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cibula11 11-27-2009 12:05 PM

Best way to insulate concrete slab
Our garage will be converted. I plan on placing 2x8 as joists for subfloor.

Could I add 6mm poly, followed by rigid foam board and then lay the 2x8 on top of this?

itsreallyconc 12-02-2009 04:18 AM

of course - its your house,,, poly 1st, joists, & foam inside the joists on the floor

cibula11 12-07-2009 08:23 PM

Wouldn't it be easier to lay the foam and then the joists on top? I didn't know if there was a reason you suggested.

mudmixer 12-07-2009 09:49 PM

Might be easier, but not as good.

How level is your existing garage floor?

cibula11 12-10-2009 05:24 AM

Its pretty level....not perfect. I guess I was thinking it would be more air tight, but if you guys think it's better to do joist by joist, I can go that route too.

Towards the bottom of the page is what I was thinking:

I know its for crawl spaces, but similar principles apply.

inspectorD 12-10-2009 05:52 AM

Always go for the continuos barrier.
I think your on the right track, from my experience this method has been far superior than installing between joists. The thermal differences are big between the two materials.
Follow the building science recommendations for your climate.

cibula11 12-11-2009 08:12 PM

Thanks. Do you have an idea on the size of rigid foam I would need. I'd like to keep it to a minimum to keep ceiling height at a premium.....but I'm guessing I'll need 2" foam.

inspectorD 12-12-2009 06:24 AM

Two inch, and get the best R-value you can find. Compare your choices available,And you can infill between the joists for a little better floor if you need piece of mind.It will help.

cibula11 12-12-2009 09:53 AM

Just to be sure, it would be okay to lay the joists directly on the rigid foam?

I thought of even adding unfaced batt insulation between joist cavities where possible. With a small gap between the top of the insulation and the subfloor

inspectorD 12-12-2009 01:49 PM

Joists or sleepers?
Joists are installed upright, sleepers lay flat on the wide side.
If you are talking about sleepers, forget the insulation under the will flex with point loads and large people. If you are going to use sleepers use a 2x4 and nail it into the concrete.

For Joists you need a ledger board at the wall section to carry the wood upright, for those you can install a ledger board (like on a deck)and joist hangers with the foam underneath...and fiberglass if you want in between. I think fiberglass is just a srewup that got perfected....use some other blown in material for better results.

Hope this helps.

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