Rim joists above existing diaper wall
Hello GBA,
I am wondering if there’s a rim joist insulation strategy I can use in my basement given that my basement walls are already built, “diaper style” by a previous owner, probably 30+ years ago.
The walls have tar paper in front of the concrete foundation wall, followed by fiberglass-filled wood framing, 6 mil poly, then drywall. The sill plate has a capillary break between it and the concrete wall.
The rim joists are accessible since the ceiling is a drop ceiling. There were paper-faced batts on them before (30+ years old) and I’ve just begun removing them and caulking the gaps in between wood components. So far I have not seen a sign of mold or rot.
This basement is relatively dry in the sense that I’ve never seen water and it doesn’t smell, but it does have high indoor RH in the summer (~70%) for 2-3 months. I have been running a dehumidifier during the summer to keep it in the 50-55% range. In winter the house runs in the 30-40% range.
For the rim joists and end joists, I’ve been thinking of rockwool batts plus a smart vapour barrier like intello plus, taped to the poly from the wall. I’ve also been thinking of 2″ EPS rigid foam, followed by a rockwool batt.
I also have a challenging ‘void’ at the 2 ends of the house which I’m not sure what to do with. I was considering removal of the bottom half of the joist that’s blocking access, so I can do the air sealing and insulation, then sistering in a new joist. But realistically I probably don’t have time for that.
My wife is encouraging me to give up and just “put it back the way it was” which I actually might end up doing. But hoping that there is a good way to improve the insulation here without going too crazy.
The house is from 1966 and located in Toronto, Ontario, Canada.
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Replies
For the rim joist cavities, after caulking the gaps in the wood components, my current idea is to install R22 rockwool batts and then a smart vapour barrier taped at the top edge to the subfloor, left/right edges to the joists and bottom edge to the protruding poly behind the existing drywall. Looking at using Majrex 200. Thoughts on this approach?
Tom,
The advice typically differs on how to deal with rim-joists on top of concrete stem-walls, and those on the floors above. Because of the difficulty of air sealing the former, and the increased moisture which can come from the concrete below, the usual recommendation is to use pieces of foam board sealed into place with canned foam.
That said, what you are proposing works fine with diligent air-sealing of the membrane in each joist bay.
The poor access to the rim-joist at the end drives me crazy. When I frame floors I do my joist layout starting in the middle to avoid that problem. It's not rocket science. I wish more framers did that.
Thanks Malcom!
I think I will proceed with the batt and smart VB... the foam sounds okay too but I like rockwool and I'm pretty detail oriented so the taping will likely work out. Fortunately my sill plate / foundation interface has a capillary break but yes there is likely going to be some moisture that migrates through the concrete.
Yeah that void is driving me crazy alright! I guess there is no easy solution there. I am wondering if I were to insulate and tape the smart VB over the 'inner' joist if it would create a major condensation risk inside the void?
Tom,
I don't see why air-sealing the inner joist and packing the cavity with rockwool wouldn't work. One thing to look out for would be holes up into the walls above for wiring or plumbing you would want to seal to stop them bypassing your air-barrier.
Yes that is a good point. The void is not really accessible so I wouldn't be able to pack it with rockwool. The subfloor is diagonal plank so there is a chance that the gaps between the planks are somewhat connected to the space above. I managed to slip my phone into the gap under the 'inner' joist to get a pic of the space and the subfloor above... Hoping this might trigger some new ideas!
Tom,
Rather than remove part of the joist, you can drill holes without compromising the structural integrity:
- Holes must be at least 2 inches away from the top or bottom edge.
- The diameter of a hole cannot exceed one-third of the joist's depth.
- Multiple holes must be spaced at least 2 inches apart from each other.
You can then pack the cavity with insulation.
Thanks Malcolm, I think that would work for insulation! How about air-sealing though? If I just packed in insulation without air sealing, warm air might migrate from above through those diagonal planks and condense on the now-cold end joist. What do you think, do I have a legitimate concern?
