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What makes more sense, insulating crawl space vs. under the floor?

According to this page:http://energy.gov/energysaver/articles/where-insulate-home
there are about 3 ways of insulating the bottom of a house... If the house is built on grade, you insulate the sides of the slab and possibly underneath the slab.
If the house is built so that there is a crawl space, the crawl space can be of the ventilated type where the underfloor of the house is insulated with fiberglass batts or spray foam.
The final way is to make the crawl space into an unventilated crawl space, i.e. a "conditioned space" place a plastic sheets all around, make provisions for drainage around the foundation, either outside or inside for excess moisture, seal off everything, then have a light duty ventilation system for Radon, then insulate the foundation walls instead of insulating under the floor. The primary benefit of this supposedly is that you can use much more insulation around the foundation walls, and get more of the house envelop insulated for lower cost vs. just between the joists in the house.

So I guess my question is, the idea of turning the crawlspace into a "conditioned space", is that really better than insulating underneath the floor, between the joists? I live in Northern California (SF Bay area) and the ground temperature isn't all that warm, if anything a bit chilly. So if I insulate the foundation but not the ground itself, turning the crawlspace into a conditioned space, wouldn't the cold from the earth seep into the house, making the house cold? Yes there is a plastic sheet so there shouldn't be any air movement and while I should really being looking at this from the perspective that it's the heat that is moving to a cold space, not the other way around, wouldn't the same still apply? That the earth would absorb the heat from the conditioned space, making the place effectively colder?

I have an older house that I want to retrofit that has zero insulation in the floor and I'm trying to decide whether it's a better idea to insulate the floor vs sealing off the crawl space and insulating the foundation instead.

Asked by Dave R
Posted Feb 8, 2013 9:42 PM ET


8 Answers

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Here's a link to an article which will answer most of your questions: Building an Unvented Crawl Space.

Answered by Martin Holladay
Posted Feb 9, 2013 6:50 AM ET


Yeah I've read that article before but it's almost as if the only justification for having a non-ventilated crawl space was humidity problems causing the insulation to collapse. If there isn't a humidity problem then somehow it IS better to have a ventilated crawl space. What they don't seem to go into is the impact of the earth's temperature on the house. I know the ground temperature in the south is higher than it is in the north so it's a possibility that higher ground temperatures plus higher overall outdoor temperatures don't really matter.

btw one of the hyperlinks in that article is dead, here is the updated hyperlink that works:

Answered by Dave R
Posted Feb 10, 2013 10:42 AM ET
Edited Feb 10, 2013 10:46 AM ET.


Thanks very much for notifying me about the dead link, and for providing the updated link. I have corrected the link in the article.

Since you live in California, which has a less humid climate than the Southeast, you can have a ventilated crawl space if you want -- as long as your climate is mild enough to keep your plumbing pipes from freezing.

Of course, if your crawl space includes any ductwork, you will suffer a major energy penalty due to the fact that your ducts are outside of your conditioned space.

Answered by Martin Holladay
Posted Feb 10, 2013 11:44 AM ET
Edited Feb 10, 2013 11:49 AM ET.


Dave R, your question is partly an energy modeling question--is there more heat loss if you insulate the floor, or if you insulate the foundation walls? If you insulate the floor and vent the crawl, your delta T is from interior to exterior at the floor line, a straightorward boundary, easy to visualize. If you insulate the walls, your delta T is from interior to crawl, which is a combination of soil and concrete walls, which are exposed to soil at various depths AND to the exterior air. I've never seen anything definitive on the various heat loss scenarios, and where the efficiencies are (probably because I've never looked far), but I wish someone would fill me in here, because there are times when an insulated, sealed crawl seems a lot easier and probably better from an energy standpoint.

My own house has an insulated floor over a 36" crawl. The joists are 2x8 and my insulator dense-packed them using canvex. I figure there's an average of about R-28, because the canvex billows quite a bit as it's packed. I have no ductwork and almost no supply piping in the crawl. There's nothing to think about unless we get an extended period in the teens.

