How would (or *Would*) you insulate this space?

[DerbyDad03]

I've spent the afternoon insulating the rim joist bays around my basement. I'm filling the bays with R-19 fiberglass insulation, but I've got a 10' section that I'm having trouble with.

I'm currently working on the side of the basement where the rim joist runs parallel to the floor joists and for the most part it's open and easy to insulate.

However, there's a 10' section that contains 2 heat ducts and a bunch of wires. There's really no way to get any appreciable amount of insulation behind, over or under the ducts.

I suppose I could figure out a way to insulate the first joist bay interior of the rim joist and seal it to the top of the block wall, but is it OK to seal the heat ducts in the cold(er) space?

I'm not sure if it matters, but that side of the house is virtually blocked from any wind since it is both on the primarily leeward side of the house *and* blocked by the house next door if the wind ever comes from that direction.

Should I just forget about it?

[cubby]

Probably a dumb suggestion (and you may have already discounted it based on circumstances) but what about foam (my answer to most things insulation)? Even if you get through 10 cans, it will do the job better than fiberglass. I don't like using fiberglass for rims. This may not be typical, and may be down to mice or bad installation, but when I pulled the ceilings in the basement of my 30 year house much of the fiberglass had fallen away from the rim enough to be pretty useless. Judging my the cold spots on the joint between ceiling and wall on some of the exterior walls upstairs, a common problem. I ended up putting in new batts but wedging them in and making airtight with a layer of pink board on the inside edge (sealed around the edges by my favorite foam of course). Worked a treat.

I wouldn't forget about it. I'd rather stuff fiberglass in with a drywall knife of something. Heat loss could be significant (oh I forget to mention that I am in MN, which may explain my foam obsession). Wouldn't there also be condensation issues when the AC is on?

[aemeijers]

What is on the outside of the rim joist at that point? Brick, real siding, or vinyl? If the latter, mebbe you can tuck a strip of foam slab along the wall under the siding without it being too obvious. If sheathing is celotex, replacing the bottom 2 feet with the fancy foil-face foam board (taping the joint of course), would do you almost as much good as the inside insulation. Simply caulking the cracks above and below sill plate can make a big difference all by itself,

Hard to say without seeing the duct, but what would it take to pull that run loose and fine-tune reality a little bit? Put foam board behind, and rejigger the coupling up the the floor registers so you can stuff it back in the bay. And while I don't like flexi-duct too much, maybe you could switch that last run of hard duct with something a little more squishable?

[DerbyDad03]

My concern with foam is that I would be embedding the wires that run behind the ducts and up into the living in foam. There is a mixture of plastic covered NM and some original 1950's fabric covered NM.

Is it OK to bury the NM in foam?

I would also obviously be covered most of the duct work with foam, but I guess that's OK.

[cubby]

I can see why you're concerned about the old stuff. I am 90% certain there is no problem foaming NM. I just had a bedroom rebuild with the new structure insulated with spray foam, and most of the wires to the ceiling lights were either buried in it or well coated. Electrical inspector was perfectly happy with it. I guess that if you were worried about it being overloaded due to wrong breaker or something, then it would be a concern from a fire/fumes POV.

[DerbyDad03]

***

Ready?

From the outside in:

Vinyl siding Some light, flaky cardboard-like backer board behind each piece of siding Foil covered foam insulation The original cedar shakes

I don't recall what, if anything, is between the cedar shakes and the sheathing/rim joist and it's too dark, cold and snow for me to check right now. ;-)

***

That might be doable, but 1 of the 2 runs run above the bathroom ceiling before going upstairs, so I don't know where the joint is right now. I'd have to move some storage shelves just to check. They also run between the I-beam and the sub-floor after making a right angle turn from a main trunk. There are others ducts off of that trunk in the same area and depending on what order they were put in - including the trunk - will determine if I can remove the 2 in question without major dismantlement.

***

I don't have floor registers, I have wall registers. When the round ducts turn up they transition to flat duct work that is flush up against the rim joists. I don't know if a rejigger is even possible in that area.

[benick]

Wow , tough one Derby..Did you ask at HD ??JUST KIDDING..Without pulling the duct and replacing it with the insulated foil covered smaller flexible duct and packing insulation in front of it before putting it in and then packing insulation around it I don't see an easy answer other than having the space foamed and even then you wouldn't know how much you got behind it..I see no easy or quick fix...Good luck...