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I'm remodeling the upper floor of a ~100 year-old house near Portland, OR. It's down to beams: all the drywall (and plaster) has been removed, along with all the old insulation. I'm about to replace the roof which will require a complete tear-off and new decking (or is it called sheathing?)

The roof is framed with old-style 2x4s (actually 2" x 4"). The roof decking will be installed on the top, and sheet rock to the bottom, so there isn't much room to add insulation and ventilation.

There is, actually, a small attic space about three feet under the peak of the roof. But most of the ceiling is shared with the roof. The pitch is quite steep.

One contractor told me I could fill the entire 4" depth with sprayed-on liquid foam insulation to maximize the R-value. He said it was allowed by code because this foam couldn't absorb water.

A second contractor said I should use the traditional soffit/ridge venting even though it would reduce my insulation by half.

I'd prefer the better insulation; it gets hot up there in the summer.

Are both of these viable? Are they both allowed? I need to tell my roofer if he should install soffit vents...

  • There are two categories of spray on foam insulation: closed cell and open cell. Which does your contractor propose to use? – Jim Stewart Aug 18 at 12:26
  • Since you asked, "decking" sometimes refers to old-style board roofs, but can also refer to modern plywood or OSB sheathing. It's a fairly general term. – isherwood Aug 18 at 13:57
  • The thing to remember is that "better" insulation can be undermined by being in contact with an extremely hot roof surface, and roofs don't tend to last as long without ventilation of the deck. Have you considered increasing the depth of the rafters on the slope? This obviously reduces interior space, but may get you closer to your energy goals. – isherwood Aug 18 at 13:59
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    As an Oregon resident also I would want the roof to breath we don’t have the extremes that some parts of the country do but our daily cycle swings from fog in the morning to 100 deg on the roof surface and this is hard on the roof as Isherwood mentioned . I would add the vents the roof will help it last longer in some cases 2x depending on material used. If we were in a hotter or cooler climate my answer would be more efficiency targeted but our climate close to the coast and Columbia and Willamette rivers is quite mild and the cycle from moist to hot is hard if the roof if it cannot breathe. – Ed Beal Aug 18 at 15:46
  • A new type of roof sheathing now commonly installed has reflective foil on one side to reduce IR radiation into the house from the hot outer layer of the roof. This foil is presumably impervious to water vapor. The space between ceiling of the living space and roof must not trap moisture. It must have pathways to dry out. Read the recent post on this site about the water in the wall of the house in Galveston. – Jim Stewart Aug 18 at 19:59

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