GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Seal crawlspace or leave alone?

joenorm | Posted in General Questions on

This is regarding a newly built home. The standard around here (PNW) for whatever reason are vented crawl spaces. When I set out to build this house I got so much push back with the idea of a sealed crawlspace I caved and went vented. I was a new builder on a budget and I only had so much energy to go against the local practices. 

So currently I have fiberglass batts in the cavities of a my subfloor along with the standard black plastic vapor barrier over a dirt and gravel crawl. 

As it turns out I get a bit of seasonal water down there. I noticed it early on and went to the effort of adding drain-rock where the water would collect so at least it would be kept below the surface of the granular fill and not make its way above the vapor barrier. That strategy has worked. The crawl space looks great after a winter with no pooling water anywhere above the barrier. 

My question is about doing a sealed retrofit. I plan to put down a heavier, continuous vapor barrier and bring it part way up the stem walls with tape. At that point I am wondering about the detailing around the rim joist. Should I remove the batt insulation around there and add foam? The insulators did a really great job down there with batts but I imagine its the air sealing around that is important? Correct?

The other question is whether this would be worth it at all? Vented crawl spaces in this zone 4C actually can look pretty good over time. This climate seems to be forgiving for them. I definitely want to put in the more robust and better sealed vapor barrier, just not sure if sealing the crawl also is worth the extra effort? I do like the idea of lowering the potential stack affect in the house. Seems like it could help with what little draft exists. 


GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member
    Dana Dorsett | | #1

    In 4C the relatively low summertime outdoor dew points (averaging < 50F) makes vented crawlspaces do just fine, but they're not very energy efficient, and nearly impossible to make air tight. It's really hard to make the floor (crawlspace ceiling) truly air tight, even with copious amounts of foam on the subfloor.

    It usually takes less foam (and does a much better job) to air seal the crawlspace WALLS from the vapor barrier on up to and over the top of the foundation & band joists.

    In a PNW climate it would be fine to leave the batts in place when foam-insulating & sealing the crawlspace walls. A dehumidifier to control the humidity level would meet code (and basically never actually run), or a modest amount (1 cfm for every 50 square feet of floor) of exhaust-only ventilation would work.

  2. brian_wiley | | #2

    I just converted our crawl to a conditioned space, and live in Boise (cz5), so I may be able to tell you about some of the benefits we enjoyed in hopes that it addresses your question about the value of fully conditioning the space. It's worth mentioning that our crawl had all of our mechanicals, which made conditioning it more of a necessity. I couldn't quite tell if that's your situation as well, so I'll touch on that in case it is.

    Relative to our mechanicals, I noticed a drop in the frequency of filter changes once we had encapsulated the space. We also noticed a pretty dramatic improvement in allergies as well, and I attribute that not only to the mechanicals, but also the stack effect dragging stuff into the living space as well.

    The humidity in the house is far more stable as well. It's fairly dry in Boise, but the indoor humidity was relative to the outdoor humidity for the most part. With the crawlspace encapsulated, its buffered from wild swings when a storm rolls through. That may be more or less pronounced in a marine climate, but I can't say for certain.

    When I sealed the space I ran the liner up the walls, and used r-19 rigid insulation on them. Previously it lacked a continuous, sealed liner, and had Homesote fiber insulation on the walls at about an inch thick, so basically no substantial r-value. Once it was fully sealed we're nearly always about 5 degrees cooler in the crawl relative to the main house's temp, and about 5% higher in humidity down there. I would never describe the floors as 'warm' as in Warmboard/in-floor heating, but they are within a degree or two of our thermostat's temperature without any under-floor insulation.

    All in all, and even with energy savings aside, I've been really happy that I did it.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |