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Pacific Northwest crawlspace insulation: Spray foam or encapsulation?

Gereim | Posted in General Questions on

My wife and I just brought our first house, a great 1,200 sq ft starter home with one issue: the batt insulation in the vented crawlspace has been ripped up and soiled by rodents. We live in Tacoma, Washington, which is just south of Seattle.

I’ve seen two compelling solutions in these Q&As: 6” of open-cell under the subfloor or encapsulate by air sealing and insulating the foundation walls.

-What is the best solution for our situation and why?

Our situation:

-Crawlspace has only about 2 ft of clearance
-Crawlspace contains rigid ductwork
-Don’t want to make a home for rodents again as the batt insulation did (Regardless, we will be hiring an exterminator and blocking the crawlspace entry ways with wire mesh)
-Want to improve the air quality for the home as my wife has allergies
-Want the most cost-effective insulation and air seal possible
—-I’m open to paying a bit more upfront if we can recoup the costs in 2-3 years with lower heating bills. We may outgrow the house in 5-6 years, so the return on investment timeline would have to be relatively short.

Any advice on how to tackle this and how much it might cost?

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  1. ohioandy | | #1

    Hey Gereim, costs for things like this vary WIDELY by region, so your first move should be to get a few quotes to see what the market is like in your area.

    I'm not sure a thick layer of open cell spray foam is a good strategy here. If you've already got ducts down there, it's nuts to insulate the floor--with anything. Go for the encapsulation (membrane on floor, rigid foam on the interior of foundation.)

    If you're hiring the whole thing out, there is no possible way to recoup the costs in a few years, or really in many years. This would be a $12k+ job where I am, and it's definitely a long-term investment. But see, it's not only efficiency but also comfort and resiliency and pestlessness, which pay dividends immediately. At resale a good real estate agent could talk up all this AND the equity boost of crawlspace encapsulation.

    Whatever you decide, congrats on the new place and welcome to the joy and anguish of home ownership.

  2. pnw_guy | | #2

    Personally I would encapsulate the crawl space rather than spray foaming the floors. Keeps your crawl space clean in addition to keeping your floors warm. Plus, read up on spray foam horror stories. The vast, vast majority of installations go well if you use experienced installers, but the fraction of a chance it goes bad? You're in for a world of nuissance and pain. So it would not be worth the risk to me in your case, where encapsulation is a viable non-spray foam alternative that would have the additional benefit of giving you a clean crawlspace.

  3. andy_ | | #3

    No opinion on the insulation, but a heads up if you're going in there for inspection or work to definitely get a tyvek suit and a N100 respirator. If the rodents have been living in there for a while you don't want to be breathing in their dry pee and poop. All sorts of nastiness can result from that.

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