How to insulate a bedroom exterior wall that is also the attic to a porch?
In thinking about a retrofit insulation to my 72 year old, 2 storey, load bearing brick home in Baltimore, Maryland, I am not sure what to do about the porch roof, which is a straight line continuation of the house roof. This results in a triangular ‘attic’ to the porch, with a porch ceiling, roof and a ‘kneewall’ that separates it from a second floor bedroom. The ceiling to this bedroom is sloped, leaving a gap between it and the rafters that form the roof. In the attic, one can see through this gap into the void of the porch attic.
I would like to insulate this wall of the bedroom. To do so from the inside means losing some square footage, so I wondering if I could fill this 100 sq ft (~450 cubic ft) porch attic with cellulose. But I don’t know whether to consider this a “wall” or a “roof deck.”
As I understand the code, if what I am doing is insulating a bedroom wall, that happens to have with a porch-attic sized cavity filled with cellulose insulation and a sloped cladding that looks awfully like a roof, then I don’t need to worry about condensation. As I read Martin Holladay describe it, “If you are building a house in one of the warmer climate zones — zone 1, 2, 3, or 4 (except for 4 Marine) — you don’t have to worry about the thickness of your foam. Any foam thickness will work, because your sheathing will never get cold enough for “condensation” (moisture accumulation) to be a problem.” (https://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-minimum-thickness-rigid-foam-sheathing)
However, if I consider the porch roof to be an attic roof, since it is continuous with the attic, then I am required to maintain a certain ratio of cellulose to foam insulation to prevent condensation, or R-15 for an R-38 total (http://www.buildingscience.com/documents/guides-and-manuals/irc-faqs/irc-faq-conditioned-attics?searchterm=cellulose+attic). This would make me need to put a whole lot of foam above the porch roofdeck to balance the cellulose that is up to 8 ft deep!
I can’t imagine I am the first to deal with this problem, so I would appreciate your suggestions or known solutions.
As an added detail, the fireplace is on that wall, extending into the porch, and the chimney goes up through this porch attic. Cellulose would thus be in contact with the chimney. Also, there would be heat loss from the bedroom into the chimney.
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