropriate footing with drain tiles. My thinking is to place insulation and a vapor barrier under the slab. Two questions:

1. Given the first floor is not livable space, does insulation under the slab have any real benefit?
2. Is there value (to the building performance) in insulating any portion of the foundation walls?
Thanks,
Steve

 

[oldestguy]

My experience. In two cases when I built a new Wisconsin house for myself I placed 2" foam insulation under he lower floor slab (one a cellar and the other a walk out basement. All I can say those spaces were much nicer to be in than other places I have been with no slab insulation.

 

[jayrod12]

and if you're providing a heated slab, you retain much more of the slab heat with insulation. You'll pay for the cost of insulation a few times over in heating costs without it.

 

[oldinspector]

First floor heated. Go with the insulation under it.

 

[SteveMatt]

thanks folks. Anyone have any thoughts about insulating the below grade walls?

 

[oldestguy]

On walls a common technique is apply the sheets of foam before backfilling and be sure to coat the part exposed so the stuff doesn't deteriorate there. Some coatings come off in time, so check carefully. Some of my houses had an asbestos board for the covering. No problem however. Not something used now.

 

[oldinspector]

Absolutely. Sorry I missed that. You can put 4 by 8 sheets--if 4X is too wide, use 2X--around the perimeter, the top being just beneath the flat styro. I'd use 2 inch on both layers. I put mine on the inside of the foundation wall as opposed to outside. It sounds like overkill, but you are heating the thing. I did this on a small duplex I built in 1980. (Along with some other insulation upgrades) Amazingly low heating bill.

 

[SteveMatt]

4x8 is what I was expecting to use, and I am inclined to put it on the inside of the walls too.

Thanks!

 

[r13]

I recall there was a paper, or guide, that discuss the need, the type, and efficiency of insulation on subgrade. It requires insulation on both horizontal and vertical (interior) elements in contact with soil that is susceptible to freeze and thaw.

 

[SteveMatt]

If you happen to come across, please share. But that is what I plan to do - both horizontal and vertical insulation.

 

[r13]

Steve,

It's been a longtime since early 90's. I followed the suggested calculation, and provided insulation foam on the SOG, and the perimeter grade beams. I'll share, if I am able to find it.

 

[CarlB]

Steve,

Insulation on the wall (and horizontal "wing" insulation) definitely aids in maintaining soils beneath the foundation in a thawed state, which is desired if footing depth is less than maximum expected frost penetration depth. It has secondary value to reduce building heat loss. If the floor is also insulated, that increases the amount of wall insulation necessary to achieve the same effect without floor insulation.
The design of Frost Protected Shallow Foundations is addressed in ASCE 32.

A similar guide was prepared by HUD:

https://www.huduser.gov/publications/pdf/fpsfguide.pdf

 

[SteveMatt]

Carl, that looks like a nice resource. Thanks for sharing the doc and your thoughts on the approach.