30x40 foundation insulation

[kylemac]

Hey Guys,



So after my last thread on my 30x40 build on debating to go block or ICF for the foundation ive landed on doing block with rigid insulation on the inside of it. Im doing a full block foundation with 4 feet below grade and 2 feet above, gonna add radiant piping in the floor for heat and doing a 4" slab in between all this. I was also gonna do 3" rigid under the floor to help out a bit as well, I have added a photo of my plan for how im gonna insulate my block wall and just wanna know if im going about this the right way or should be doing something different, I don't have it in this photo but there will also be poly and crushed gravel under the 3" rigid, would be great to know if im on the right track or not, thanks!

 

[Chris705]

Kyle - you have a good grasp on insulation in the lower part of your build design. Insulation on the inside down to footing is great! Knowing that you want to continue that rigid up to top of your stem wall is also great to keep the heat at slab edge within your building. This is the easiest to overlook or ignore because it is sometimes difficult to detail. You will ideally want to protect that exposed rigid insulation that is above the slab line. With plywood or dense glass sheathing, fastening thru to block or maybe using ‘Z’ furring. I was able to find used 3” blue board that had been used in an IRMA roof system- 2’x4’ boards for $3 each on Rochester’s Craig’s list. I installed my rigid just like you’re showing but my stem wall stops at the slab line. I then foamed the balance of walls & roof. So very tight & easy to heat and with the in-slab system....my feet never get cold. Would be great if you can find used insulation. I like the insulation on inside of foundation & under slab, that combination is the ultimate. Compaction of your backfill on the inside will be important for slab edge support.

 

[yeldogt]

You can certainly do that .... but, it's diminishing returns on the inside of the block under the slab. Soil is a good insulator --

Better putting a 1" thermal break on the outside.

The earth has heat -- in Europe they would have insulation horizontal away from the building to trap and protect the foundation. Take a 4/8 foam sheet and place it on the ground .... around me the soil will never freeze under it

 

[Chris705]

Problem is with insulation on the outside you have to protect it from degradation and then get it back in line with your wall framing above. Or hang the wall framing out the same distance of insulation and protective skin applied to the stem wall. On Net zero homes the rage is using insulation on the entire exterior surface. It makes sense but going down on the inside accomplishes protecting the sub slab grade from freezing and expanding outward with out the need to protect it.

 

[maxpat82]

outside rigid insulation!

here's how I did my 28x44:
2" outside rigid pannel down to the footer. 5" slab with radiant pex tubing and 2,5" under slab rigid foam)
https://imgur.com/XJi9nW9

https://imgur.com/q4sZQAo

you need to "protect" the insulation for look only...it will survive many decade on it's own whitout anything!

I didn't want to have to put anything on the concrete inside since I weld and the insurance need steel everywhere.... I put door core evrywhere (which add R15 on top of the R20 in the wall)
https://imgur.com/0yKrkk9

 

[rwa2004]

+1 on insulation on the outside. Another idea may be to cover the exposed insulation above grade with aluminum flashing.

Why did you decide against ICF?
I expect I will also go with block like you due to the cost difference.

 

[yeldogt]

Problem is with insulation on the outside you have to protect it from degradation and then get it back in line with your wall framing above. Or hang the wall framing out the same distance of insulation and protective skin applied to the stem wall. On Net zero homes the rage is using insulation on the entire exterior surface. It makes sense but going down on the inside accomplishes protecting the sub slab grade from freezing and expanding outward with out the need to protect it.

How will it protect the subslab .. it's earth on both sides?

You want to stop the heat from moving out of the exposed wall ... in my area ... I just do a thin thermal .. and cover it ... insulate the inside and the sill.

In really cold climates there are ways to cover hide .....

 

[kylemac]

+1 on insulation on the outside. Another idea may be to cover the exposed insulation above grade with aluminum flashing.

Why did you decide against ICF?
I expect I will also go with block like you due to the cost difference.

I decided against the ICF for the same reason as you, cost, would love to do it but the father in law is a stone mason so all im paying for is the block and mortar which is working out to a lot cheaper than doing the ICF foundation

 

[Chris705]

Max pat- what did you protect your rigid with? Cement coating of some sort? Will it stand up to a weed eater? Applications I have seen tended to be prone to Spaulding/chip off. The aluminum coil stock option may be viable. Also I kept a sheet of the 2x4 Dowboard insulation outside for about 3 years after my build and the surface certainly had some UV degradation. Was able to poke my finger into the surface layer by about 1/4”.... so it is not a long term exposure material.

Yeldogt - yes earth on both sides,(I think in both options the insulation is keeping the heat inside the foundation wall, protecting it from freezing if the sub slab soils became wet, that expansion can damage the stem wall) what is the difference in preventing heat moving to cold on the outside of foundation wall vs. from the inside? I just see the insulation being damaged easier on outside than when protected on the inside. If the OP was going to put rigid up on the outside and then maybe put up vinyl siding I could see where running that layer straight down may not be an added cost?
Trying to understand the cost justify when OP already is somewhat concerned of cost and nixed the ICF’s. Don’t get me wrong I am aware of the current trend to run insulation board top to bottom on net zero homes, it does come at a premium though.

 

[climb.on]

I would strongly encourage you to do more insulation the portion of the foundation that is above grade. The difference in temperature above grade is far more significant than it is below grade. I did 2" XPS (R-10) under slab, and 4" XPS (R-20) on the perimeter of the slab, R-40 in my walls and R-60 in the ceiling, of my house. My shop is similar except I used SIPS for the walls. I was able to cantilever my walls over the edge of the slab to maintain a plumb plane with the foam on the outside. Many way to skin this cat, but R-value locations is the important aspect here.

 

[climb.on]

Max pat- what did you protect your rigid with?

Here's a good thread on this topic. I posted how I protected mine. It's a challenging but important detail to get right. Several decent options. https://www.garagejournal.com/forum/showthread.php?t=389855&page=3

 

[kylemac]

Thanks for all the opinions here guys, got some good information on some products I didn't know about, still don't know how I feel about using a stucco type product on the exterior of the rigid, like most are saying I feel it would be quite brittle. The menards product that climb on above has suggested seems like a pretty good system, but we up in the great white north here home depot doesn't seem to carry anything like that so I will have to keep searching, I think my final decision will be to put 2 inchs of rigid on the exterior and I am also now contemplating on doing 1 inch on the interior as well, as for how I am going to offset my wall I haven't figured that out yet, either I overhang the outside to meet up with the rigid but then have a bigger ledge on the interior, or just leave the wall centered on my block and do some sort of L Flashing at the bottom of the wall to cover the top of the rigid, this was one of the reasons I was leading more towards the inside block wall with the rigid as I wanted to carry the siding straight down, decisions decisions