To add to what Malcolm said, that rigid foam you use should be something a bit vapor open, so EPS or fiber faced polyiso. I usually use EPS, since it's easy to get and relatively inexpensive. I like to use a better grade of canned foam to foam in the panels, usually Loctite's (now GE's) TiteFoam product, and I bevel the edges with my hot wire cutter so that the panels stay in place. Cut pieces of ~12 gauge solid copper wire can act as temporary spring clamps to hold the panels in place while you apply foam and let it cure. Cut the pieces about 1/2" longer than the distance between the top of the mudsill and the underside of the subfloor, then push the middle of the piece tightly against the foam panel. The wire will bend and lock into place on it's own, but is easy to remove after the canned foam cures. This is how I do it, and it works great.
That trouble spot is tricky. I have a few of those in my own home too. The best option for those is spray foam, but even then, it's difficult to do a good job due to the limited access. I've tried the Krakenbond stuff you can get on Amazon for this purpose, and it works, but it's hard to know if it's fully covered the rim joist without leaving a gap down around the mudsill.
BTW, what is that metal vent pipe? in the back right of the third pic?
Bill
Thanks Bill!
I'm actually now looking to use rockwool batts with a smart VB such as majrex, in order to avoid foam altogether. It feels like a similar amount of time/effort but more green and also seems like I could more easily inspect inside there if needed. I do like the copper wire spring clamp idea though, I'll definitely remember that if I end up using the rigid foam.
Re: the trouble spot, yes very tricky! What do you think of my idea of slicing the bottom half off of the blocking joist in order to access the void and air seal/insulate the real end joist. Then sistering in a new joist? It seems like a ton of work though.
The vent pipe is an hvac duct leading up through the corner of the main floor living room into a 2nd floor bedroom floor vent. I had to remove the section of horizontal duct in order to access what I thought was the end joist. The plan is to reinstall that duct, maybe with an insulation sleeve on it, after the end joist is air sealed/insulated.
Batts and a vapor barrier is going to be a LOT more work than foaming in cut pieces of EPS. I think it will also be very difficult to get all the details right, and it's a lot more critical with your proposed assembly than it is with rigid foam. I don't think you're gaining any "greeness", either, just changing what the process is (blowing agents for the foam (which are mostly recaptured during manufacture in the case of EPS), or high energy inputs in the case of mineral wool). I think your proposed assembly increases risks and doesn't really gain you anything over using rigid foam here.
I suppose you could cut a big chunk out of that joist, but as you said, it would be a lot of work. Concerns I would have would be can you fit a new full length joist into the space? I don't mean right next to the original joist, I mean can you get it through the house and access door to the crawl space after allowing for things like turning corners? When the house was built, they would have dropped the joists in from above before the walls were up. You don't have that luxury now that the house is built :-)
The other concern I would have is if that joist is carrying much load, you may need to put in some temporary supports to prevent the wall from sagging, and then jack up the old one a small bit to "preload" it before sistering in the new one. That might be some added complexity too.
Bill
Thanks Bill, it's good to know about the foam not really being much greener than the rockwool. How about resistance against critters and moisture, does rockwool fare better? Batts and VB would also make it easier to open and inspect the wood if I have any concerns in the future, so I was thinking the extra effort taping might be worth it. I will take some time to consider the foam though...
For the void, would you drill holes into the joist face at a certain interval and then fill with foam from there? What about cellulose? I am pretty reluctant to use a ton of spray foam. The mudsill is kind of accessible from the crack in this section (see this video: https://www.dropbox.com/scl/fi/zytgk6mumgs2w8tzhtza6/IMG_1460.MOV?rlkey=a2ojy6dxb2fg5czjygmed0nac&st=lupthrha&dl=0). In another spot (on the other side of a supporting beam) the joist is staggered inward so that section will be harder....
