# Insulating a post tension slab



## T-Bird (May 9, 2013)

I am developing construction documents for a building with a P.T. slab (wood framed stacked flats over concrete parking structure) and I'm not sure the best way to develop insulation details. I was anticipating applying rigid boards or batts to the underside of the structure, but my concern is thermal bridging. Anybody have practical experience in solving this?


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## Mark K (May 9, 2013)

Suggest that you consider the potential for condensation.  Suggest that the you consider the impact of your solution on the way piping and conduits are attached to the under side the slab.


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## T-Bird (May 9, 2013)

Insulating above the slab I imagine using a high density rigid insulation and topping slab with built-up wood plates at interior walls and a formed concrete curb at exterior walls.


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## mark handler (May 9, 2013)

Seal the top of slab with xypex What is Xypex Crystalline Waterproofing Technology?

add homasote flooring to top of slab Sub floor insulation sound proof floor underlayment


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## RLGA (May 10, 2013)

You can use a sprayed polyurethane foam.  ICC-ES has several evaluation reports for these types of products.  Make sure you select a product that is acceptable for your construction type, as many are only approved for Type V construction.  Also, some systems require an intumescent ignition barrier coating (these are mentioned in the ICC-ES evaluation report) to comply with the code's combustibility requirements regarding foam plastics.  I'm currently specifying this on a similar project with a Type III building over a Type I building using Section 509.2 (2009 IBC), and there's a parking garage in part of the Type I building that requires thermal insulation at the underside of the PT concrete slab, which is the floor of the apartments above.

There are two types of sprayed polyurethane foam:  closed-cell and open-cell.  The closed-cell type is virtually identical to sprayed polyurethane foam roofing systems.  This type is the most moisture-resistant and is considered a vapor retarder.  The open-cell type is also a vapor retarder and is moisture resistant, but not to the extent of the closed-cell type.


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## mark handler (May 10, 2013)

Ron

the issue with spray foam in a garage is flamespread Type I typically

LA fire won't allow it.

It also reduces the headroom height in the garage.


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## RLGA (May 10, 2013)

Mark:  I don't know about LA's product approval process, but sprayed polyurethane foam insulation meets the requirements for flame spread per Chapter 26.

Additionally, the intumescent ignition barrier prevents flame and temperature from reaching the foam like it does for steel.

And, yes, the foam will reduce headroom height, but so will any insulation type added to the underside of the slab.  Polyurethane foam has an R-value per inch that is higher than any other type of insulation on the market; so using another product will reduce the headroom even that much more.

Insulation could be added to the top-side of the concrete deck, but it should have a fairly high compressive strength (like that used under plaza deck pavers), to withstand the loading with minimal deflection.


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## Mark K (May 10, 2013)

First I would find out what is done in the pacific north west since that appears to be where the project is.

Insulating a PT slab would only be needed when it gets cold enough over a long time to overcome the thermal mass of the slab.

The insulation on the bottom might work if the whole area of the slab is covered by conditioned spaces.  Many projects will have exterior public spaces on top of the PT slab which would allow the slab to be cooled from above which would allow the cold to migrate horizontally into the perimeter of the units.

In situations like this your options are is to find out what is done locally and whether there have been problems, hire a consultant to study heat flow and condensation issues, or  to use your best guess and hope for the best.


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## T-Bird (May 10, 2013)

We talked to a building envelope and water intrusion consultant briefly and he suggested applying the insulation to the bottom of the slab and warned against insulation on the top of a slab. He is aware of projects where the living units have had unknow leaks and the water had no where to go. Locating the leak can be involved and intrusive, and mold has been a big problem.

We anticipate 6" of insulation and will increase the garge height 6" to accomodate it. The slab cantilevers in some locations and we will add insulation to the face of bottom of these slab areas.


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