I have a walk-up attic here in Southern NY. I also have 2 air handler units in the attic that provide both A/C and Heating. I was thinking of enclosing the entire attic with JM’s – CI Max 4″ Polyiso rigid board. It is foil faced on both sides. I was going to foam and tape all seams. The roof joists are 2×10 and would be the chase for my soffit vents venting up to the ridge vent in my roofing. Since the attic will be an essentially sealed space, should I remove the attic floor insulation or can I leave it? The board has an R-24 / 26 rating. Do you see anything wrong with this plan? Great site and thank you. Very informative!
Yes, that’s true, but remember that moisture rises with heat and the moisture you create in the living space — showers etc. — has to go somewhere?
Very interesting concept. You didn’t say how much insulation is in the floor. Are you doing this work with or without a building permit? Are you concerned with the building code requirements for attic insulation in your area? Codes aside, I think you’ll find the results of your plan excellent. I think I would leave as much insulation on the floor that will fit beneath the floor boards. That combined with the R-values in your Cl Max, should get you to code requirements.
What about attic air space ventilation? Seems like all your ventilation is above the insulation boards. How will you handle the moisture that migrates from the living space to the attic? Could result in some condensation issues.
Ventilation is not my specialty. Perhaps a roof fan that could be activated by humidity.
Keep in mind that my expertise is with reflective insulation and radiant barrier. I just mentioned the issue because it seems logical to me that you’re plan might cause condensation. That’s just a guess, not a certainty.