“I'm insulating my basement using 2" rigid foam on the concrete walls. I keep hearing to also put foam up in the rim joists, what is the purpose of this?”---------
----- This is to air seal the wood rim from outside air infiltration because the wood shrinks/expands with the seasons: http://www.paintsource.net/pages/sol...ood_shrink.htm
See the "Calculating shrinkage" chart on page 51; http://books.google.com/books?id=iwS...rafter&f=false
“I have plumbing and HVAC up there that fit ok with the existing fiberglass insulation, but there isn't room for 2 inches of foam in a lot of places.”-----------
----- even 1/2" of foam is better than any amount of fiberglass (air permeable) insulation; read last paragraph (though the foil-facing would not permit any drying to the interior); http://www.buildingfoundation.umn.ed...oistphenom.htm
”Since the rim joists are above grade, why should I replace my existing insulation there?”----------
------ because the foam moves
the condensation plane from the wood to the foam surface, to air dry inside (and won't rot when wet): "Condensation control"; http://www.buildingscience.com/docum...l_seal_rev.pdf
The foam is cold on one side, warm on the other; this is the "first condensing surface"
and is warm enough to prevent condensation, IF the foam is thick enough for the location (temperature wise); http://www.buildingscience.com/docum...bout-diffusion
The wood rim is cold on both side with an R-value of 1.25 per inch; standard rim= R-1.9.
Clothes taking longer to dry?
Clean the dryer screen in HOT water if using fabric softener sheets.
They leave a residue that impedes air-flow, costing you money.
Clean the ducting in the last six months? 17,000 dryer fires annually!