Insulating a floating slab

[amrrenos]

Hey everyone,

I am building a 24x32 garage this spring and I was wondering about the best way and products to insulate before I pour. My thoughts were to severely compact the area, lay 6mill vapour barrier, then followed by 2" blue styrofoam. If this seems correct please let me know, if not, your ideas will really help.

Thanks

 

[nehog]

Floor heat or no floor heat?

If no floor heat, consider perimeter insulation (goes down a ways below the slab, but not under it.

 

[MN BIANCHI]

I used 2" high density foam on both the heated and unheated sections of my building. I also used perimeter insulation on both sides. The unheated side is insulated and despite temps this winter well below zero for most of the last month the unheated side is still warm enough that snow will melt from the snowmobile when I park them on that side.

 

[Boyd Who]

I live in the frozen wastelands of the Canadian prairies and when we built our garage last summer we insulated under the slab as well as the perimeter. It works quite well, the slab is cool this winter but not frigid. I did not go with in-floor heat.
The poly goes between the insulation and the slab, as you can see in this pic...

 

[Bones35]

this works and you don't have to use blocks, clean your forms and use as headers.

 

[GreenInsulation]

If you are using radiant to heat, should use CreteHeat modular green panels as you get can 2" which gives you R 10 or 3" which gives you R 14! Snap shiplap joints together, walk tubing in place and pour! Already has the poly moisture/radon protection built in as well as the knobs to hold the tubing in place, no need for wire tying the tubing. Quick install time but most important, retains the R value much longer than XPS and will NOT crack so no moisture/cold can seep up in thru cracks. Shipped factory direct for lowest pricing. http://www.barrett-inc.com/crete-heat.html

 

[AMCguy]

On my 32'x32' I used 2'' high density styrofoam and a 6 mil vapor barrier. I'm very glad I did.

 

[NXGTS]

Floor heat or no floor heat?

If no floor heat, consider perimeter insulation (goes down a ways below the slab, but not under it.

Not trying to hijack but how do you insulate the sidewalls if they're exposed. I will have at least 16" of exposed concrete above the slab. On the outside, the building will be backfilled up to the wood structure. I would assume this will cause a huge heat sink inside the garage. Any suggestions? Is there a way to cover up that exposed insulation on those walls?

 

[southernfriedcj]

How does having 2" thick dow board under the concrete affect the compression strength of the floor?
I just seems like it would weaken the floor, but I'm sure it is acceptable if everyone does it.

 

[DE@N]

What is the purpose of the perimeter insulation? Does the ridgid foam serve as the form? Is there an insulating property provided by this as well?

 

[bczygan]

What is the purpose of the perimeter insulation? Does the ridgid foam serve as the form? Is there an insulating property provided by this as well?

Yes and yes. Heat always goes to cold. In winter freezing temperatures extend a certain depth into the soil depending on your location. Far enough underground temps are a steady 55 deg. F. Perimeter insulation to the depth of the foundation slows heat loss from the building to outside the foundation wall and into the colder earth beyond. Under slab insulation slows heat loss from the warmer slab to the colder ground underneath it. Well designed insulation systems provide a continuous blanket of protection, eliminate leaks and drafts, and slow heat loss by the most evenly cost effective means. Heat rises, so more resistance is needed in ceilings, then walls and finally floors. If floors are heated they need more protection. If perimeter foundation walls are insulated on the outside they act as a heat sink and help develop a heat loss and gain cycle that can match and counter the day and night temperature fluctuations and moderate their effect.

 

[bczygan]

How does having 2" thick dow board under the concrete affect the compression strength of the floor?
I just seems like it would weaken the floor, but I'm sure it is acceptable if everyone does it.

Most under slab insulation has a compression strength of
between 40 psi and 80 psi. This translates to between 5760 PSF and 11,500
PSF. Since we typically design for an allowable soil pressure of between
1500 PSF and 2000 psf, you can see that the insulation is much stronger
than the soil.

 

[surfsup]

Most under slab insulation has a compression strength of
between 40 psi and 80 psi. This translates to between 5760 PSF and 11,500
PSF. Since we typically design for an allowable soil pressure of between
1500 PSF and 2000 psf, you can see that the insulation is much stronger
than the soil.

So why overkill the foam density? Foamular 250 is 25psi which is 3600 PSF, still overkill, really, yet 1/2 the cost of higher density 40 psi board.

 

[mrolds88]

So why overkill the foam density? Foamular 250 is 25psi which is 3600 PSF, still overkill, really, yet 1/2 the cost of higher density 40 psi board.

I would think 2 things. First is availability. Second is the trials and tribulations that the foam goes through during the build prior to the pour. I used 2" roofing foam under the pour at my house. Glad I did as I have in floor hydro heat But the greatest thing I can say about it it is sturdy. I had a piece break off on the trip home. It landed on the road and a tractor trailer ran over it. My first thought was that piece was finished. Went back and picked it up, the track could be recognized, but wasn't compacted.