Possible mold in unconditioned crawlspace

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  #1  
Old 07-19-04, 02:24 PM
reyesuela
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Possible mold in unconditioned crawlspace

I have a main house (approx. 1200sqft of floor above the crawlspace) and a guesthouse (approx. 1000sqft). Just discovered that unconditioned spaces are no longer vented properly because of joist insulation covering the vents (which were put between joists instead of in the foundation wall like they should have). There is possible mold/mildew/something under the guesthouse, which is very moist (but not dripping!), and the main house crawl is slightly moist, too.

We are on the top of a mountain and the slope of the ground surrounding the houses is correct. This house was built in 1983, and there is no central heat or air, nor will there be forced heat. With the existing insulation between the joists, with the fact that we have no plans to ever install ductwork in the crawlspace, and with the lack of preparation at construction, it seems wisest to keep the unconditioned/vented scheme.

Step #1 is to install PROPER vents and to completely close off the vents that are in the wrong place. Step #2 is to install a vapor barrier under the guesthouse (there is none at all) and to fix the one under the main house so that it is taped correctly and continues up the foundation walls the way it should. Step #3 is to install rigid foam insulation against the undersides of the joists, over the existing insulation.

I have two questions.

1) If there is mold, should I bother to treat it since I'm about to seal it under a vapor barrier? I'm going to put in the vents very soon, but the vapor barrier won't go down until winter so there's less of a chance of meeting a snake under there or getting bitten by hundreds of spiders. *g* This will give it an extra chance to dry out first.

2) The vapor barrier under the main house is some sort of black plastic. Should I put down one of the new materials on top of it instead of trusting it?
 
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Old 07-20-04, 01:01 AM
Wrenchforhire
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For the cost of plastic sheeting, I would replace any exsisting vapor barrier.

I would also be careful with the foam board, you don't want to trap moisture in with the floor joists.
 
  #3  
Old 07-20-04, 08:23 AM
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Unconditioned Spaces

For uncondtioned spaces, the correct insulation scheme is to install the insulation in the joist spaces...NOT against the foundation walls.

Your basement is currently insulated correctly and should not be altered.

I do share the previous poster's concerns about placing rigid foam insulation over the bottom of the joists because these boards will trap moisture that will be entering the unconditioned space from the floor above.

If you insulation between the joists is blocking the vents, the vents are in the wrong location.

Vents need to be in the foundation wall, not in the rim/band/box joist....so install new vents as needed.

The dark plastic vapor barrier should be fine as long as it is at least 6 mil thick, but changing the plastic sheets won't hurt and won't break your wallet.
 
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Old 07-20-04, 11:23 AM
reyesuela
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Originally Posted by homebild
For uncondtioned spaces, the correct insulation scheme is to install the insulation in the joist spaces...NOT against the foundation walls.
Building Science Corp recommends a poly barrier around the foundation walls, too. I wasn't talking about insulating it.


I do share the previous poster's concerns about placing rigid foam insulation over the bottom of the joists because these boards will trap moisture that will be entering the unconditioned space from the floor above.
I see your point, but I believe that the insulation currently has a vapor barrier facing the floor above. The side below has no barrier, though, so water can get into the insulation from above.

Within the next few years, there will be radiant heat put in on the floor above, so if rigid foam were applied after the heat's been on for a month, there shouldn't be a problem in trapping anything.

This is the system I want to follow:

http://www.buildingscience.com/resou...rawlspaces.pdf

(the unconditioned vented version)



If you insulation between the joists is blocking the vents, the vents are in the wrong location.

Vents need to be in the foundation wall, not in the rim/band/box joist....so install new vents as needed.
*sighs* I was planning on installing all new ones, but after I walked around the perimeter of the house, I realized that the foundation walls are buried too deeply to add the number I ought to. So I'm going to add the ones I can and push back the insulation and add a piece of wood to hold it away from the vents that are already there. Not ideal, but I'd mess with the dainage if I dug!


The dark plastic vapor barrier should be fine as long as it is at least 6 mil thick, but changing the plastic sheets won't hurt and won't break your wallet.
Ok!
 