I think using any batt or other air-permeable product here is asking for trouble, even with the vapor barrier. The rigid foam is close to being a fully adhered insulating material in this case, due to it's relatively small size and the fact it will be against a very nearly perfectly flat surface. You end up with an insulating material that leaves no voids, and creates it's own pretty much perfect air barrier, which is very helpful in this situation and you can't get that with batts and a vapor barrier regardless of how careful you are.
I don't think either material is inherently better or worse in terms of critters. I've seen critters burrow in foam, and I've seen them in mineral wool too. Mice would probably prefer the mineral wool to the rigid foam, insects probably don't care much either way.
Note that "canned foam" (one-part foam) IS NOT the same as "spray foam" (two-part foam). All the issues you here about with spray foam problems are not issues with the canned foam, so don't worry about using it. The downside is that it takes a lot of cans of canned foam to use the stuff as an insulating material and not just a gap filler.
In your difficult spot, I think Malcolm's idea of drilling a series of holes is a good one. I would try the Kraken Bond canned foam to insulate and seal in that case. The Kraken Bond canned foam comes in an extra big can, and is intended to be used as insulation and not just a gap filler. BE SURE to use their foam gun so that the applicator tips they include with each can can snap onto the gun. Their tips will NOT snap in place on the common Dow
"Pro" gun you can get in the box stores. I've used this product a few times in the past and it does a pretty good job. Lessons I've learned using it: use a relatively LOW flow rate (adjustment knob on the gun dialed down), it works better when applied more slowly. It expands a bunch AFTER application, so it's going to "grow" more than you expect from the initial application. The can is big, and makes the gun difficult to maneuver in confined spaces, so keep that in mind here. Lastly, you'll need more cans than you'll think you do, so be sure to have some extras on hand.
Bill
Thanks again Bill, I have a tough decision to make now! You've made a strong case for foam. It's also good to know that the canned foam is safe. How does canned foam do during expansion/contracting of wood components though? I read that it can crack, which would allow that warm house air to enter in behind the foam....
Also, something I was hoping to do was connect the smart vapour barrier to the wall poly to create a continuous air barrier. If I did rigid foam plus batt, the gap at the top of the wall assembly would still be open to the interior... not sure that's a good thing.
Regarding the joist, yes it would be possible to get a new joist down into the basement pretty easily. This is because my house is built in 3 sections with 2 steel I-beams. I would only need a 12 foot section (approx) of 2x8". But even then, I cringe just thinking about the effort and risk of doing this. Filling the void with foam seems more straight forward. It doesn't seem like the kraken foam is available in Canada though... how about something like Sika Boom-121? And how would I ensure the void would be completely filled? It would be like flying blind a bit...
Reply to post #13:
I haven't seen canned foam cracking from expansion/contraction myself, but I've heard of it. I think using it in the rim joist area as we're discussing won't be an issue. I'd expect this type of issue more in areas where two beams are butt jointed, places where different parts of the structure come together relatively loosely. With the rim joist, a lot of things are locking the pieces together, so any relative movement between them should be very minimal, making for little risk of seal failure. Note that I do like the better grades of canned foam for this application. I usually use the TiteFoam product, but I suspect the Sika product is probably similar in terms of being a high quality product.
If you want to keep the air barrier continuous, I'd use 1/8" masonite or other cheap panel product to bridge the gap, then bury the edge at the rim joist in the same bead of canned foam that you use to seal in the EPS panel. Use sealant to seal the other end to the framing. You'll have to see what works for you. 1/2" polyiso would be another option here. You just need something that will make a good air barrier. I personally prefer panel products over membranes for this sort of thing, but it does depend on exactly what you have to do, and I have obviously not seen your situation in person. Use your judgement here. The big disadvantages to membranes here is that they will flop around during installation, which can sometimes complicate things, and they are easier to damage compared to panels.
BTW, the Sika product you mentioned is very different from the Kraken Bond product. There is also a Vega Bond product that is pretty much the same thing. Maybe you can get one of those? They are all extra large cans of one part canned foam, and they come with special tips for their foam guns so that you can spray the canned foam instead of using it in the more traditional gap filling role.
Bill