How to insulate vented crawlspace ceiling to prevent rot & mold under vinyl flooring?
I am in the Seattle area (Zone 4 Marine) and mold is my worst nightmare, and, somewhat separate from that, I do not want to seal and condition my crawlspace for air quality concerns (namely the risk of mixing air from the existing crawlspace — which is gross– with air we all breathe in the house, even if we attempt a negative pressure design). I *also* have vinyl flooring under my hardwoods, which will create a reasonable barrier to drying-to-the-interior from crawlspace/floor-joists to the conditioned home space.
How do I properly insulate my floors to code-required R30 without creating dew-point-related wood rot in my floor joists a la [this BSI writeup](https://buildingscience.com/documents/insights/bsi-009-new-light-in-crawlspaces)?
As I understand it, rigid foam over the entire underside of the floor joists should be fine, but I don’t understand how the joists can dry if they *do* happen to get wet if I have vinyl flooring above them and rigid closed cell foam below them. And can I use rigid foam on the inside of the crawlspace walls as well as on the crawlspace ceiling (underside of floor joists) under fire code? I might be misunderstanding, so maybe this is a non issue if I get some properly backed foam?
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Hi finePNW, and I'm sorry no one responded to this yet. I have a similar issue with an uninsulated, unconditioned, vented crawlspace even though I'm in the milder CZ-2.
I'm not a building scientist, I've just been reading GBA for years, and if I understand correctly, if you want insulation, then encapsulation is your only option, for precisely the reasons you mentioned: anything else limits the ability of the framing to dry and increases the chances of mold and rot.
However, I don't think you need be concerned too much about "gross" crawlspace air. Encapsulation includes putting a plastic barrier over the soil, that that will likely stop most of what you're concerned about. Encapsulation turns an icky space into a nice, clean one. You do need to address possible radon when encapsulating, though.
finePNW,
"I do not want to seal and condition my crawlspace for air quality concerns (namely the risk of mixing air from the existing crawlspace... with air we all breathe in the house.
With the crawlspace ventilated I would bet you are getting much more air exchange with the house above through the stack effect than you would with an encapsulated crawlspace using one of the conditioning strategies that doesn't share air between the spaces.
@Malcolm, I suppose you may be right, though I am currently working to seal the crawlspace from the rest of the house as much as possible, so hopefully the stack effect between crawlspace and conditioned space is much diminished.
I am also working to create a very low ACH in my conditioned space, so if I were to encapsulate, would I need to actively provide an HRV connection to my crawlspace (vs. relying on a negative pressure situation)?
I was thinking I could insulate my stem walls, insulate floor joists with foil-faced polyiso, and seal off the crawlspace from the house per Figure 7 in the BSC article I noted (https://buildingscience.com/documents/insights/bsi-009-new-light-in-crawlspaces), but has the thinking on being able to do this changed?
finePNW,
No that still works - and the PNW is one of the few climates where ventilated crawlspaces don't usually have problems.
Whew! Thanks for taking the time to reply. Very good to hear that.
Thanks @michaelbluejay for kicking off the responses here! I think you gave it a bit more visibility :).
No radon here! I tested (WA state provides a free test kit; I highly recommend residents take advantage of it!).
I guess I'm concerned about the 100 years of asbestos saw dust (there were plenty of asbestos tiles cut directly through with dust falling into the crawl by the previous owner) and black mold (an incident we had a while back... that was remediated, but I'm confused about whether spores were fully destroyed where they "sprayed" affected wood on the underside of our joists / ceiling of crawlspace). Any errant asbestos dust would ideally be covered, and hopefully the mold spores were toasted by the certified remediation company. So maybe not a big deal... but I'd also have to flip flop with my contractor, and my pride would hurt!
Most importantly, I guess I'm concerned about the noise of an always-on fan and relying on the fan not to fail (and then create back flow into the house). Any experience with fan noise/vibration or reliability?
I also was hoping to follow Figure 7 in that link I shared (I actually attached it to my reply to Malcolm below). I'm curious why this might not be an effective option per your comment "encapsulation is your only option", if you have any specific thoughts.
Thanks!
Michael's 100% right. Encapsulation is cheaper, easier, more effective and more resilient than what you're proposing, anyway.
Thanks @ohioandy. I'm curious what makes it cheaper? Seems like insulating crawlspace walls to a high R-value and installing a fan would be on par with insulating crawlspace ceiling in terms of cost and effort; does the un-insulated floor not drain heat from a house in winter; and is the longterm cost of an always-on-fan + maintenance not typically substantial? Any gut feeling or potential writeup somewhere on cost?
FinePNW, maybe I shouldn't reply to posts on a Friday evenings; I got carried away with superlatives in advocating for encapsulation. But the only one you challenged was the "cheaper." First of all, I'll allow that all crawlspaces are different, and encapsulation can be far more difficult and expensive in some than in others. But in terms of total insulation/membrane costs and labor, run the numbers for your situation; even if it's a wash the superior overall result of encapsulation should sway you that way. I don't have specific information about fan operation costs or noise, the jobs I've done either had leaky ductwork down there or we just relied on passive circulation. Of course this would be part of your deliberation, but I doubt ventilation awkwardness would come close to trumping the overall advantages.
You mention asbestos dust. A 15- or 20-mil liner laying on top of the crawlspace floor would put that out of mind.
Good luck with sorting out the project!
About asbestos: I would run an air scrubber before, during, and after the work. Some of it is probably already circulating, and doing the work will disturb some of it and make it airborne. You can rent an air scrubber, but if you need it for more than a few days, or you ever plan to use it again, it's a good investment. The HEPA 1000 from JonDon is a decent value, even if it's not awesome. A cheaper option is to tape an HVAC filter to the back of a 20" box fan. It's not as effective as a true scrubber, but you can just run it for longer.
You talked about asbestos dust being covered, but I hope you know that once the tiles were cut, a lot of the asbestos became airborne, and might still be there. The smallest asbestos particles are so light that they float in the air forever. Think about the sun shining through the window in a dark room, when you can see all the particles floating around. They ain't falling.
Not a bad idea! Thanks for the suggestion. I’ll suggest it to my contractor. I’m actually a trained air pollution scientist (not asbestos or mold though), so I should have thought of that, haha!
It seems so me you have mentally boxed yourself into a corner and decided you have no good option and only two bad options to pick between. Let’s open our minds and consider other possibility.
You don’t have mold upstairs so if you keep the air in the crawlspace at more or less the same temp and humidity as upstairs you will not have mold. Clean Remove whatever contamination you can cover the rest on the ground with a thick plastic vapor barrier if you have room for a thin concrete slab even better. Seal the exterior vents, insulate the exterior walls and install a mini split set to heat and cool the crawlspace to the same temp as upstairs.
Clean the crawlspace so it is as clean as the rest of your home.
Walta
Walta,
I think he has two good options. He can build a vented or un-vented crawlspace. Done correctly, both work well in the PNW climate.