Q’s about crawspace encapsulation. Management of moisture post encap. While avoid air transfer(makeup air) between crawl and living area, as well as attic(vermiculite)
Hello,
I am located in BC canada. I just bought a 1952 home, bit of a fixer upper.
One big area of concern is the vermiculite in the attic and how it may be disturbed by various other jobs needing to be done in the home.
One of the more pressing jobs is the crawspace. It has a very bad odor and the humidity is hovering around 80%. Currently it is a dirt floor. Luckily no visible mold.
I have approached a basement system company. They have recommended full encapsulation which includes the membrane and spray foam up to the subfloor. I do not believe they air seal the subfloor. They also recommend a single exhaust fan that would either pull air through various cracks in the subfloor or install a dedicated air transfer grill. The grill would meet BC code. The exhaust fan would be on a timer for 8 hours a day.
I have a couple concerns with the plan. I am not sure whether they are valid or not.
1) With a exhaust fan (100cfm) would this not create a slight neg pressure and potentially pull air from the attic?
How can I be certain makeup air is not coming from the attic?
2) The fan only runs for 8 hours a day. With the fan off could lingering smell/radon/soil gases enter the home. Especially if an air transfer grill is installed ?
What I am really asking is, how can I properly manage the moisture in my crawlspace, while maintaining a balanced air system throughout the house. To be clear, I absolutely do not want the possibility of contaminating my home with soil gases from the crawl or asbestos fibers from the attic
Please advise. Thank you
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Replies
Westcoastgrind,
The fan and transfer grills work well in our temperate climate. The Stack Effect means you would need really high CFMs, and a lot tighter walls than I suspect you have, to draw air down from the attic.
Rather than a 100cfm fan on a timer, why not use a 30 or 40cfm fan used continually? That would alleviate any concerns you have about odors (although once the space is encapsulated I bet they disappear), but also provide a constant slight pressure imbalance, rather than a periodic larger one.
HI Malcom,
Thank you for the reply. Sounds like a good idea to me.
Do you recommend trying to air seal the subfloor in the crawl and then providing makeup air via the grill? The subfloor is some kind of cedar planks, lots of gaps.
Inside the home is drywall and fairly air tight except for the chimney pipe, outlets and potentially baseboards/wall. What I am hearing from you is there would still be a stack effect in the home, despite the encapsulation, and that a 100cfm would be unlikely to overcome this effect.
I talked to another contractor today and he said that encapsulation might not be necessary and I should try and save my money. He suggested laying some poly on the dirt floor, cutting a 6 inch vent hole in each corner of the crawl and installing a intake fan In the middle of the crawl with ducting to exhaust outside.
Thoughts?
"Inside the home is drywall and fairly air tight except for the chimney pipe, outlets and potentially baseboards/wall."
Have you done blower door testing? If you're not measuring, you're guessing. By modern standards, what you've described doesn't sound tight at all.
Correct me if I am wrong, but I don't belive it is safe to do a blower door test on a home with vermiculite in the attic.
I agree with you this home is not well sealed. It was built to 1952 standards, that is the concern.
Westcoastgrind,
Encapsulation really only means providing a continuous air and vapor-barrier on the floor and walls of the crawlspace. That's going to do the heavy lifting in terms of reducing excess humidity. Adding insulation to the walls, whether foam or batts, will reduce energy use, but may not be a priority for you right now.
The transfer grill kills two birds with one stone. It reduces humidity, and helps condition the space by drawing warm air from the house above.
The PNW is one of the few areas where vented crawlspaces can work - but really aren't a good idea. As Walta said, you don't want to draw outside air into the crawlspace, through passive or powered vents. It will reduce humidity, but makes the floor the boundary of your conditioned space, meaning you should be insulating and air-sealing the floor, when it is much easier to just include the crawlspace as part of your conditioned house.
I am hearing what you are saying regarding bring in outside air. I guess this would make the crawl "unconditioned" despite managing moisture?
I would prefer to have a conditioned crawl but my initial concerns stand. I do not want any contaminated air entering the home from either crawl or attic.
Westcoastgrind,
Once encapsulated, with a low CFM fan and transfer grill you will not have the living space contaminated by air from either source.
Assuming you are in coastal BC, if you bring in outside air, unless you condition (heat) that air, it will track the outside RH pretty closely. And the outside RH is often in the 70% to 90% range.
If the cost of the first option is too much right now, I would get the poly done to air-seal the crawlspace walls and floor, and forgo the insulation until later.
Crawlspace encapsulation, the way I read the code book encapsulation is not found in the list of approved options. Maybe the CA book is different.
