Seal (without insulating) sill plate?
April 25, 2015 8:47 AM   Subscribe

I've got a crawlspace in a 1940 home that is reasonably warm and dry, but I want to seal against airflow and spiders. Is it crazy not to insulate as well? Everything I've found talks about sealing AND insulating. I feel like I can get the sealing done now, and might insulate at a later date.

Hello,

I've got a crawlspace in a 1940 home that is reasonably warm and dry, but I want to seal against airflow and spiders. Is it crazy not to insulate as well?

I feel like I can get the sealing done now, and might insulate at a later date.

If I go this route, what is best for sealing the sill plate and rim joist (not sure if I'm using those words right...)? Caulk only, or Great Stuff? If I use Great Stuff I imagine the process being easier -- it's a tight crawlspace -- but I'm not sure if that works as well.

The other concern is if Great stuff will get in the way of future insulation (I'd probably use rigid foam and re-seal around the edges -- but honestly insulating might not ever happen because I don't want to spend much more time down there).

FYI hardcore professional sprayfoam is probably not on the table as an option right now.

Thank you for any input.

(if a photo helps, the idea is to do this, but without the rigid insulation at this point -- I'm thinking caulk or Great Stuff only for now)
posted by powpow to Home & Garden (11 answers total)
 
What they're showing in those pictures is for a full basement. What you probably need is insulation under what is your floor (i.e. what would be the ceiling if you had more than a crawl space). Where are you located? Climate makes a difference in this.

I doubt that you'll be able to get rid of spiders, and if you somehow manage to make your crawlspace so tight that spiders cannot get in you'd also be keeping moisture in.

I had some kind of tiny spiders that were biting me, I set out several glue traps and they're gone.
posted by mareli at 8:57 AM on April 25, 2015


When I had a crawl space it was vented on all sides (like most attic spaces are). Sealing it up -- making it conditioned space -- would have required installing a vent fan for moisture control. Your house may be constructed differently, but be careful to not create more issues while solving this.

An energy audit may be a helpful start point in considering what changes make sense and in avoiding harmful consequences.
posted by Dip Flash at 9:02 AM on April 25, 2015


Response by poster: Thanks all, I'm basically in the Southwest, where it is quite dry. The crawlspace is currently not vented, and everybody who's been in there comments on how dry/dusty it is.

I had an energy auditor come, and he suggested the sealing/insulating down there to keep cold air out in the winter.

Eventually there will be a radon system in there (membrane, pipes) and a fan sucking air out of the crawl space. I'm trying to do the sealing before the membrane goes down so I don't mess up the radon system.
posted by powpow at 9:17 AM on April 25, 2015


Someone else will know better, because I really don't, but if the radon system will be sucking air out of the crawlspace, does the makeup air come in from the exterior? That may affect where you eventually insulate (because the crawlspace will, by definition, end up cold because of the makeup air).

That said, I can't see any reason not to seal up. You'd be sealing up air leaks, not insulating really at this point, so something like silicone caulk would be fine. Or a fine bead of expanding foam caulk.
posted by BillMcMurdo at 9:28 AM on April 25, 2015


Response by poster: My understanding is that the makeup air is supposed to come from the soil. But there's no way that's 100% the case. There will be pinhole leaks and imperfectly sealed edges, so some of the air comes from somewhere -- either exterior (which I don't think I want because freezing pipes, etc) or the living space above.

That's all part of the reason I want to keep cold air out. So stuff in there isn't chilly and attracting condensation.
posted by powpow at 9:33 AM on April 25, 2015


I would look carefully at the value of sealing the edges of the crawlspace versus sealing and insulating between the crawlspace and the house. Crawlspaces are kind of weird, but in your climate, where freezing of pipes is likely not a concern, you want to keep the heat in the house itself and prevent it from leaking into the crawlspace, not try to keep the crawlspace warm.

The other upside to sealing between the crawlspace and the house is that it will reduce your radon risk, whereas sealing the outside of the crawlspace will actually increase your radon risk (if the crawlspace is better sealed, radon concentrations will increase).

