Q: A client has a sunroom with an uninsulated concrete slab that is 5 inches below the house floor. What is the best way to insulate and raise the floor to match the hardwood floor in the main house?
A: Steven Baczek, a residential architect from Reading, Mass., who specializes in designing durable, low-energy homes, responds: With that 5-inch height difference, I assume that the walls are built on a stem wall, which is not uncommon. You will need to build up the sunroom floor in layers to seal and insulate the floor, while eliminating thermal bridging and creating a nail base for the wood floor.
The first thing to determine is whether the slab is relatively flat and consistent in height. A long level, a straightedge, or a couple of strings can help with this step, but the best tool for checking the floor is a laser level. If the concrete slab is in bad shape, it might be easier to take it out and start from scratch.
If you start with the 5-inch difference between the top of the slab and the finished floor of the house, and you subtract 3/4 inch for the strip floor and another
3/4 inch for the subfloor, you’re left with 3 1/2 inches for a supporting frame. Because the slab is uninsulated, it is likely to transmit moisture from the ground below, so first I would cover the slab with 4-mil cross-linked poly (see Insulating a Sunroom Floor, below). Extend the poly up the walls a couple of inches and make sure that it is pressed tightly into the juncture between the slab and the sunroom walls. Tape the poly to the walls with waterproof flashing tape.
To build the supporting structure, start by installing treated 2x4 sleepers on the flat on top of the poly. Place the first sleeper 6 inches in from the edge of the walls and space the rest 24 inches on-center. Fill the spaces between the sleepers and around the perimeter of the room with 1 1/2-inch rigid EPS insulation.
For the next layer, cover the entire floor space with 2-inch-thick rigid insulation. That layer will take the height of the sunroom floor to 3 1/2 inches above the slab.
Next, install the 3/4-inch subfloor on top of the rigid insulation. Drive 3 1/2-inch screws through the 2-inch insulation and into the sleepers below; that screw length should anchor the subfloor without penetrating the poly barrier. With the subfloor installed, you should be able to continue the hardwood strip flooring from the main house directly into the sunroom, uninterrupted. Essentially, this is a floating floor system that is R-15+, sealed, and thermally broken. The insulated floor will make the sunroom warmer, drier, and less likely to cause problems with indoor air quality.