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attic insulation in an old house

greenehead | Posted in General Questions on

Hello all,
I’ve read through several GBA articles and am hoping someone can direct me to the right answer/article.  I think I need to create a sealed attic and I am unsure how to go about it.
In zone 3, I have a large but simple hipped roof with true 2×4 rafters, irregularly spaced with 1/2″ osb decking with asphalt shingles.  The attic is 12′ tall at the ridge (supported by purlins/braces) and is partially vented.  There are 2 gabled dormers (one on each side) with gable vents and there is 1 solar powered vent on the roof near the ridge/center.  (I understand the solar power vent is useless and I plan to get rid of it.)  There are no soffit vents and there is no ridge vent.  I can easily seal the 2 gable vents. 
There are many copper water lines, a tankless water heater, and a gas-fired furnace with ducts in the attic.  The attic floor is insulated poorly with batts. (I can remove these easily).
The roof shingles are newer so I’m not planning to add insulation on top of the roof decking.
I have no plans to finish the space in the attic; it is used for storage only although the access is great with a full set of stairs.  I can replace the gas-fired furnace (it’s about time to do this anyway) with a sealed unit.  I considered switching the system to ductless so I could just vent the attic and insulate at the attic floor but I’ve got the copper water lines and water heater up there so I think I need everything to be inside the conditioned space.
Does all of the above make sense? 
If so, my question is how do I seal and insulate at the roofline with the 2×4  rafters?
Install foam board under the rafters?  How many layers?  (This seams super expensive?) Fur down the rafters to make them bigger and spray foam between and under the rafters?   Open cell or closed cell or a combination?
I hesitate on installing spray foam under/between the rafters since it is a historic house with original framing.   A preservation architect recommended installing a felt layer (or house wrap) under the rafters/osb before spraying the foam but I wasn’t sure about that.  It seems foam board under the rafters would be better from a historic preservation point of view (could be undone relatively easily later).
The house is uncomfortable and inefficient for lots of reasons (old windows and doors in addition to poorly insulated, etc) and I’ve been slowly trying to tackle various issues while trying to keep true to the original architecture/materials. 
Thank you in advance for any insight you can provide,
Jennifer

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Replies

  1. user-2310254 | | #1

    Hi Jennifer,

    You might want to read Allison Bailes’ article on combustion appliances in conditioned attics: https://www.greenbuildingadvisor.com/article/can-atmospheric-combustion-work-in-a-spray-foam-insulated-attic.

    FWIW. I had a similar situation in my townhouse. I decided to eliminate all the gas appliances before converting my attic to conditioned space. It just seemed safer all around. If it’s time to replace the furnace, I would consider a ducted mini-split. I’d be tempted to replace the tankless WH with a heat pump water heater or Raheem Marathon. (If you could relocate the WH, all the better.)

    If foaming the sheathing, it’s usually safer to install a flash of closed cell foam followed by open cell. But it can be difficult to find an installer to agree to this best practice (at least in my area). I have R-30 of open cell on my sheathing, but I have a supply duct in the attic and monitor humidity conditions during the cooling season.

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