How much moisture can a crawlspace/air gap introduce?
This house has 400 sq/ft of basement and 300 sq/ft of crawlspace, and despite my best efforts i had to install a dehumidifier. It needs emptying 2-3 times a day (30 pint/day model) meaning its pulling 20-30 pints a day. The crawlspace is not encapsulated (and shallow and hard to traverse) and the energy audit showed almost 15ACH50, much of it coming from the crawlspace, i have been in there a few times but could not locate the air leaks but i could not access all of it.
I know this is a hard to answer with any accuracy question, but how much contribution would the dirt be to the high humidity and how much the air infiltration (which i assume exists from the audit, though i have been told the air may be being pulled through the dirt).
I am near Toronto Ontario.
Bonus question, i suspect the brick foundation only extends 2-3 ft below grade, how deep can i excavate the crawlspace if i am able to to do it someday?
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Alan,
You need to excavate soil from your crawl space to allow better access. This work is usually done with a short-handled shovel (or an entrenching tool) and Sheetrock buckets.
If you are worried about undermining your footings, create a trench down the middle of the crawl space, and slope the soil up from the trench toward the exterior crawl space walls. If you have serious doubts about the wisdom of this work, consult an engineer.
The amount of moisture contributed by air leaks depends on (a) the weather, (b) the size of the holes, and (c) the extent to which the crawl space is depressurized. This last factor depends on the number of air leaks in the upper envelope of the house, and the operation of exhaust fans and your HVAC system.
Suffice it to say that it is very important, if you have a dirt-floored crawl space, to (a) cover the dirt with 6 mil polyethylene, and (b) to seal air leaks in the walls and rim joist area. One way to seal the air leaks is with spray polyurethane foam.
For more information, see Building an Unvented Crawl Space.
The 300 square feet of dirt in crawlspace is definitely NOT the source of 20-30 pints of water in the air. It's the outdoor air leakage. Until you treat the leakage, nothing you do to the dirt floor will have very much effect.
The drier air of late fall/winter will reduce the amount of airborne moisture for the season, but it'll be back again when the outdoor dew points rise again in spring.
Thanks for your replies Martin and Dana
I'll assume i can't dig all the way to the base of the footings (assuming its not just brick on dirt with no footing), is there a formula for how much dirt has to be left in place?
Also getting rid of that much dirt is another challenge, i have nowhere to put it and it would cost a lot of money to pay someone to haul it away.
There are no exhaust fans in the house, and the rest of the house is relatively airtight (not very, but not bad at all except for the attic access but i have that temporarily sealed). I have a chimney vent water tank and would like a closed system one but its unlikely to happen on my budget, so i may end up with another chimney vent or a powervent if i can put together the money.
My previous HVAC was a chimney vent 80% efficient, indoor combustion air, the new one is a 95% high eff double piped. The winter humidity rose since the replacement, went from 20-30% in winter to 25-50%.
I have no confirmation there is actually air gaps to the outside, i am just assuming there is from the 15ACH50 blower door result and noticing how much of it was coming from the crawlspace. Since i bought the dehumidifier this past spring i don't have any fall/winter data on how much water it will produce.
How much water per day would be likely from 300 sq/ft of dirt Dana?
The 300' of bare dirt might put out as much as a liter under extreme circumstances, but not two. Depends a bit on the temperature of the soil (and it's moisture content), relative to the dew point of the air in the crawlspace.
The wintertime humidity rose with the power-vented version because you no longer have a passive 24/7 draw of dry winter air into that space. In winter the outdoor air is much dryer than you'd find inside of conditioned space, but from the beginning of June through mid-September the outdoor air is much more humid, if you pull up a dew-point graph for Toronto and zoom & scroll to see a whole year:
https://weatherspark.com/#!dashboard;a=Canada/ON/Toronto
A dew point of ~55F/13C is at the high end of the healthiest range of conditioned space air, and that would result in a fairly high RH in a cool 15-16C basement or crawl space. The July/August mean outdoor dew point in Toronto is about 61F/16C, so outdoor air being cooled in that crawl space is nearly at saturation, with a very high relative humidity. In mid winter the outdoor dew points are average about +17F/-8C, which is much colder than the interior air, and infiltration air being raised to 15-16C in the crawlspace would then have a very low relative humidity.
