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Community and Q&A

Elevated concrete slab over crawlspace

grid14 | Posted in General Questions on

Long time reader.. first time poster

I am currently renovating a 1941 brick veneer cape cod revival home in climate zone 4 that involves addressing some existing issues with a bumpout at the rear of the home. As best I can tell the space was originally an unconditioned sun room built atop an elevated concrete slab that sits about 5-1/2″ below the finished floor of the house.  Below the slab is an unvented crawlspace that is accessed through an opening in a partially finished basement. The crawlspace has a vapor barrier over the dirt and a couple inches of closed cell sprayed onto the concrete surfaces (both the vertical foundation walls and the bottom of the horizontal slab).  At some point over the years the previous homeowner tried to condition the space by adding sleepers atop the concrete slab to bring it level with the main house and routed ductwork through the rim joists into the sleeper cavities and terminated them at floor registers.

Even with the added air conditioning/heat from the forced air system, the room is still quite uncomfortable during the as the wall/roof insulation is very poor and there is no thermal break between the slab and the space above.  The initial plan is to tear the space down to the studs and address the roof/wall insulation as well as new windows and door. However I am struggling to come up with a gameplan for the floor.  Looking around on this website and others it seems like the typical way to address a slab on grade is to lay down a vapor barrier and building up the floor with EPS foam board and pressure treated lumber.  In my case I have the added complications of the ductwork and the crawlspace below.  I am worried about doing something that might trap moisture in the floor or walls.

I would like to get some opinions on a best course of action to help address some of the challenges with the concrete slab.  I realize tearing down the bump out and rebuilding it would be best but that’s not a financially feasible option at this point.

I have tried to put together a sketch to help illustrate some of the existing conditions.

Any input would be greatly appreciated.

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, your plan to address the walls and roof is a good one. If you pay attention to air sealing and improve the R-value of these assemblies, that will make the space more comfortable.

    Your slab is not as bad as some slabs, since it is insulated on the underside with spray foam insulation. That insulation is a good thing. The major problem with the slab insulation is that whoever did the work forgot that you need vertical insulation at the perimeter of the slab. In your case, that means that you need exterior rigid foam on the concrete stemwall associated with your slab.

    For more information on insulating concrete foundation walls with exterior rigid foam, see this article: "How to Insulate a Basement Wall."

  2. Expert Member
    Peter Engle | | #2

    Martin's right, you do need exterior insulation at the slab edge. But that's not easy to do with a house that has brick veneer sitting on the edge of the slab. There's no easy way to insulate the thermal short caused by the brick veneer itself. Still, some exterior foundation insulation will help warm the edge of the slab. Paying attention to the walls and roof will make far more difference in comfort and energy use.

  3. grid14 | | #3

    Thanks for your responses. With the bump out being brick veneer and original to the house its probably not feasible to add any exterior insulation.

    I will focus on the wall and roof insulation and see what I can do about adding some rigid insulation below the sleepers.

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