Attic insulation – modified cut & cobble – cathedral ceiling – climate zone 5a
Hello, we are in the middle of an attic remodel product, and I have decided to attempt the cut and cobble method of insulating our existing rafter bays with polyiso insulation boards. I would like some addvice or confirmation/critiques on my insulation strategy.
We have existing 2×6 rafters in our 1.5 story, 1941 brick home. We are located in climate zone 5a. I realize I still can’t get to the code-required insulation value with a 5.5″ depth. It’s a small space (see photo) so I really think we can only fur down and additional 1/2″. I have no interest in closed cell spray foam for a myriad of reasons. We also have no soffit/overhang on our roof. I will have a ridge vent coupled with a deck vent (smart vent) system to create ventilation channels in the rafter bays.
I can get my hands on recycled, paper-faced polyiso in 4″ and 1/2″ thicknesses for a great price. My thought was to stack 1/2″ foil faced with a 1″ air gap (facing the the roof sheathing) followed by 4″ friction-fit paper-faced, followed by 1/2″ paper-faced over the rafters to create an air barrier with taped seams. My thought process here is that I could skip a majority of the excessive “great stuff” spray foam-ing. I have atttached an insulation detail to show what I’m describing as well as a photo of the space. Am I missing something here? Any concerns with this assembly?
I should also add that the floor joists (ceiling joists of our main level) will be filled with R-30 rockwool insulation.
Any help or guidance is much appreciated. Thank you.
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Replies
connorjw,
If you are venting the roof, and it has a warm-side air barrier, I don't see why it matters what type of insulation you use, or whether it is air sealed with spray foam.
The only problem I can see with what you are proposing is the batt insulation at the floor. Attics insulated at the roof-line need to be fully conditioned to avoid moisture problems. If you do that (keep the attic at a similar temperature and humidity as the rest of the house) the batt insulation brings no benefit. It's like insulating between a first and second floors.
We will have a ducted heat pump installed in the attic. The goal with the rockwool insulation at the ceiling joists would be to separate the zones somewhat thermally so that we can keep the attic cooler than the rest of the house at night for sleeping.
connorjw,
As long as you control the relative humidity in that cooler space it will be fine.