Planning a new house in Pender County, North Carolina, Jason Dennis finds the aesthetics of a crawlspace appealing, but not the potential problems. Instead, he’s thinking of pouring a slab over compacted fill inside his foundation walls — a raised slab.
The Climate Zone 3 house will be a simple rectangle, 50 feet by 39 feet, with a wraparound porch on two sides.
Dennis wants to know whether the raised slab approach is a good idea, and how such a foundation should be insulated.
“Would insulating the slab be necessary for my climate zone?” Dennis asks in a Q&A post. “If so, how should insulating be done? Rigid foam underneath like with a non-raised slab?”
Those are the questions that get us started on this Q&A Spotlight.
Insulate the slab perimeter
In referring Dennis to an earlier article on this topic, GBA Editor Martin Holladay says it’s possible to eliminate the horizontal insulation underneath the slab. But in Climate Zone 3, Dennis should not skip the vertical insulation at the slab’s perimeter.
Holladay had written: “While it could be argued that insulation might be useful in Climate Zone 3, it really isn’t needed in warmer climates, where an uninsulated slab helps lower air-conditioning bills compared to an insulated slab.”
How deep should the insulation go?
If Dennis adopts that plan — rigid insulation vertically against the inside of the stem wall but no insulation underneath the slab — the question becomes how far down the insulation should go.
“The insulation should extend down to the footings, or if they end up being very deep, a couple of feet below the exterior grade,” Malcolm Taylor replies.
The foam can be beveled at the top edge so it’s not visible on the inside while bringing the concrete all the way to the…