Moisture in a Crawl Space

[phamENG]

The title is a little misleading. This is a house design I'll have coming up. Located in the coastal Mid-Atlantic. It's in a flood zone and requires the house to be elevated several feet above grade. As a result, the design calls for it going all the way up to fit a garage underneath with a "crawl space" throughout the rest of it...though there's no need to crawl with 8' of head room.

Everyone loves to hate a crawl space these days, but given the height and proper detailing, it seems like it should be a non-issue. Clean, poorly graded sand to an elevation higher than the exterior to prevent ponding, vapor barrier, and adequate ventilation should be fine. Anyone have any doubts?

There's also a desire to use trusses on the first floor. If this were 2 or 3 feet above grade I'd say no, but again...8' above grade...I'm not really concerned.

What does everyone think? Am I being too cavalier? Should I be more concerned about this? Peak equilibrium moisture content in the area is 14.9% in August. So even if it's slightly higher under this house, I have a pretty sizeable margin to get to 19% and start worrying about wet service.

Thanks for the sanity check.

 

[jayrod12]

I'm ok with all of that. What does Mid-Atlantic mean in terms of Winter temps? Are you just planning on relying on some passive ventilation or will it be actively ventilated by mechanical equipment? I've been in crawlspaces I can walk in without ducking (I'm a hair over 6 feet in the morning, 6 feet on the nose by the end of the day) that have been moist. Granted those were buried in the ground, not above grade like yours.

 

[phamENG]

This is the southern edge of the Mid-Atalntic, so "average" lows are just above freezing year round - say 34 or so in January. Occasionally down to single digits, though.

I don't think there's a need for mechanical equipment, though fan equipped vents could be used to prevent build up of stagnant air.

 

[jayrod12]

Yeah then I don't really see an issue with what you're proposing, and using the trusses themselves don't really get me worried either. I'd just make sure that the vents are well below the framing to prevent wind driven rain from getting on the underside of the floor framing.

 

[LuK13]

pham,

tl;dr: I started by writing a long reply and then I had a moment of clarity. Would you second guess this if it were an unconditioned garage with a dirt floor? You will probably be fine. I kept all of my original text below if you want to make this more of a thought experiment.

If you have time I would recommend reading Water in Buildings by Bill Rose. It goes into great detail about the history of the ventilation requirements for attics and crawlspaces that we currently have in the ICC model codes. Long story short, the ventilation ratios don't have a strong basis in science.

I have also seen some independent testing of passive vents that suggest they allow much less air flow then the listed net free ventilation area may lead you to believe. I seem to recall a study that found fewer, larger vent openings performed better than many, smaller openings, but I cant locate it at the moment. Of course, the performance of two large vents at opposite ends of the structure can be dependent on prevailing winds, think gable vents for attics.

At the end of the day the point of ventilation is moisture control by dilution. If enough emphasis is placed on eliminating sources of moisture, there will be much less moisture you need to dilute, and any ventilation scheme is much more likely to succeed. By having a tall, above ground "crawlspace" and not a traditional crawlspace, you are ahead of the game on that one.

Vents should be high enough up the walls that they are not letting surface runoff in. I see it all the time when only one course of CMU is above grade, but you have plenty of height to play with.

Detailing of the crawlspace ceiling/first-story floor needs to prevent condensation on the underside of the first-story floor framing. I'm talking air, vapor, and thermal barriers. I've been in an ~5' tall vented and unconditioned crawlspace where there was no insulation or vapor barrier along the first-story floor framing. The floor sheathing was covering in biological growth and the hardwood floors were so buckled it was like a roller coaster.

Another plus of all that head room is the contractor is less likely to cut corners. Seen a lot of crawlspaces that are more like caving and I have to breath in to squeeze under framing. I'm not shocked when I find a half assed install of poly along the floor in those crawlspaces, how are you suppose to work in a space that tight? I'm lucky I'm just there to take a look and snap a few photos.

IDK how much building envelope stuff you get involved in, its usually more of the architects domain. I do mostly forensic work now so I have started to educate myself more in building envelope performance as I get a lot of work related to moisture problems.

 

[kipfoot]

I think you'll be fine.

It's a big volume so the air won't be as likely to be get as cool as a small space under an air conditioned house. Just make the vents window-sized, because that's what they'll want when they decide to put a slab and a pool table down there.

 

[DBronson]

phamENG:

I'd be very careful with this approach - if the house is air conditioned, you'll almost certainly have moisture condensing on the underside of the floor sheathing with the resulting conditions LuK13 mentioned. The moisture comes from the outside air that's brought in through the code required vents. I'd highly recommend you consult the Building Science Corp website. Joe has several articles on the proper design of a crawl space. Link. Look on the Articles and Papers page and click/search for crawlspaces.

Regards,

DB

 

[phamENG]

LuK13 - thanks. Especially for the tip on airflow and vent size. I'll make a note to be sure that's addressed.

kipfoot - yeah, I've done a few FEMA house raisings where they "just wanted it to look normal." Yeah right. Thanks.

DBronson - excellent source. I've poked around on there before but had forgotten about it. BSI-128 Designing For Floods is certainly useful. I like the detail showing bat insulation with continuous rigid insulation and foil below it with a protection board for the moisture problems. I'll have to discuss it with the architect and see what we can do with it. Only thing I don't like is the inability to inspect the framing. So if the detail works, awesome - it's just like a second floor that you can't inspect - but if it doesn't...

 

[LuK13]

pham,

With that detail the sealant or gasket around the perimeter of the ceiling is key. Any airflow that sneaks into the floor framing through the ceiling-to-wall intersection is going to carry way more moisture than would ever diffuse through the ceiling.

After bulk water (a leak), air flow is enemy #2. I've see similar issues with parapet walls where air leaks along the perimeter of the ceiling makes their way into the parapet, condensing and destroying the parapet from within. Joe from BSC coined the term "parapetits" since its so common.

If the air sealing is done well you can sleep at night. I see a number of people debate permeable vs non-permeable foam insulation in situations like this. The permeable crowd is of the opinion that if you do have a moisture problem it can at least make its way through the foam and damage the sheetrock to tip off the occupants that something is wrong. Of course code is still probably going to want a vapor retarder somewhere in that assembly.

 

[Enable]

Crawl spaces are really not much different than anything else in terms of detailing. The issues usually come from people seeming to think that building science principles cease to apply just because people call a space by another name. You either consider them part of the conditioned space or you do not, and there is no in-between.

Check these out, they're from the leading expert on building science pretty much in the world. Though you either really enjoy his humor / writing or hate it so. I tend to enjoy it espeically since his blogs are technical parodies of the equivalent ASHRAE publications they are based on.

Crawl Space #1
Crawl Space #2
Crawl Space #3

EDIT - Just noticed you already referenced Joe yourself, ha! My bad, it's the long weekend and didnt read the entire thread prior to responding. My apologies.

 

[XR250]

I practice in central NC - where most houses are constructed on crawlspaces.
My experience is:
Ones with proper venting, vapor retarder and height seem to do better.
Floor trusses hate high humidity and tend to creep more than other products.