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Insulating a vented crawlsapce

m854 | Posted in Energy Efficiency and Durability on

I’m helping a friend do some energy improvements. The house is from the 50’s and seems to be made of concrete block with stucco on the exterior and plaster on the interior. I don’t think there’s much or any insulation in the walls.

The crawlspace is vented, and the furnace is down there, so not an ideal situation. I think it would be worth encapsulating at some point in the future, but I think it’s too much work/cost to do this year.

The floors get cold in the winter, especially near outside walls. Would it be worthwhile to insulate the rim “joist” (it appears to be just a concrete block) while leaving the venting in place? That would provide some insulation between the rim “joist”, which is the coldest part of the crawlspace, and the perimeter of the floor. Obviously the crawlspace is cold because it’s vented, but it’s not as cold as outside because it gains heat from the ground, from the uninsulated floor above, and from the leaky heating ducts. So although insulating the rim joist might not save much energy, I am hoping it would make the perimeter of the floor a little more comfortable.

We’d use closed cell spray foam to seal and insulate the rim joist. That is one of the steps towards encapsulation, so even if it doesn’t make a big difference now it could still help when it’s encapsulated later.

The house has humidity issues, so doing an incomplete job of encapsulating the crawlspace would make that worse. That’s why I want to leave the venting open until we can get a well sealed vapor barrier in place.

We’re in Colorado, climate zone 5A.

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Matt.

    As you noted, if the plan is to seal the crawlspace down the road, and this is how you would treat the rim joist at that point, then maybe there is no harm in trying. In my experience working on an energy retrofit that took many years to complete, not much changed in terms of comfort and energy savings until I started to close in on finishing the project. So, I'd be skeptical that you would see too much improvement. But who knows, if the rim is where the most heat loss is occuring, the insulation may help to keep the floors a little warmer around the perimeter in the heating season. I guess I don't know, but I'd be very interested to hear the results if you do decide to try it.

  2. the74impala | | #2

    Don't try to halfway do it. Get on YouTube and do a search for crawlspace encapsulation. Watch 20 or so of them and you should get an idea of what is actually best, the nutty ones will be obvious when you watch a bunch of them. Should make a pretty big difference in the comfort of the house too. Insulate with eps foam and seal as you go. The feet on the floors above with love it.

    1. m854 | | #3

      Yes, I'm aware of the right way to do it, but it's a lot of work. For example creating a continuous vapor barrier on the floor is easier said than done when there's a furnace and some framing sitting directly on dirt. Any suggestions would be welcome! I haven't seen good videos on what to do about that. As I learned with my house, having a vapor barrier that's not well sealed in a crawlspace that's not vented is going to increase the humidity more than just leaving it vented. In Colorado vented crawlspaces do seem to work, but they are definitely not ideal for energy efficiency (whereas in hot humid climates, vented crawlspaces will definitely grow mold).

      It makes sense to encapsulate this crawlspace at some point since it contains the furnace and leaky ductwork. But I don't think it's something we'll be able to get to this year. I think insulating the rim joist is one of the things I can start with, without significantly increasing the risk of moisture or other problems. The question is whether it would be worthwhile or not, in terms of improving comfort.

      It seems weird to use so much foam to insulate the crawlspace when the walls have minimal or no insulation.

  3. the74impala | | #4

    Until the while project can be done, just doing the rim would do nothing besides strengthening the area from coming apart...something you don't need. Spray foam seals and insulates. You still have active airflow and no other insulation. there would be no point except to say it is there. If you go through YTube you will find where issues like yours are addressed. It may involve lifting the mechanicals, and it will all be harder the shorter the head room is you are working with there.

    I would wait to spray until you do thre the rest so you can seal all of it up with the foam as you go.

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