I'd like to do a bad attic insulation job - will this work?
I'm only sort of kidding. House is very large, 100 year old brick center hall colonial in historic district in central Virginia. No exterior modifications will be allowed. Venting is by gable vents, around 4 sqft at west end, 8 sqft at east end. No soffit or other vents are present or possible due to historic district regulations (not to mention practicality). Large 3rd floor is built into the slate roof with dormers. Ceiling of 3rd floor is not even close to sealed. Extensive hvac in attic is not sealed or insulated even vaguely correctly.
Currently there is some mineral wool between the rafters forming the 3rd floor wall/ceiling. I propose to fill all rafters with wet blown cellulose, seal the gable vents, and insulate exposed brick gable on interior with flash and rigid foam. I will then add both a return and a feed from the hvac to the attic, making it conditioned space, and dealing with moisture.
I am not willing to have high density foam sprayed, as the failure risk is way too high, hence the need for mechanical moisture control - there's no practical way to spray reasonable lifts.. I think that's effectively what's happening now, due to the woefully insufficient venting and poorly sealed hvac and 3rd floor ceiling. In fact, I'd bet the interesting Venturi design of the gable venting (previously fan assisted) effectively depressurizes the attic nicely.
Obviously this is not ideal. But I'm not going to be able to make this perfect, or even close, considering my constraints, which are ironclad. Moisture permeable assembly and mechanical ventilation/ humidity control seem to be the only solution - no million dollar renovations removing 24 squares of 12/12 slate are practicable...
My thought is that interior moisture in the winter will be dealt with by the heat pump/low ambient humidity, and the high exterior humidity in the summer will be somewhat mitigated by the high heat load of the insulated slate roof. I of course expect no slate failure from roof overheating... Any moisture making its way into the attic through the insulation will be handled by the hvac, which is sized for dehumidification
Posted May 28, 2012 10:31 AM ET
Edited May 28, 2012 10:41 AM ET
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