I find vented, power vented and two flavors of conditioned. The way I see it the only thing dumber than vented is power vented. So, you are left with conditioned as the only sane options. It sounds like you are unwilling the connect the crawlspace and the living spaces so you will need to commit to buying separate equipment and spending the money to run that equipment necessary to keep the crawl space at more or less the same temp and humidity as the rest of the home. Some people find installing a dehumidifier in the crawlspace will provide enough heat to keep the air in the crawlspace above it dew point (conditioned).
To my ear it sounds like they have talked you into power venting your crawlspace. Let’s think about this the air in your crawlspace is more or less the same temp as the rest of your home. Now you are going to use a fan to blow that warm air outdoors to control the moisture in the crawlspace. Ideally the air will be replaces with conditioned air from the living space and unconditioned air will find its way into the house to replace it. Generally, this will be at the lowest leak where the air is coldest. You are blowing heated air out of your house deliberately that is totally crazy.
Consider having the attic insulation tested for asbestos and having any asbestos removed.
After any asbestos is gone get the house air sealed with blower door testing before during and after.
Walta
Hi Walta,
I am doing my best to follow what you are saying but I am a little lost.
In your scenario where I run a dehumidifier, would there be no venting or air flow at all? Even with a dehumidifier is it critical that the crawl stays above dew point? I am not very familiar with dew point but I understand it's the tempa where the air will release it moisture?.
The #1 do it properly option, by a very reputable and expensive company is to do full encapsulation with mechanical ventilation. Specifically pulling air from living area and exhausting outside. It's pretty clear that you are against the idea of exhausting conditioned air.
What do you suggest as a good alternative?. Keep in mind things have to be realistic given the age of the house and my limited budget. I can afford full encapsulation and a dehumidifier if needed but this is already more than I was hoping to spend. I definitely cannot afford a new hvac system or wildly expensive alternatives. I cannot afford to remove the vermiculite at this time. All said and done with new insulation this would cost me 20k
One thing I dont not understand is if you are against exhausting conditioned air then how is fresh air entering the home? One more consideration is any lingering odors or soil gases with mechanical ventilation would these not be mitigated? My home is in a low radon risk area but it is a thought.
I recommend you get that vermiculite tested. Sometimes it has asbestos in it, sometimes not. If it doesn't contain asbestos, then it's not as much of an issue. Get it tested, and maybe rest a little easier.
Regarding the crawlspace, once it's encapsulated you'll have less humidity issues since the membrane and sealing work will keep that moisture out. I'd run the membrane up the walls and insulate with rigid foam (polyiso is great for this), then you only need spray foam for the rim joist -- and sometimes that can be done other ways too. You have to have some ventilation through the space, or you can put a dehumidifier in there. Either way works, but ventilation is cheaper to operate over time.
I don't think the fan would draw from the attic, but you'd need to provide more detail on the layout of the space to be sure. Usually all you want to do is circulate some air through the crawlspace so that it gets conditioned air from the living spaces, preventing stagnant air from sitting in the crawlspace.
Bill
Hi Bill,
Thbak you for your input.
To further complicated things, I was talking to a friend today. The fact that I don't not have perimeter drains came up. He mentioned that I better make sure I dont have a moisture issue related to lack of drainage. He mentioned that if I encapsulate and do not adress any potential drainage issue that I could trap water behind the membrane and this could damage my foundation.
Do you know anything about this? So far I have no seen any standing water but it has not rained heavily yet. How would I go about determining if perimeter drains are absolutely necessary. The house has done somewhat okay for over 70 years without them. How do I determine if drains are needed?
Foundations don't care if they get wet. They can leech out minerals in some cases if water migrates through them, but your interior-side membrane won't do anything here, and you can only fix that kind of problem effectively from the exterior (grading, gutters, possibly french drains). I wouldn't worry about the foundation being "damaged" here.
I would worry about bulk water if that's a potential issue. If that's the case, and you can't fix it properly from the exterior, then I'd consider putting a drain around the interior perimeter and put in a small sump with a pump in it. Before you go to all that effort though, be sure you actually have a risk of bulk water getting in.
Bill
Thank you
Almost certainly the crawl space has been drying into the house. You want to stop that, you want to keep that moisture out of the house, nothing good can come from it. It's unlikely, but possible, that enough moisture flows into the crawlspace to undermine the foundation, and that drying into the house is the only thing keeping the foundation from collapsing.
A sheet of polyethylene plastic on the floor of the crawlspace will stop the flow of moisture. I wouldn't even begin to worry unless you started seeing visible liquid water under the sheet after installing it.
Yes the house is higher humidity as well around 60%.
Do you recommend full encapsulation?