Will this insulation assembly work?
Hello,
I have a situation and I was wondering if I could get some input on an assembly I may be doing. Please refer to the PDF drawing I have attached to this. it will explain a lot more than words can. I am in Washington State in Marine Climate 4. I am working on a remodel of my house.
I realize there are many better ways that this assembly could be done, but that isn’t what my questions are about.
Top two drawings of attic cavity:
1. Will the settled cellulose and the air gap associated with it create problems with mold and rot down the road? The foam insulation above it is enough to keep condensation from occurring at the roof deck, but it could still occur on the rafters. There would be limited drying potential to either side.
2. I will be doing the blow in and the only available blower rental in the area is from Home Depot. I have read many accounts on this website about how their blowers are not powerful enough to dense pack a wall. I have noticed that Home Depot does show on the machine how to adjust it to get it to dense pack. The posts I have read normally say that these machines are under powered/under maintenanced to perform this task. However, as you can see this is a different situation than a wall. If I netted off each 16″ O.C. ceiling bay I would have around 5.7 cubic feet of insulation per bay. Could I get this to dense pack with a Home Depot blower?
Bottom Drawing (Rafter Detail):
Will my Foam/Fiberglass sandwich work? My concern is that while there is a thermal break between the conditioned space and the rafter there is no thermal break between the exterior and the rafter so the rafter would protrude into the air filled fiberglass batt. Would this cause a problem? There would be dead air stored in the fiberglass batt cavity which shouldn’t pick up moisture unless there was a problem with the air sealing on either side of it (which is likely since there are no perfect air barriers). There would also be little drying potential in either direction since there is foam on either side. Hmm the more I am typing here the less need I am seeing for posting this as it all looks like bad joojoo mgumbo. However, this is pretty much flash and batt with the only difference being that I am adding another layer of foam on the inside of the house and lowering the drying potential. Although, if in flash and batt you used a vapor retarding paint you would end up with the same problem more or less.
Thanks,
Spencer
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Replies
Spencer,
I agree with you: "There are many better ways that this assembly could be done."
Q. "Will the settled cellulose and the air gap associated with it create problems with mold and rot down the road?"
A. No.
Q. "Could I get this to dense pack with a Home Depot blower?"
A. It's hard to say. The answer depends on the type of blower rented by Home Depot, its condition, and your skill.
Q. "Will my Foam/Fiberglass sandwich work?"
A. I don't like it. My usual advice: if you like foam -- either spray foam or rigid foam -- choose where you want to put it, but put it in just one place. That way the assembly can dry in at least one direction. The problem with this type of "sandwich" is that you are creating an assembly that can't dry out readily. If the sandwich starts out a little damp, it stays damp.
Q. "My concern is that ... there is no thermal break between the exterior and the rafter."
A. You're right -- it would be much better to put a continuous layer of rigid foam on top of the roof sheathing.
Q. "Would this cause a problem?"
A. It's hard to say. But if I had to build up an assembly like the one your are suggesting, I would probably stick with several layers of rigid foam, and I would skip the fiberglass. That is fussy, time-consuming work, of course -- but after all, you have already stipulated that "There are many better ways that this assembly could be done."
Q. "It all looks like bad joojoo mgumbo."
A. Back to the drawing board...