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Should balloon-framed houses with open crawlspaces be sealed?

Given a "historical" (ca. 1910) wall assembly, plaster/lath -> framing -> siding (no insulation, sheathing, or vapor barrier), does it still make sense to seal the crawlspace? Is it risky, much like adding insulation w/o also rebuilding the exterior wall to add a rain screen can be?

Asked by Matt Goff (zone 2A)
Posted Jan 14, 2013 5:17 PM ET
Edited Jan 14, 2013 5:19 PM ET


6 Answers

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The short answer is yes.

Anything that you can do to lower the air leakage in your home will reduce your energy bills. If the work is performed properly, sealing your crawl space will reduce your home's air leakage rate.

However, halfway measures won't work -- so you need to address possible water entry issues, the dirt floor, and so on. More information here: Building an Unvented Crawl Space.

If you have true balloon framing that extends uninterrupted from the sill to the attic, it's important to do your best to seal the bottom of the stud bays as well as the top. If possible, see if there is access to the top of the stud bays from your attic, and take measures to seal those openings up.

Answered by Martin Holladay
Posted Jan 15, 2013 6:54 AM ET


Unsealed crawlspaces in almost all of the eastern half of the US would bring more moisture into the house than it would purge. Ground moisture needs to be controlled with good bulk water management, vapor retarders & insulation on the foundation wall, and at a minimum a vapor barrier on the floor.

There is no down-side do doing this, even in the drier climes west of the Rockies. But in the sticky-humid swamp called climate zone 2A it's of HUGE benefit for reducing summertime latent loads, and lowering the overall mold risk of the house. An air-conditioned room above a vented crawl space in your climate easily turns the subfloor & joists into a mold-farm when the outdoor dew points are hanging in the 70s. By sealing the outdoor air out of the crawlspace the air conditioning becomes a drying factor for the wood, rather than the cooler temps making it a wetting factor.

Answered by Dana Dorsett
Posted Jan 15, 2013 6:06 PM ET


Thanks to both for the input. Martin, I've probably read your article five times in the past year, and it's the primary reason I'm researching the details of sealing my own crawlspace. I'm a believer!

I guess my specific concern, which I did not clearly articulate in my original post, is that I have read that this wall assembly relies on the stack effect to dry incidental water intrusion (hence why many retrofit insulation projects end up causing new problems). I'm taking steps to R&R all window flashing and siding-- is this sufficient? I don't see any signs of bulk water in the crawl, but that's not to say the wall cavity isn't getting thoroughly wet. I have not been able to verify yet if there is mid-floor blocking. What is the preferred method to seal the lower and upper openings?

One other follow-up question: the foundation is a rubble stem wall with rubble piers (all local limestone). All of the articles I've read assume a flat foundation wall surface for adhesive and battens. How can I work around my very uneven wall? The only option I see (assuming battens are critical) is to build forms and superficially extend the interior of the foundation out a few inches to provide a consistent fastening surface. I've done a lot of uncomfortable jobs in the crawl, but that one would take the cake....

Answered by Matt Goff (zone 2A)
Posted Jan 15, 2013 6:54 PM ET


Any time that you fill the empty stud bays of an old uninsulated house with insulation, you reduce the ability of the wall assembly to dry. In most cases, however, the wall still performs well -- as long as there are no gross flashing errors leading to water intrusion.

Q. "What is the preferred method to seal the lower and upper openings?"

A. If you find that you can look into the stud bays from the crawl space or the attic, the easiest way to seal the opening is probably with a two-component spray foam kit (a Froth Pak kit).

Q. "The foundation is a rubble stem wall with rubble piers (all local limestone). ... How can I work around my very uneven wall?"

A. As I noted in one of my articles ("How to Insulate a Basement Wall"),"If your basement has stone-and-mortar walls, you can’t insulate the walls with rigid foam. The only type of insulation that makes sense for stone-and-mortar walls is closed-cell spray polyurethane foam."

Answered by Martin Holladay
Posted Jan 16, 2013 2:15 PM ET


Martin, one last follow up. You addressed insulating the stone-and-mortar wall, but do you have any guidance regarding the vapor barrier? Your article advises that it should be attached to the wall with battens-- this is obviously not going to provide much of a seal with my rough wall. Should I batten anyway and use spray foam as grout/caulking as required to fill the gaps?

Answered by Matt Goff (zone 2A)
Posted Jan 22, 2013 11:43 AM ET


I advise you to attach the horizontal batten to the stone-and-mortar wall as best you can, with occasional masonry screws. Once the batten is secure, seal the air gaps between the batten and the uneven wall from above, with canned spray foam.

Answered by Martin Holladay
Posted Jan 22, 2013 11:56 AM ET

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