I was in Cheapo Depot looking at insulation and found some rolls (1/2″ thick) rolls of insulation that had foil on both sides. I am insulating my new house which has kneewalls, actually 5′ exterior walls and a bevel ceiling on the second floor. I really want to avoid ice dams on this house (in the mountains of western Maine) so I’d like to augment the insulation on the north side of the roof at this area. I was thinking that stapling this foil stuff on the bevel part would be of some benefit and some me time and money furring out. The rafters are 2×12.
Anyone with any experience with this stuff?
Replies
Rather than fool around with the foil, focus on the basics: ventilate the roof from eaves to ridge, fill the rest of those 2x12 bays with insulation, install a good vapor barrier and plug all possible air leaks into the roof system from the heated space.
Al Mollitor, Sharon MA
I would vote no. Too much trouble for minimal benefit.
If you really wanted to supplement the insulation in this way, you might consider a continuous layer of rigid foam under the drywall. This creates a thermal break with the framing.
The problem with kneewalls is that they are halfway in and halfway out of the thermal envelope. Why? Because the 'floor' in the kneewall is generally over a heated space. This makes for an inherently difficult air-sealing situation. Where is the line? Blocking off the joist space for the lower level is tricky. Invariably, someone will want to cut a hole in the building envelope so that the kneewall can be used for storage or mechanicals.
For new construction in particular, there is a convincing argument to be made for bringing the kneewall space 'inside' by insulating and using airtight drywall approach ceiling to floor, then treating the kneewall like an interior partition.
The most important thing is to block all air leaks. Most homes lose the bulk of their heat via convection, and this really is the root cause of most every ice dam.
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For more info on sealling kneewalls, see: http://homeenergy.org/archive/hem.dis.anl.gov/eehem/95/950309.html
Agree with both.
Insulate the entire length of the rafter bay and bring the kneewall inside the building envelope. Better thermal performance year-round, and "conditioned" storage space (in typical ciinstruction) behind the kneewalls as a bonus.
If you have any HVAC in the attic, there's another efficiency gained.