I have a new-construction 9' poured concrete basement (1600 sq ft) with top bearing floor trusses. About 18" of the basement wall is above grade. There is 2" high density EPS under the slab. Despite more rain this year than in any other on record for our area, the basement has had no leaking/seeping of water. The house is in south-central PA.

I would like to insulate the the basement interior walls with rigid foam. I could glued it onto the concrete wall, fitting it up to the top of the wall between the trusses. But I'm concerned about possible moisture issues for the non-treated wood of the trusses very close to the concrete & on the "cold" side of the insulation (if I cut the rigid foam to fit it around the trusses). I have a good bit of the sub-slab EPS left over and could use it for part of the wall insulation if it is acceptable for that use.

For the sill and blocking on the top of the concrete wall I was thinking of caulking the joints between the wood and insulating with rock wool carefully cut to fit. I prefer to avoid spray foam DIY and the rock wool would be removeable for periodic inspection of the sill/blocking area (i.e. termites, moisture, mold, etc).

I'm just guessing on this approach. I have to do something (insulation-wise), that's for sure.

Thanks!


First floor top-bearing trusses seen from basement.


Rather dark (sorry), trying to show mud sill and blocking on top of concrete wall (at top 3rd of photo).
Mud sill is on a rubber gasket. I propose caulk & rock wool for this area.

I know you mentioned avoiding spray foam, but I think it really is the best option for areas just like this. It seals everything up nicely. I recieved quotes for 5" of open cell foam for 190 LF of mud sill area for about $500.

Yes, closed cell foam is appropriate for the sill area. That will be cold, and you don't want a porous insulation for the job, as interior humidity will diffuse easily to the cold surface and condense.

Do the floor trusses butt up against the concrete? If not, I'd see if there is any way to slide a strip of poly or some other stiff vapor barrier material between the wood and the concrete. If the strips are wide enough, they can wrap the ends of the trusses out past the rigid foam to go on the concrete wall and isolate the wood from moisture in the concrete. If you have to use a stiff vapor barrier material so that you can drive it up between the truss and concrete, you could then paint the sides of the wood with something, or even use strips of Vycor or other peel/stick membrane to protect the sides of the wood out past the foam.

Peel & stick vapor retardent membranes inhibit the drying of the wood, and may increase rather than decrease the risk of problems, depending on what the rest of the stackup looks like. Closed cell foam and wood have about the same permeance at a given thickness, which makes it a near-ideal solution here.

A 1-2" shot of closed cell on the foundation sill and band joist, sealing it to the top of the wall foam is preferable to any open cell solution in a PA climate. Open cell is fairly vapor open, and you could run into wintertime condensation/high moisture at the band joist if you went that route. With 1" of closed cell foam you can fatten up the interior R that spot with 3" of wet-sprayed cellulose (to conform around the I-joists) and it would not need much in the way of an interior air-barrier or vapor retarder. With 2" of cc foam on the band joist you could make that 5". The foam then becomes a non-wicking condensing surface, and the cellulose wicks moisture away from the structural wood, further protecting it.

A loose cut'n'cobble of the EPS would work too if you then seal the edges with 1 part foam (or 2-part from a FrothPak kit or similar) to make it air-tight, blocking convection transported moisture from reaching the band joist. But since EPS is fairly permeable (though not as permeable as open cell) you have to consider what it takes to protect the band joist from vapor diffusion. It's still OK as long as the exterior siding outside the band joist is back-ventilated (rainscreen gap, vinyl siding, or metal on purlins/furring), which gives the band joist significantly more drying capacity toward the exterior. But if the siding is tight to the sheathing, which is again tight to the band joist you need to have a lower-permeance interior to avoid wintertime moisture buildup.

Open cell can work if the siding is back ventiliated too, but you need to get the permeance of the stackup between the interior air and the band moist down to under 5 perms, (under 3 is even better). In warmer climates open cell might be a preferred solution, but So. PA is still US zone 5.

I think your plan is fine. Standard unsealed fiberglass batt over 1 1/8" osb rim or 2x will sometimes get wet depending on alot of factors and then dryout in the summer. I have seen rot and mold but moreoften just some water staining.

In your case you have what looks like at least 2 2x4 blocks between floor joists at least 3 1/2" thick which gives you a somewhat better buffer, plus you'll air seal the cracks in each bay and then your batt. If you mind your indoor humidity you will likely have 0 problems, although you may need to use foam in the bays where you cannot adequately air seal such as duct boots, pipes, exhaust vents, electrical conduit, ect where it is not feasible to adequately seal with caulk. Another tricky spot for you is the top plate of the floor truss where it lays on the mudsill, you need to seal that joint which is hidden by the vertical board closest to the basement wall, this junction is a good canidate for expanding foam as you wont be able to get sealant back there.

