bassboat1 t1_jef73y9 wrote on March 31, 2023 at 3:56 PM

If there's no need to pay to heat the attic, insulate the ceiling and save the $$.

haxxer_4chan OP t1_jef7mnb wrote on March 31, 2023 at 4:00 PM

Thank you, out of curiosity, how does that save money if we don't heat the attic (which we do not)? I would have assumed that keeping insulation closer to the floors we do heat would have been more efficient, but I am asking here because I admittedly know very little about it.

bassboat1 t1_jef9lhv wrote on March 31, 2023 at 4:12 PM

You'll be heating less surface area and volume.

In my area (zone 5), ceiling R-value requirement is R-49. The job would require one run of tunnel vents at the eaves, some fiberglass batts to block them up and 18"-20" of blown insulation on the flat ceiling. To get the same R-value on the rafters, you would need either a hybrid of spray foam/batts/rigid board insulation, or build down the rafters for batts. You'd also have to apply R-19 to the gables, and consider a venting solution.

haxxer_4chan OP t1_jefab37 wrote on March 31, 2023 at 4:17 PM

Thank you, this makes a lot of sense. Much appreciated!

shootdang167 t1_jef900u wrote on March 31, 2023 at 4:09 PM

Foam is expensive compared to batts or cellulose

haxxer_4chan OP t1_jef995g wrote on March 31, 2023 at 4:10 PM

Ah right, I totally misread ceiling as meaning rafters and thought they were suggesting the opposite. Makes more sense now.

DarkLink1065 t1_jefgbix wrote on March 31, 2023 at 4:56 PM

There are actually some very good reasons to include the attic in the heated space (e.g. if your hvac conduits are inside your conditioned space rather than running through a blistering hot or freezing cold attic, you can prevent fairly significant losses and actually save money even though you're heating a larger space), but you have to make sure your attic, roof, and HVAC systems are set up for it. You shouldn't just slap insulation on the bottom of the roof deck and assume it will work, it may not and it also has the possibility of creating things like moisture damage problems in your roof depending on your climate.

BullOak t1_jefgtu9 wrote on March 31, 2023 at 4:59 PM

This is one of those counterintuitive building science things. Done properly, a house with a conditioned attic will require less HVAC energy than a house with a vented attic. Generally speaking, there's enough gains in latent loads, stack effect control, and keeping the ducts in conditioned space to offset the additional conditioned area.

but vented or conditioned is much less important than getting the details for either correct.