Concrete Home: To Insulate or Outsulate?

Posted by Leslie Jackson on January 13, 2010
Concrete Home: To Insulate or Outsulate?

Yesterday, we got a letter from Martha: "We are in Texas. We have an all concrete home made of blocks and concrete roof. How can we insulate it? The home is about 2000 sq ft. And we cannot get it warm or cool enough with a new 5-ton central AC unit. Please help.

As a natural builder, my first response is wrap it in straw bales .  The technique is described in Carol Venolia and Kelly Lerner's book, Natural Remodeling.

Resident scientist Steve Greenburg says, "My sense is that all that mass should be inside the insulation, so maybe rigid insulation on the outside of the concrete with e.g. stucco finish on the exterior walls and maybe a sprayed-on foam roof (with high albedo), if the roof is flat.  Does it really have a concrete roof? (Oh, maybe concrete tiles?). Another issue is properly sealing and insulating the ducts. If the roof can't be externally insulated and the ducts are in the attic, spray foam on the underside of the roof might be the way to go. If the ducts are in a basement or crawl space, then sealing the ceiling, insulating the attic floor, and ventilating the attic might be better. Pay attention to the windows—exterior shading or at least Energy Star windows for summer, let the sunshine in in the winter—is also important. HE has done articles on all of these. Find a house doctor and have them do some diagnostics, look at the specifics of the home and site, and make recommendations on that basis.

Danny Parker of Florida Energy Center adds: "You are right Steve. Simulation suggests that insulating on the outside looks to be the most favorable option. This can be done with EFIS as we did in two experiments now, nearly 20 years ago. Stucco goes on the exterior of the insulation. Even R-5 on the exterior changes performance dramatically.

My guess is that this house has other problems. Ducts? Yes. Many homes in Texas have air handlers and ducts in the attic so likely it's leaky, with lots of conduction losses as well. A four-ton HP should cool and heat that size of house otherwise. 

But if it has a concrete roof and the ducts are inside, then the HP itself has trouble--such as with a five-ton air conditioner outdoor unit matched with an old and poorly functioning indoor unit. (This is a terrible problem in Florida because AC contractors love to change the outdoor unit and not change the indoor one).

As for the concrete roof, it can be insulated on the exterior as well and covered with a white protective membrane, metal roof or other such assembly. R-30 is typically recommended.

What would you do?

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