Spray foam or eps chunks is a better option in my opinion but your plan will not make your house rot.

Unfaced batts against millied timber rim joists will usually work out OK, but against the abutting OSB of the subfloor you're asking for mold & rot to get started. Unfaced batts will also only perform at about half their rated R without an interior air-barrier. It needs to be air-impermeable to mitigate the wintertime moisture accumulation in the band joist & cold edge of the subfloor, with the OSB being far more susceptible than full timbers.

Dana, I'm saying in the real world beyond pen and paper this is not necessarily true (asking for mold and rot) and I can say so in a cold climate from first hand, own eyes inspections of probably hundreds of homes at all times of the year in over 10 years of remodeling and home performance contracting. Homes from new to a hundred years old, good construction to crappy, engineered product or stick framed. I've pulled back batts in lots of homes to do spray foam to find absolutely zero signs of any damage and I've never seen issue on the subfloor, only the rim joist. Overall, the consistent worst when I find damage is engineered rim between the rim joist material and the exterior sheathing.

So I wouldn't fear monger unless you have first hand experience with a consistent percentage of damage out of a sizeable sample of homes with the standard batt only, my experience says otherwise.

The newer and tighter the home, the more wetness I've seen. Early winter, early spring seem to be the worst times in terms of condensation here in WI.

Hello All and thank you taking the time to respond. From your explanations I understand more about the potential risks with the batts and also there are several interesting ideas described above that I'm thinking over before I post any more on my specific case.

I can see how spray foam comes up as the no-brainer answer. And I'm not dead set against it ... I don't even mind paying someone else to do it because this I will not DIY. I've used spray foam cans for air sealing tiny odd spots in the house. Since there is no heat yet, I warm up the can before applying but it is still a little tricky to get a good job, imo.

In the link below an owner of an energy efficient house under construction describes his DIY experience with spraying the rim joist. Kudos to him for accurately describing his experience so that others can learn and benefit! I'm sure a lot of people get through DIY spray foam just fine ... but if something goes wrong ... you get a lota' wrong that's rather hard to clean up.

http://uphillhouse.wordpress.com/2011/05/05/foam-continued/ 

Now some follow-up questions:

Fast forward to a pro spraying the rim joist in my basement, in March, let's say. It's going to be a lot cooler than 60 deg F. What do (should) pros do to routinely get good results in colder than optimal temps?

If there are some pin holes here and there in the foam application, due to missed spots or shrinkage, do you just assume that these little holes are so small that moisture damage would be self limiting because of the tiny surface area invovled?

I've read cautionary tales about how beads of caulk or foam (like around pipes) can fail over time with the normal movement of wood frame buildings and hence the recomendation for gaskets instead. Does that same logic apply to spray foam in a sufface application like a band joist? Why or why not?

Thanks

Pros use a "cold weather" formulation of the 2-part chemical mix that makes up the foam, and adjust the chemical temp controls accordingly. I'm not sure how cold they can go and still get good results, but even 30F or lower isn't unheard of with the cold-mix. If the basement is heated the surface temp of the band joist would almost certainly be above 50F,maybe even above 60F in your location in March, barring a late-winter cold snap. The professional spray equipment has a HUGE advantage over the DIY kits in the form of tight mixture and tight (even separate) temperature control of the 2 chemicals, usually heated in the hoses with the temperature feedback for the control at or just before the mix-head in the spray gun.

By conrtrast, with the DIY kits the best you can do is keep the tanks in the right temperature range by keeping the tanks in a tub of water maintained at the right temp. But the tanks cool as the pressure is released by spraying, and the hoses are subjected to the ambient air temperature- it's nowhere near as tight a control over the process.

Don't sweat the pinholes & minor separation gaps unless they're big enough to allow real convection currents, but ideally the foam would stick to the joists subfloor and sill making a good seal. A real shrinkage gap of any width or length can be sealed with 1-part foam after the fact. Shrinkage issues with closed cell foam are more likely to occur when spraying in lifts thicker than 2" at a time, creating a much higher temperature difference between the center of the foam and the surfaces as it cures. (Most vendors spec 2" at a time, and violating that has also resulted in house fires.)

greentree: "fear mongering"? Moi? I can't put it on a percentage basis since I'm not looking at 100s of situations. Yes, newer-tighter (like this place)==higher wintertime interior humidity==higher mold risk. The north side of the house is more likely to have these issues than the other sides where solar heating on the exterior introduces periodic drying events and more rapid drying in the spring, etc. But aside from the mold risk the lower than rated performance of batts without interior air-barriers is reason enough to go one better. I've seen exactly one instance of an OSB subfloor being affected with black mold on the cool ~1" strip where it met the band joist when a low-density R30 was stuffed in there without an interior air barrier- call it a 3-sigma outlier if you like, and there may have been other factors increasing the risk in that house.