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Community and Q&A

Insulating crawlspace in a 200 year old house

sGiMZWEUo3 | Posted in Energy Efficiency and Durability on

We have a project involving a 250 year old house built on the side of hill that, 17000 CFM @50 pascals. The customer does not have a lot of money, so we are trying to figure out the best way to air seal/insulate the crawlspace. When running the blower door, there is noticeable air leakage through the floor boards and along all interior wall bottom plates. The oil furnace is in the crawlspace and although I would like to figure out a way to condition it all, the crawlspace is uneven and broken up by rock walls as part of the foundation, storage areas, and 5 foot high dirt piles. Our second and cheaper option was to remove the existing R-13 craft face batts in between the log floor joists and spray foam 2″ of cc to the joist bays, the homeowner cannot afford the 2″, so we are down to 1″. However, since we are not doing 2″ and eliminating the batts, I am wondering if we should flash the floor at 1/2″ cc, reinstall or use new batts. My question is twofold, first, are there any other suggestions, and second, if we went with the 1/2″, we would need to re-install the batts to increase R-value, however, I am worried about using the craft facing will somehow create a moisture problem between the foam and the batt. If I take the craft facing off, I will have nothing to staple to the existing log joists. The crawlspace has no moisture issues.

The house is a historic house and it appears that the walls have been re-insulated (guess R-13). There are some other problem leakage spots that we are going to fix (i.e. seal and insulate behind large stained glass attic access at the top of the stairs). Windows are original with storms.

There is also a pantry that has no insulation underneath the floors with frozen peaches on the shelf as proof. The house is in Maryland. Thanks in advance for any advice.

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  1. bdrfab | | #1

    Is the homeowner able/willing to do any work themselves?

  2. user-869687 | | #2

    Air sealing is useful, but you're talking about very low R-values here. Nobody on this Q&A forum will recommend doing this half-assed, and a thermal boundary over an unconditioned (and impossible to condition) crawlspace needs at least R-30. Obviously it's not helpful to locate a furnace in an unconditioned space, so it would be best to either relocate any mechanical equipment and ducts into the conditioned space or to partition the crawlspace, so you end up insulating around the furnace. Reinstalling the existing R-13 batts might be cheap but not such a good service to the homeowners, because they're evidently hoping to reduce their energy consumption. I'd suggest telling them what it will cost to do this properly.

  3. GBA Editor
    Martin Holladay | | #3

    Trying to help out a client with a low budget by proposing a halfway measure is always a mistake. Do it right or don't do it.

    Someone needs to excavate the dirt piles and clean out the crawl space floor to make it level and to provide good access. The crawl space walls need to be insulated with at least 2 inches of closed-cell spray polyurethane foam.

    It makes no sense to insulate between the floor joists.

  4. wjrobinson | | #4

    Jim, airsealing is always first and is always the best choice if that is the budget. Your idea could and should be added to in the future as the above radical elements suggest.

    Welcome to 2012 and the hugely polarized modern Twitter, blog, Facebook, texting, smartphone world, me included!

  5. Danny Kelly | | #5

    Jim - if your customer is on a tight budget, some low cost strategic air sealing can do wonders.

    You seem to be focused on the crawl space since this was where you noticed the most air leakage during your blower door. a blower door is a great tool but can sometimes be a bit misleading. Just because there is a "hole" in your floor, doesn't necessarily mean that air will infiltrate that hole. Remember the building science basics - stack effect/heat rises and one out = one in. If there are not any holes in the top of your structure for the air to exfiltrate out of, there will not be any air infiltrating at your floor.

    Spend your clients money by air sealing at the top of the house. This will cut down on the air leakage at the bottom of the house even without sealing the holes at the bottom. When on a budget, every little bit counts. If you seal 50% of the holes, you will eliminate 50% of your air leakage - this does not need to be 100% perfect in order for it to work.

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