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Raising a sunken floor

We have a home with a sunken living room and we wish to raise the floor approximately 9 inches to make it level with the surrounding rooms. The floor of the sunken living room is slab on grade. On three sides the adjacent floors are over vented crawl space, the fourth side is an exterior wall. To raise the floor I intend to build a frame with a plywood sub floor. With the raised floor should I mimic the surrounding floors, ie create a vented space with insulation up against the plywood sub floor. Or should I create a completely sealed and insulated space with rigid insulation on the slab.

Asked by Molly McCabe
Posted Wed, 11/23/2011 - 02:35


5 Answers

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I can't think of a good way to vent a crawl space that's only 9" high. Plan B is the way to go.

Answered by James Morgan
Posted Wed, 11/23/2011 - 09:29

Helpful? 0

I would make sure your subfloor on the ventilated crawl space is well air sealed and insulated as well. After that, I would install rigid foam over the concrete slab and then build-up the floor; but which ever method you use, the flooring material (underlayment & finished) should be carefully selected. If you install tile or stone floors, you may have expansion and contraction problems due to different material properties; wood and carpet are more forgiving.

Answered by Armando Cobo
Posted Wed, 11/23/2011 - 11:21

Helpful? 0

I think the solution is somewhat dependent on the conditions. What is the size of the sunken area? How is the perimeter defined where there is crawl space adjacent--are there existing joists/rim joists, concrete stemwall, or what? What will be the insulation detail around the edge of the new floor area?

Answered by David Meiland
Posted Wed, 11/23/2011 - 11:54

Helpful? 0

One side is an exterior wall. Two adjacent sides are from the original structure with 4"x6" beams and 2"x6" T&G as the subfloor. The final side is a later addition with 2"x10" joists & 3/4" plywood subfloor. Rim joists of the existing floors sit on top of the crawl space stem walls. I haven't demolished yet, so don't know if the stem wall is flush with the slab or a little above it. Edge insulation detail will depend on what I find when I demo. I have thought about laying XPS on a vapor barrier on the slab, then framing on top of that to match the height of the surrounding floors. Anticipated finish floor is 3/4" quarter sawn red oak to match the existing floors.

Answered by Molly McCabe
Posted Thu, 11/24/2011 - 00:05

Helpful? 0

My advice: build the level up with rigid foam (Type IX EPS or XPS) and pour a new slab on top of the foam.

If you don't want to pour a new slab, you could install two layers of 3/4-inch T&G plywood, screwed together, on top of the foam, with the lower layer of plywood adhered to the foam with foam-compatible adhesive.

Answered by Martin Holladay, GBA Advisor
Posted Sat, 11/26/2011 - 07:26

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