Insulating the crawl space perimeter vs. insulating the joists–is there a clear winner?
Pondering this question in zone 4 marine, northwest of Seattle, where the wet winters fool people into thinking the humidity is high, and almost no one has air conditioning.
For better or worse, I look at a lot of crawl spaces with an eye towards energy improvements. They are often in poor condition insulation-wise, with a low quality batt installation that’s falling out, was not installed in contact with the subfloor, and is either R-11 or R-19. Even the newer R-30 installs have the air gap above the batts. Many of these crawls have an air handler and ductwork installed in them, and very few of these systems look well-insulated or well-sealed. Only the newest ductwork has mastic.
The choice is between insulating the perimeter concrete walls and insulating the subfloor, and I currently explain both options to the owner but ask that they acquaint themselves with the pros and cons of both approaches by doing their own research. I talked to the person who heads the one program operating regionally that does subsidized weatherization, and he explained that they are more likely to do the perimeter than the floor, BUT, then proceeded to tell me that the main reason was that they had to pay a very high prevailing wage for any duct sealing or insulating work, which made the job too expensive… so their default is the perimeter.
Is there a solid rule of thumb that can be applied here, something that compares the effectiveness of a well-insulated floor to a well-insulated perimeter?
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I'm in SW WA, zone 4 marine and have been trying to justify a conditioned crawlspace (flawless vapor barrier, rat slab, perimeter insulation, ductwork in the crawl) and kept getting hung-up on the cost of the rat slab but then I saw Fernando Pagés Ruiz mention in one of his books what's called flowable fill. It has more sand and water (and maybe some fly ash) and less cement than a regular mix (3000psi-ish) but is a little cheaper, self levels and doesn't need to be raked or floated.
From what I can tell, all that's needed is a line pump (not boom pump) and a couple guys on the line. Total cost might be less than $1.00/sq ft. for material, pump and hiring one helper.
There might be more cost savings realized by downsizing the HVAC b/c of the increase in efficiency and no worries over whether the insulation is in contact with the subfloor.
So, if all the costs and savings associated with a conditioned crawl are higher but close to that of a vented crawl, I'd go with the former.
PS. I'd love for the resident experts to correct any of my assumptions and conclusions.
It's far easier to reliably air seal at the crawlspace perimeter walls than at the subfloor & joists, which have many more electrical / plumbing / duct penetrations.
In addition to the potential ductwork leakage and air infiltration issues, crawl spaces in the NW also usually house plumbing, which can freeze up solid and break during the extended cold snaps if you insulate the joists & ducts well in a vented crawl space. (Ask me how I know... :-( )
I agree that in most cases the perimeter is going to be easier and cheaper, but which saves more energy? Under my house, I did a half-day of guided air sealing and then bagged the joists and dense-packed them. I don't have ductwork down there, so it made no sense to me to do the perimeter with less insulation (a lot of people seem to do 2" of rigid on the perimeter) and have nothing insulating the soil. Looking for some Michael Blasnik-style analysis here, some real numbers.
David,
This article does a pretty good job of summarizing what we know about crawl spaces in different climate zones: The Scary Crawl Space.
My instinct is to favor the unvented sealed crawl space, especially in locations where frozen plumbing pipes are a possibility. If you have the same instinct, you will be insulating the crawl space walls. From my understanding, there isn't any clear evidence that sealed crawl spaces save energy west of the Rocky Mountains, so I wouldn't try to base my argument on energy savings.
As with many situations where there are two possible approaches, and no clear winner, what really matters is whether the details are done right (no matter which approach you take). If your instinct favors one of these two approaches, trust your instinct; just make sure that the work is conscientious.
In my experience, it is hard to insulate between joists in a way that keeps the insulation in contact with the subfloor over the long term. If you have workers who can do that -- and do it well and do it neatly -- then insulating under the subfloor can work in your climate.