In most places code dictates that the footing be at or below the 100 year frost depth unless they are frost-protected by various insulation methods. Simply leaving it exposed or very shallow risks frost heaves. A quick & dirty web search indicates that in Toronto that's about 1.8 meters below grade:
http://www1.toronto.ca/wps/portal/contentonly?vgnextoid=6d02aa55ee66f310VgnVCM10000071d60f89RCRD&vgnextchannel=6534aa55ee66f310VgnVCM10000071d60f89RCRD
"Watermain pipes are buried in public streets, about 1.8 m deep, just below the area where frost ends or “the frost line.” "
Alan,
I'm not an engineer, but I don't see any structural reason why you can't excavate your crawl space down to the bottom of your footings (assuming that your walls have footings). That said, the money you spend consulting an engineer will be money well spent.
As to how you get rid of the dirt: your problem is a little like the problem that the prisoners of war faced in The Great Escape. Maybe you can put the dirt in your pockets and release it during rec time, when you are out in the yard.
Seriously, you can put a sign out in your yard ("Clean fill -- free") or ask around in your neighborhood. A lot of people would love to have some free fill.
If you are stumped about where the air leaks are located, you can buy a two-component spray foam kit and invite your blower-door contractor to return to your house. A blower door will reveal where your leaks are.
Any idea how much an engineer would cost?
It would seem that the process of elimination shows there is air leakage, i don't like process of elimination answers because unknown circumstances or miscalculations can lead to wrong conclusions, but i'll have to do what i can to get it dug out and sealed up.
I do hope the footings (if they exist) extend below the frost line, i am not optimistic (this is a very odd duck house). The floor over the crawlspace was ice cold last winter, in previous years one of the vents feeding a bedroom had separated and while the room was cold the crawlspace was kept quite warm and toasty. I had blocked it up last fall to get more heat to the living areas. On the plus side even though we went below the 99% temp for several weeks last winter, none of the water pipes in the crawlspace froze which was a real concern after cutting off the heat source.
Also Hot2000 says the heat load would go from 45,000 btu to 34,000 btu @ -2F (-20C) if i reduced the air leakage rate from 15ACH50 to 5ACH50 so thats my goal.
What's the point? If you have nothing in the crawl space that will be negatively affected by the humidity, then isolate it from the building by using a bullet proof membrane on the crawl space ceiling/first floor joists (insulate it too). Apply a Cu-bor treatment to the rim joists and any other untreated lumber that remains exposed (below the vapor barrier) and in contact with the damp air and concrete sills/walls.
You can protect the crawl space framing with a one time, life time Cu-bor treatment; no need to run a dehumidifier. You can stop wasting heat and AC by isolating the house from the crawlspace.
Mechanical and passive venting of an enclosed crawl space is the fastest way to (1) increase stack effect, (2) drive moisture into the framing and into the above conditioned spaces, (3) get harmful seasonal condensation and (4) waste energy by conditioning below and compensating for conditioned space affects above.
PS
http://www.coppercare.com/Product/CuBor
In Boston, I could hire a structural engineer for $125/hour, $200 for a site visit. No idea if that's high or low, but it ended up a tiny portion of the overall costs of renovating my house. In your case, I would imagine you could resolve everything with a site visit and a couple phone calls if you find something strange while digging. So, hiring an engineer would probably cost less than $300.
@ Flitch That is a very interesting solution. In my case i have heat/return vents, water pipes, drain pipes, electrical wires and gas lines running through the crawl, and i would like to reduce the radon (if any), and deal with heat loss and massive air infiltration its causing (and be able to reach things when they need repair) but this is a very interesting idea that i will keep in mind because i am sure i can use this product elsewhere in the future. Thanks for mentioning it :)
@ John, thanks for the numbers, i expect them to be higher in Canada (if you add 50-100% to standard US pricing your in the ballpark on most things). I was concerned it might be a few thousand dollars so under 1k as a starting point is very good to know