The best way forward is to have the underside of the floor spray foamed, which covers insulation and air sealing at once. If you look at the value of labour (even if it is you working for free) to otherwise insulate and seal underneath a floor, I think you'll find the spray foam looks much more affordable. Cutting material to fit between joists or sealing plastic between joists is finicky, time consuming work and even worse in tight quarters.

I think you'd be better off saving your money and time until you can do the job properly, rather than doing some air sealing now, which is unlikely to have much effect.
posted by ssg at 9:43 AM on April 25, 2015 [1 favorite]


Response by poster: Thanks, I hear you on the tradeoff of time and effort and money -- I've been very tempted to just call somebody. The idea is to try a quick seal and see if good enough is good enough.

Freezing pipes are definitely a concern here unfortunately. So I hesitate to keep the crawl space too "fresh." Ideally the radon system will take care of the gas.
posted by powpow at 10:00 AM on April 25, 2015


Another term for what you're looking to do - especially because your crawlspace is already nonvented - is 'crawlspace encapsulation'.

Spiders are following other bugs into your crawlspace. If you can control the other bugs, the spiders will go elsewhere to hunt. I bought good stuff from Do My Own Pest Control and put a barrier spray around my house. They have good guides as well.
posted by bookdragoness at 11:28 AM on April 25, 2015


Best answer: I just did this to my house last autumn- and I live in the southwest. It is a huge(!) improvement in air quality and heat retention. Vented crawlspaces are of the devil.

You can do a quick seal up of the major holes with great stuff and caulk, but as cheap as foam insulation is, it's hard to recommend against just doing the job right the first time. It took a weekend to do mine, and went quickly - most of the joist spaces are the same, so a template makes cutting easy. The foam blocks are glued into place and then great stuff around them.

I also had a radon system installed, and a good vapor barrier placed on the floor and glued to the stem walls. This is important, because you actually increase your radon risk by sealing the space.
posted by Pogo_Fuzzybutt at 5:08 PM on April 25, 2015


Response by poster: Interesting Pogo_Fuzzybutt, thank you. So what you're saying is it isn't that much more trouble to stick blocks in.

I wasn't sure if you could pre-cut lots of them the same size and get away with it. Maybe if you undersize the blocks a little and fill any excess with Great Stuff... My main reason for not insulating is laziness and wanting to minimize time in the crawl space (again, the pipes haven't frozen yet), but if you can pre-cut 99% of it outside with more elbow room, that might change my mind. My crawlspace is very hard to work in (as are many, I'm sure), but maybe it's worth it.

Good to hear it was successful.
posted by powpow at 6:28 PM on April 25, 2015


The best way forward is to have the underside of the floor spray foamed, which covers insulation and air sealing at once.

Freezing pipes and such are still a risk throughout much of the southwest. In addition, what happens is that warm daytime air, dry as it is, is drawn into the crawlspace, where the cooler space causes condensation. The use of swamp coolers adds to this effect as it pushes moistened air into the space. The moisture that gets down there basically never escapes - and this becomes a habitat for critters and mold. The spores then get drawn through the house via the stack effect.

Recommendations are now to seal the crawlspace and treat it as a basement. Radon is a risk, so it is best to install a mitigation system.

Maybe if you undersize the blocks a little and fill any excess with Great Stuff...


It's best if you make the fit as snug as you can, but yeah that's the idea. If you look at your rim joist, you'll find that the floor joists are connected at regular intervals, so probably 90% of your cuts will all be the same. You can do those cuts in the back yard or whatever, and then install them. That goes fast. The rest of the work is more time consuming, but foam is really easy to work with, so it isn't that bad.

But yeah, it was a big quality of life improvement. Critters and bugs have been exorcised and the floor is much warmer through the winter. We saved about 10% or so off our heating bills.
posted by Pogo_Fuzzybutt at 6:42 PM on April 25, 2015


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