Spray foam strategy for 1930 house (vapor barrier, ignition barrier, thermal bridging)
Greetings. I have been studying this forum over the past month in the hopes of understanding how to fix the building my wife and I moved into in July.
The house, built in 1930 as a bungalow, was renovated in 2004 with a new second floor and large attic, and then renovated once again in 2009 after it was purchased out of foreclosure. The renovations included replacement of all the windows, but not much else from an insulation perspective. We are located in northern Mass (climate zone 5) and the total square footage is 900 per floor.
The walls and windows in the living space appear well sealed. The problem with the building is that it has a fieldstone wall in the basement, 18″ thick, uninsulated on the slab, the walls, and the ceiling. Meanwhile, the heat produced by our gas furnace is quickly moving through the ceilings to a vented attic, and then is lost forever.
Basement walls, as of december 10, are already at 32 degrees, while it is still 68 degrees in our vented attic!
Because the air handler is in the attic, we want to start by moving the attic into the conditioned space (hot roof approach). Sealing the attic will reduce the “stack effect” that draws the cold throughout the entire building. This could be done with cellulose (we have generous 10″ rafters in the attic), but we are hoping to use foam, for superior air ceiling. Both approaches qualify for federal and utility company incentives.
One installer recommended 5″ of closed cell on the roof deck and 3″ on the gable ends. The other installer planned to completely fill the ceiling and wall cavities with open cell, giving us 9″ on the ceiling and 3 1/2″ on the attic end walls. So the alternatives from an R value perspective are:
Closed Cell: R33 on ceiling, R19 on gables (Bayer Baysystems)
Open Cell: R36 on ceiling, R14 on gables (Demilac Agribalance)
Both solutions require an ignition barrier to meet code — intumescent paint — if we don’t plan on finishing the space with drywall this year.
1. THERMAL BRIDGING. I am concerned that by partially filling the rafters, thermal bridging would be a lot worse on the roof with the closed cell option. The exposed surface of each roof rafter would be a full 9 1/2″ as opposed to 1 1/2″ so they would act as “heat sink fans” with closed cell.
2. FLAMMABILITY. With open cell the foam will be inches from the gas furnace, with close cell about 12 inches. The furnace is a 2004 model. I read here that the furnace should have “sealed combustion” for safety… how can I tell? I am thinking that a sheet of drywall right above the furnace would be an important investment in any case.
3. VAPOR BARRIER. I have read here that open cell might need a vapor barrier. Considering that all fans vent to the outside in the building, and that we will have no A/C, do we really have to worry about condensation with either solution?
My proposed solution is:
A. go with open cell
B. add staggered studs to the gable ends, coming out 2 inches to make a 2×6 wall. This would reduce the thermal bridging and permit an R value of 22.5, which seems better matched with the R36 on the ceiling. The lumber to do this would be only about $75… my carpenter could do this in a few hours.
C. spray on the intumescent paint myself to save on cost. There is is this one available locally:
Is this as good as the DC315 paint that is marketed for use with foam? link:
Both paints cost around $750 for 10 gallons, and I am assuming we would vacate the house for a few days to let the foam breathe before applying the ignition barrier. Spraying the paint ourselves, we save at least $1000 — but how can I ventilate the space for safe application once the foam is in place?
D. My hunch is that the paint could also reduce the ability of moisture to penetrate the open cell foam. Is this correct?
Well, that is enough for starters. I will describe the basement situation in more detail later on. And what ever happens, I can provide data to this forum on our natural gas usage both before and after the application of the foam.
Thanks in advance for your help!!
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