  #5  
Old 07-20-04, 01:56 PM
reyesuela
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I bought a cheap hygrometer yesterday. It POURED last night, and the ground is still wet, so if I'm ever going to have humidity problems in my crawl, I should now.

I checked the main house cawlspace more carefully and discovered that the vapor barrier *was* installed correctly.

The guesthouse has no vapor barrier.

The relative humidity outside was 39% at 85 degrees. Inside the main house crawlspace, it was 65 degrees, 71% humidity. I looked at a psychometric chart and determined that this was the humidity expected when air at 39% RH and 85F is lowered to 65F--that is, there WAS no moisture infiltration from either the house or the ground.

I checked the guesthouse and found that at a slightly higher teperature (68F), the humidity was at 78%. Looking at the same psychometric chart, I determined that part of that moisture was definitely a problem from the lack of a barrier.

But even with a barrier, I still have a moisture problem. Traditional wisdom recommends venting my crawl more than it is now, but looking at that idea critically, all that would do would be to introduce MORE moist, warm air into an area that is being cooled by the earth. So that approach would only work if I managed to warm up the entire earth around the house (unlikely), put something with insulative value on the walls and floors to keep the earth from cooling the space (a bit more practical, would be pricy, though, and would negatively affect my heating and cooling because the air is hotter than the earch in the summer and colder in the winter), or if I managed to get the air exchange rate up SO high that the cooling effects of the earth wouldn't matter (again, bad for bill, would definitely require a fan). And if I miscalulate, more vents could actually make my problems worse.

All in all, I'm less than thrilled with this approach.

The alternative, though, is to block out all external air. After all, it's the cause of all of my problems under the main house--and will be the only cause under the guesthouse once the poly goes down. I could put down the poly, block off all the air vents, and permenantly install a couple of programmable dehumidifiers, one in each crawl. They'd turn on whenever the moisture content got above 45%.

I think that this approach, which almost certainly violates code but which most building science experts would recommend *g*, would probably be a much more effective (and cheaper) long-term solution to my problem.

Does anyone see any problems with it?
 
  #6  
Old 07-25-04, 08:00 PM
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Crawlspace

Blocking warmer moister outside air from entering by eliminating vents is correct.

Blocking ground vapor from penetrating upward by installing 6 mil or more plastic sheeting is also correct.

The additional point you miss and code requirement is to insulate the walls and/or floors of the crawlspace to keep the space at a temperature above the dew point so that condensation will not or cannot occur.

Codes require insulation for crawlspace walls continuing down at least 1 feet below grade or where grade is at floor level, insulation must continue in along the floor towards the center of the crawl for at least 2 feet.

Codes also require that unconditioned crawlspace ceilings be treated just like attic ceilings with an equivalent amount of insulation. If your region calls for r-49 in your attic, it also requires r-49 for your crawlspace ceiling.

That said, you are on the right path, just failing to understand that by insulating the crawl it does not matter how much humidity or water vapor enters the space or gets trapped there as long as the temperature never falls below the dew point and can condense.

So the bottom line remains as the code scientists claim:

No ventilation.
Block floor vapor from entering.
Kraft vapor retarder for crawlspace ceiling up towards living space.
Insulation along walls and floors to prevent condensation.

Helpful link:

http://www.extension.umn.edu/distrib...ng/DK7051.html
 
  #7  
Old 07-26-04, 05:02 PM
Ed Imeduc's Avatar
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I cant recall all the homes that we have had to go in and try to clean up the mess from people putting insulation up there in the joist. Even the gov www. say dont I know of no building code that calls for it. I know for sure not here.
6 mil poly over lap 2' tape all seams up the wall 1'. 2"or 3"polystyrene on the wall up to the joist. Small cut pieces of R19 up in the joist space there on the sill plate all around the home. All vents closed and sealed.
Try http://aboutsavingheat.com/crawlspace.html

permenantly install a couple of programmable dehumidifiers, one in each crawl. They'd turn on whenever the moisture content got above 45%.
Yes put one down there for now. When we have the duct work down in a crawlspace like this we put two small register in the duct and one in the return. This way it also work as a large heat sink for you.

ED
 
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