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sandyman720 05-22-2007 01:26 PM

Insulating Joists over concrete
Hello. I have a concrete porch slab that I will be putting an addition.

I will run 2x8 joists 1.5 inches above the concrete for a raised floor then insulate between the joists.

Should I put a poly barrier on the concrete?

Should I worry about anything else?


concretemasonry 05-22-2007 01:49 PM

Insulating Joists over concrete
I would put poly (6-10 mil) over the concrete.

Use pressure treated wood and appropriate fasteners.

If you are in a heating climate, I would put a vapor retarder (poly)over the insulation before putting down a subfloor. Do not worry about the "double vapor barrier" since you will probably have some circulation in the 1.5" in gab between the concrete and the joists.

Just my opinion.

sandyman720 05-22-2007 01:51 PM

Do I need to add vents in the rim joists?

I dont think I need to use pressure treated if the joists are not touching the floor correct?

concretemasonry 05-22-2007 03:33 PM

Insulating Joists over concrete
You don't have to use treated for the 1.5" if you are absolutely sure you will mot get moisture on top of the poly from the sides or around any nails. - Your call.

If you don't put poly down on the concrete, you will have to use treated for the 1.5".

Make sure you use the proper nails and attachments for anything nailed to the treated lumber.

Vents would not be necessary for such a small volume since wood construction leaks a lot of air anyway.

Ron6519 05-23-2007 10:13 PM

I'd worry that a porch floor wasn't engineered for this use. Can it support the added weight? Was it poured with the needed footings? Frostline issues?

robertcdf 05-24-2007 12:52 PM

I recomend the use of pressure treated ANYTIME something is outside. Even if its 10' in the air. In fact your local codes might require anything less than 30" above ground to be PT.

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