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Need help with insulating a block house with crawl space

Robert Fordham | Posted in General Questions on

I am remodeling an old family house that was built in the 50’s, that is structurally in great shape, and is in middle Georgia (climate zone 3). The house has block exterior walls that extend down 18 inches into the ground to the footer. The crawl space is slightly below grade, but seems to stay fairly dry because the house is sitting on a small hill. I am doing a complete remodel, but not sure how to insulate without causing myself problems down the road. The exterior walls from the inside out consist of several layers of paint , plaster, block, and couple more layers of paint on the exterior. It was stripped with 1x4s and then vinyl siding, but I am removing this because it is old and brittle. I will be replacing the windows and want to insulate it well for my area. The crawl space has two access holes one on the side and the other on the back of the house and no other vents. There is no plastic under the house and no insulation. I don’t know of any other houses in my area that are block with a crawl space, and the contractors I have talked to haven’t seen many houses like this, so I am getting varying opinions on how to insulate. My wife likes the look of the plaster walls but we can cover them up. The crawl space is only about 12 inches high, so i can get up under there to do some work but it is fairly tight. I have looked at exterior insulation, but not sure about the details with a crawl space. Do I condition the crawl Space? Would it just be easier to get the details right by insulating the interior? I’m not looking for the cheapest way, but don’t want to spend money that isn’t going to help that much either. If I go with external insulation, what are some suggestions about siding and where the siding would contact the ground. Stucco on foam?
Thanks in advance for the help.

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  1. GBA Editor
    Martin Holladay | | #1

    To avoid a major thermal bridging problem at the area between the exterior grade and your floor level, it would be best to install all of your insulation on either the interior or the exterior.

    If you choose to install exterior insulation, it would be best to excavate down to the footings, and to install one or two layers of rigid foam (or perhaps mineral wool) from the footing up to the underside of the roof sheathing (or the underside of your attic ventilation channel).

    If you choose to install interior insulation, the insulation should extend from your crawl space floor to the ceiling of the top floor of your house. This insulation would be rigid foam in the crawl space; you could choose from a wide number of insulation choices on your above-grade walls.

    For more information on crawl spaces, see Building an Unvented Crawl Space.

  2. Robert Fordham | | #2

    Thanks for the quick answer Mr. Holladay. As I read the article on building an unvented crawl space, I believe the details to get this right would be pretty hard to do on this house because of the way the pillars and bracing are done underneath. Some of the beams are true 4x10s that are only 8 to 10 inches off the ground on irregular shaped pillars. So to get the detailing right I would have to redo some of the pillars and I'm not sure I could get them out without busting them up, which is doable but in less than 12 inches under the joists would be a rather LARGE task and also with the problems that can occur with a well detailed unvented crawl space, listed at the end of the article. Would you recommend doing a vented crawl space, insulating the floors, and doing interior insulation? Or do you see the long term benefit to spending the extra time and money to make a vented crawl space.

  3. Robert Fordham | | #3

    Sorry on the last sentence should read Or do you see the long term benefit to spending the extra time and money to make a unvented crawl space.

  4. GBA Editor
    Martin Holladay | | #4

    Vented crawl spaces don't work in Georgia, for the reasons given in the article. I suggest that you follow the advice in the article for detailing a sealed crawl space. Even if you can't manage to get every detail perfectly installed, in Georgia an unvented crawl space should still perform better than a vented crawl space.

    The usual solution to a cramped crawl space is to excavate the soil to lower the floor.

  5. Lucy Foxworth | | #5

    I spent days underneath a house on piers last year (Upstate SC - zone 3 also, a little cooler than your location) insulating under the floor. And it was a relatively comfortable dry space with 3-5 ft of head space to work in. It was still awful. My neck was killing me. My head was sore from bonking it on the floor joists. Lots of spiders and wasps. Don't do it especially with the limitations you have.

    Insulate around the crawlspace, preferably on the outside if possible. Insulating on the outside could give you a continuous layer of insulation with no thermal breaks and you wouldn't have to lose your plaster walls. Since you are already taking down the vinyl siding and replacing windows, this is the ideal time to do exterior insulation.

    If you use foam you have to consider termites in our neck of the woods as serious risk to invade the foam. If the entire house is concrete block and they can't get through the blocks, you will probably be fine though termite tunnels would degrade the foam and kind of defeat the purpose.

    I really think the mineral wool insulation board is the way to go. I always recommend the mineral wool option for the south because of bugs and fire resistance. Another option is something like Foamglas, a cellular glass insulation. It's pretty pricey, but you could put that below grade around the house and then foam above. Termites and insects will not be able to get through it.

  6. Robert Fordham | | #6

    Thanks for the input Lucy, I will look into that. In my area most of the contractors are really good people but all have similar building techniques. When I ask them about a conditioned crawl space and external insulation, (trying to get information to get the details right for my particular situation) they look at me a little funny. I have a close friend that is an insulation contractor, and has been doing insulation for 15 years in my area, the houses he has had a personal hand in handling air sealing and insulation and hvac are some of the lowest energy consuming houses in the area. He says he has only seen a handful of conditioned crawl spaces (His personal house has a basement). The ones that he has seen, the installers didn't get the details right,(one they didn't leave a space for termite inspections and house was infested less than 7 years later, and another house didn't get proper drainage and was floating the membrane under the house). So it is just hard to get information on the proper way of doing things, when I'm trying to take the whole picture into consideration and making sure I understand correctly from the articles. Thanks

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