Unusual thermal envelope intrusion at roof-penthouse
I’ve discovered that the ~1987 penthouse addition to my house is partially outside the roof thermal envelope . Since it is attached (naturally) to the remainder of the house it compromises the performance of the whole house.
In a nutshell:
The penthouse was floor joisted directly on the old tar and pitch roof. Since it is a penthouse, a portion of the flat roof remains exterior space. No air or thermal barrier was installed at the perimeter of the penthouse in the roof trusses below or at the roof plane These trusses are only filled 1/2 way with insulation (9″, R-30, in 18-20″ trusses). Additionally, the uninsulated portion of the roof truss bays are allowed to communicate with air in the rest of the house due to a lack of air barrier.
The penthouse is unusably hot and cold. Conditioning it would be a ferocious expense in its current state. And it compromises the performance of an otherwise reasonably insulated building which we do care about.
I’m struggling to figure out an idealized solution to address the thermal and/or air barrier issues.
One option would be to cut access holes in the old roof deck (or from conditioned space below) and do my best to fill the roof trusses with cellulose (on top of the existing 9″ fb batts). I’m not sure what WUFI would think of a pile of cellulose in an unvented flat roof assembly. Gaps allowing communication of super heated ‘attic’ air to the rest of the house could be addressed.
Another option would be to gain access to the penthouse perimeter in the roof trusses from below, installing some combination of roll air barrier and sheet foam (a good air barrier), thus hopefully bringing the floor system and old roof deck into the air barrier.
A third option would be to insulate the floor system (~R-23) and perform some air sealing to prevent air communication. This is by far the easiest and cheapest option but it seems questionable to allow a 13’x10′ slice of unvented superheated roof space to be sandwiched inside what should otherwise be space and structure inside the envelope.
Has anyone dealt with this ‘penthouse’ problem? I have to guess this enveloper boundary problem exists for any penthouse addition.
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