A client of mine has a house with a crawl that is essentially a short basement. The block walls are a bit more than 48" tall. It is bone dry even after extended heavy rain. It has interior stairs that are big enough to handle a 4x8 sheet. It has an electric furnace and a lot of uninsulated, leaky ductwork. We're going to seal the foundation vents and insulate the walls with rigid foam. It seems like a no-brainer compared to insulating the floor (losing regain of the duct losses), and trying to seal and insulate a maze of ductwork (that would have to be done with FG and foil scrim, or possibly SPF although I am not a big fan).

So I guess in your case, the question is, what are the specifics? Do you have ductwork? Can you get rigid into the crawl without cutting it into small pieces? Do you have easy access to spray foam installers? Is your crawl space bone-dry or not? Do you need termite inspection gaps? Do you have potential radon? What is your budget?

Answered by David Meiland
Posted Feb 10, 2013 3:44 PM ET


I was mostly looking towards insulating the crawlspace because there is a lot of duct work and it's a very expansive area, over 4K square feet sort of a sprawling situation. I think there are humidity problems because there is insufficient drainage around the foundation due to plants being planted too close and so that is one thing that will need to be addressed regardless of the insulation situation. Also, due to the previous furnace and one of the water heaters in use, the crawl space is used as a fresh air intake so that likely would change if the crawlspace is sealed off. This house was built in '72 with zero underfloor insulation and while 2X8 or 2X10 (not sure, pretty deep though) was used in the construction, I can't help but be bothered by all the perforations in the house's conditioned space. Btw the house uses a stucco exterior so there is no way I can replace the siding without great expense. The house uses a heat pump for heating but due to the great expense of heating this place, it has been turned off completely in the mean time.

I guess the question I have then is, if this were turned into a passive house, what would be done and what cost? Then the second question would be, what would be the logical way to insulate the house w/o excessive expense?

Answered by Dave R
Posted Feb 10, 2013 9:04 PM ET


Dave R.
Q. "If this were turned into a passive house, what would be done?"

A. You would be in for an extremely expensive remodel, and the expense could never be justified by the anticipated energy savings.

Q. "... and what cost?"

A. At least $100,000.

Q. "Then the second question would be, what would be the logical way to insulate the house w/o excessive expense?"

A. Hire an energy auditor or home performance contractor certified by RESNET or BPI. Get an energy audit and ask the contractor for a list of cost-effective energy retrofit measures.

Answered by Martin Holladay
Posted Feb 11, 2013 7:02 AM ET


Does that mean passive houses are cost ineffective? I thought 10% increase in cost was pretty good for making a house a passive house. I mean what was the cost increase from going from '72 building codes in California to 2014 building codes? has to be more than a 10% increase right?

Also your answer doesn't address which route one would go for insulating under the floor. I'd hope that a slab of concrete isn't a prerequisite.

Answered by Dave R
Posted Feb 12, 2013 7:24 AM ET


Most Passivhaus buildings in North America have specifications that are not cost-effective. That said, some builders have succeeded in building new Passivhaus homes that are cash-flow neutral -- in other words, the additional mortgage costs associated with the incremental construction costs (compared to code-minimum construction) are balanced by energy savings.

Here's a big caveat: the same claim can be made for many energy-efficient homes that don't meet the Passivhaus standard. In fact, it's usually possible to do even better (financially) if you ignore the Passivhaus standard and simply focus on the most cost-effective investments.

When it comes to Passivhaus retrofits, I've never seen one in the U.S. that makes sense in terms of cost-effectiveness.

Concerning your question about insulating your crawl space: I believe that it makes the most sense to create a sealed crawlspace with insulated crawl space walls, for a host of reasons. I haven't done a detailed energy modeling exercise, however; in any case, such energy modeling will differ in every climate zone. If you have strong reasons why you prefer to insulate your crawl space ceiling, go ahead.

Answered by Martin Holladay
Posted Feb 12, 2013 8:18 AM ET
Edited Feb 12, 2013 8:21 AM ET.

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