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Old 04-26-2007, 09:10 AM   #1
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Insulation over concrete slab


Hello,

I am enclosing my covered concrete slab patio.

I am thinking what the best way to insulate the floor. I was going to raise the floor about 8 inches with 2x8s and then put rolled insulation in between the joists.

What do I have to worry about with moisture?

Thanks!

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Old 04-26-2007, 10:38 AM   #2
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Insulation over concrete slab


If you're going to go that route, you should probably put down a sheet of polyvinyl as a moisture barrier. Of course, you should use pressure treated wood. I would treat it like a crawl space and make sure it's properly vented.

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Old 04-26-2007, 10:45 AM   #3
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Insulation over concrete slab


so put the polyvinyl over the slab, then put the joists in.

I should put a vent on the outside of the room venting under the floor correct?

should i drill a hole in each joist to have airflow, because the joists will rest on the slab and then the subfloor will go directly on top of the joists.
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Old 04-26-2007, 01:40 PM   #4
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Insulation over concrete slab


I think you can do it multiple ways.

The best way may be to just lay the floor joists on the concrete and then staple faced insulation on top of the floor joists. Using faced insulation allows you to staple the insulation in place which prevents it from falling on the ground since the vibrations of the floor may jar it loose down the line. Unlike a crawl space, you won't have access underneath your floor. Also, the "facing" is a vapor barrier.

Before laying down the subfloor, apply construction adhesive on top of the joists. You may have to peel back some of the faced insulation in order for the adhesive to have good contact. Then use stainless steel screws or hot dipped nails to secure the subfloor in place, since you're working with pressure treated wood as the joists.

It may not be necessary to use polyvinyl on the concrete slab itself, but it definitely helps. For circulation, you can drill holes into the floor joists, but to keep the floor joists strong, I may opt instead to drill a few half circles on the edge of the floor joists on the edges that contact the concrete slab. These half circles would allow circulation without totally compromising the strength/structural integrity of the floor joist.

I don't know how flat your concrete slab is, but you may need to shim with pressure treated shims in some areas. Also, some people use fans, not just louvered vents, to actively circulate air in crawlspaces.

Last edited by handy man88; 04-26-2007 at 02:04 PM.
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Old 04-28-2007, 11:24 PM   #5
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Insulation over concrete slab


Do you have any idea if the slab is engineered for the additional load?
Ron
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