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What level of heat is recommended in an unvented crawlspace?

My project has 2200 sf crawlspace, 2/3 of which is 18" minimum and the rest on a down slope having a height up to 12' and which will house a water heater and furnace; and it has a 12" thick structural mat foundation., This seems like the ideal application for a conditioned crawlspace so that I can run forced air ductwork and piping, since there is not an attic. There is less exterior wall than there would be floor area to insulate, and just as your 2011 article says, the insulation would eventually fall out. Another plus, the floor would be warmer and I won't have to insulate the ducts and pipes, will I? And if the ducts leak a little it goes to good use, sort of.
Per your post the IRC calls for either of two options, the mechanical exhaust with a transfer grill from interior, or a register in the crawl space with some grills to exterior. The mechanical sub says the crawlspace air should not be returned because it would pick up gas and basement odors.
Are the minimum air changes assuming a delivered air temperature, or a maintained crawlspace temperature?
In my fairly mild Zone 3B (Calif Zone 12) is it necessary to insulate the slab and foundation walls as if it is occupied space, or is it enough to insulate the cripple walls (perhaps with Thermax which you mentioned)?
If the air is exhausted, how does one control the amount of conditioned air lost?
If I have a hole to the exterior for exhaust, how well should the cripple walls be sealed?
Would I need a vapor barrier and gyp on the cripple walls to keep condensation off the sheathing, or can it breath back to the interior?
Thanks,
Brian R

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Asked by brian rawlinson
Posted Wed, 03/19/2014 - 23:22

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3 Answers

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Brian,
It sounds as if you have read my article, Building an Unvented Crawl Space.

In California, if you build a properly detailed crawl space that includes forced-air ductwork, your crawl space will never freeze. So there is no need to add any space heating for the sake of your plumbing pipes.

Q. "Are the minimum air changes assuming a delivered air temperature, or a maintained crawl space temperature?"

A. The air exchange requirements found in the building code (and recommended in my article) are required to help control humidity levels, not to maintain the right temperature in your crawl space.

Q. "In my fairly mild Zone 3B (Calif Zone 12) is it necessary to insulate the slab and foundation walls as if it is occupied space, or is it enough to insulate the cripple walls (perhaps with Thermax which you mentioned)?"

A. The 2006 IRC and the 2009 IRC require that basement walls in Climate Zone 3 be insulated to a minimum of R-5. The same requirement applies to the walls of conditioned crawl spaces, and it represents good practice. Of course, you can always add more insulation than the minimum code requirement.

In your climate zone, I wouldn't bother insulating the crawl space slab.

Q. "If the air is exhausted, how does one control the amount of conditioned air lost?"

A. There will be an energy penalty associated with installing an exhaust fan in your rim joists as required by the building code. If you choose this option -- it's one of two possible ways to control humidity in a sealed crawl space -- you will need to provide “continuously operated mechanical exhaust ventilation at a rate equal to 1 cfm for each 50 square feet of crawl space floor area.” The energy penalty will be small, especially in your climate. If the energy penalty bothers you, you can experiment with turning the exhaust fan off after the building inspector has given you an occupancy permit. However, this approach would require careful monitoring of the RH in the crawl space -- you don't want it to get damp -- and careful monitoring of radon levels in your home.

Q. "If I have a hole to the exterior for exhaust, how well should the cripple walls be sealed?"

A. Air sealing your home's thermal envelope is always important. For more information, see Air Sealing a Basement.

Q. "Would I need a vapor barrier and gyp on the cripple walls to keep condensation off the sheathing, or can it breathe back to the interior?"

A. Assuming that you have installed a layer of rigid foam on the interior of your concrete walls, you don't need an additional vapor barrier, since rigid foam is already a vapor retarder. Most building inspectors will either require the use of Thermax (which has passed fire safety tests) or will require that the rigid foam be protected with gypsum drywall or cementitious backerboard.

Answered by Martin Holladay, GBA Advisor
Posted Thu, 03/20/2014 - 04:33
Edited Thu, 03/20/2014 - 04:36.

2.
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Thanks Martin. I'm trying to understand nuances of the two vent options, both of which state there should be a "return air pathway to the common area" via a transfer duct or grill (assuming that means the interior occupied space). Since I already have the ductwork in the crawl it would be easy to distribute conditioned air to all the corners and let it come back to a central transfer duct or grill, or directly into the return air side of the furnace; is that apt to put a bad basement smell (such as pressure treated lumber) into the house even if it is bone dry because of the heavy slab? The IRC doesn't seem to allow that conditioned air to be exhausted to exterior.
If I use the power exhaust system, would it be reasonable to tie it in to a continuously running bathroom exhaust near the two extreme ends of the house, and out through the roof, rather than out through holes in the rim joists, which are blocked by concrete walls for much of the crawlspace?
If properly detailed, can I confidently state to my client that this is superior to a conventional cold vented crawl, in terms of comfort and energy efficiency?

Answered by brian rawlinson
Posted Thu, 03/20/2014 - 16:29

3.
Helpful? 0

Brian,
These issues were discussed in the article I linked to.

Here is what I wrote: "It’s important to emphasize that even a well-detailed crawl space represents a problematic foundation design.... One problem with sealed crawl spaces: the air quality in such a crawl space may be poor unless the crawl space is equipped with an exhaust fan that runs continuously. Of course, such a fan can be part of a whole-house exhaust ventilation system; but when (not if) the fan eventually conks out, the homeowner is unlikely to notice. That raises concerns over air quality in the crawl space — and also in the home above.

"According to the previously cited Home Energy article, 'Additional radon testing showed that radon levels in the closed crawls — with a relatively low dilution rate — were roughly 10 times the levels measured in the vented crawls.' If a builder chooses to condition a sealed crawl space using 'Option 2' from the two code-approved options — that is, by installing a forced-air register rather than an exhaust fan in the crawl space — it’s easy for any radon or moisture in the crawl space to circulate throughout the house."

It's your choice: you can either install an exhaust fan that pulls air from your crawl space and directs it outdoors, or you can add conditioned air to the crawl space and allow some of that conditioned air to enter your conditioned space above. The latter option carries some (small) risk of air quality issues.

The third option (not really code-approved) is "watchful waiting": seeing whether your crawl space is OK without either ventilation / conditioning option.

If I had to choose, I would choose the option of exhausting air from the crawl space to the outdoors. I wouldn't have any problem explaining to a client that this option is superior to a vented crawl space.

Answered by Martin Holladay, GBA Advisor
Posted Fri, 03/21/2014 - 04:52
Edited Fri, 03/21/2014 - 04:54.

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