Insulation in the attic when the furnace is up there
Just read the link about insulation options when your air handler/furnace and heating/cooling ducts are in the attic, and was surprised to read that you should not insulate the floor of your attic if (a) it is conditioned space (which mine will be), and (b) your furnace is up there, and (c) you insulate the roof rafters above.
Apparently the idea is that the attic floor insulation causes the main space below (e.g., in my bungalow cottage) to be warmer than the attic itself, even though you have insulated the attic rafters above. This makes sense of course, because the main living area below has both the ceiling/floor joist cavities and the attic rafters as insulation, whereas the attic only has the rafters as insulation. (in my case the rafters would have about R32 or so and the 2 x 10 floor could be filled up with another 40 or so if I wanted.)
This differential insulation for the main cottage below versus the attic would supposedly cause condensation as the warm air drifts upstairs to the attic, and perhaps even the reverse in the summer as hot air migrates down from the attic.
I'm a little confused about this, so I would appreciate any comments anyone would have, thanks.
Also, if I did insulate the floor of the attic as well as the attic rafters, would I have to completely enclose the floor joists with vapour barrier, or could I just do the bottom of the joist cavity (which is also of course the ceiling of the main floor below, which is T & G Cedar with stain and shelac or something similar.) The cottage is at least 75 years old, and the 2 X 10 ceiling/floor joists are 1.75" X 9.75" - beefier than today's 2 x 10 lumber.
Last edited by Jimmybrp; 12-11-2011 at 01:51 PM.