2x4 roof truss insulation

[Geran]

Hi all,

I have a home built in 1979/1980 with 2x4 roof trusses. I am wondering how would you go about insulating the underside of the roof.

Looking at BSI-100: Hybrid Assemblies from BuildingScience, I am assuming Figure 6 would be the best option with R-15 closed-cell, and the rest being open-cell or wet sprayed fiberglass/cellulose.

I would prefer not to insulate the floor of the attic if I can get away from doing that.

 

[Dana]

How soon would you be re-roofing? Using rigid foam above the structural roof deck plus R13 - R15 fiber insulation between the 2x4 top chords is safer & greener than doing it with a combination of sprayed R15 closed cell + R34 open cell (9", installed in two passes) below the roof deck.

To get to code minimum compliance on a U-factor basis (=U0.026 max, = R38.5 "whole assembly") takes only about R35 if all the insulation is continuous, not thermally bridged by framing. With R15 fiber between trusses 24" o.c. it takes only about 4"- 4.5" of continuous roofing polyiso (R23 - R26) above the roof deck. A stackup of 4.5" of foam with a 5/8" nailer deck through screwed to the structural roof deck with pancake head timber screws can be finished off at the roof edges cleanly with a 1 x 6 facia board. This wpuld be essentially Figure 4:

The material cost of R25 roofing polyiso is on the order of $2.50-$3.00 per square foot if going with virgin stock foam, but is less than a buck a square foot (cheaper than batt insulation) if using reclaimed foam. The nationwide dealer Insulation Depot has a facility in the Baltimore area selling used and factory-seconds rigid foam. They often advertise here, as do a few others (this is one of their adverts), and almost always has used polyiso in stock in varioius thicknesses. Similarly, Repurposed Materials Inc has a facility south of Philly that regularly gets in used fiber faced roofing polyiso for cheap (but none listed at the PA facility currently at this hour.) Doing two (or even three) layers of thinner foam is generally better than one thick layer, since the foam expands & contracts with temperature & moisture cycles, and staggering the seams puts thermal breaks over the potential gaps.

A nailer deck of 5/8" OSB runs about 10 cents per square foot, 5-5.5" timber screws at 16 per 4x8 sheet adds another 50-60 cents per square foot, R15 fiberglass less than a buck a square foot, so using reclaimed foam it should come in around $2.25-$2.50 per square foot before labor. On simple roof lines without a lot of hips & valleys the labor isn't much. If there are bunch of gabled dormers or something it would require adjustments to the dormer siding and kick flashing and compound angle cuts to the foam, which starts to add up.

By comparison, R15 closed cell spray polyurethane would run about $2.50-$3.00 per square foot in my area, and 9"/R34 of open cell would run another $3.00 per square foot or more, something like $5.50-6.00, installed. For really complicated roof lines this is often the cheaper way to go, despite the higher environmental expense of the virgin-stock polymer and the HFC blowing agents for the closed call foam. There are more environmentally benign HFO blown closed cell foams out there, but 15-20% more expensive per R than the HFC245fa blown stuff in my area.

 

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[Geran]

How soon would you be re-roofing? Using rigid foam above the structural roof deck plus R13 - R15 fiber insulation between the 2x4 top chords is safer & greener than doing it with a combination of sprayed R15 closed cell + R34 open cell (9", installed in two passes) below the roof deck.

My current roof isn't even 10yrs old so it will probably be another few years. How is it safer exactly?

To get to code minimum compliance on a U-factor basis (=U0.026 max, = R38.5 "whole assembly") takes only about R35 if all the insulation is continuous, not thermally bridged by framing. With R15 fiber between trusses 24" o.c. it takes only about 4"- 4.5" of continuous roofing polyiso (R23 - R26) above the roof deck. A stackup of 4.5" of foam with a 5/8" nailer deck through screwed to the structural roof deck with pancake head timber screws can be finished off at the roof edges cleanly with a 1 x 6 facia board. This would be essentially Figure 4.

The material cost of R25 roofing polyiso is on the order of $2.50-$3.00 per square foot if going with virgin stock foam, but is less than a buck a square foot (cheaper than batt insulation) if using reclaimed foam. The nationwide dealer Insulation Depot has a facility in the Baltimore area selling used and factory-seconds rigid foam. They often advertise here, as do a few others (this is one of their adverts), and almost always has used polyiso in stock in various thicknesses. Similarly, Repurposed Materials Inc has a facility south of Philly that regularly gets in used fiber faced roofing polyiso for cheap (but none listed at the PA facility currently at this hour.) Doing two (or even three) layers of thinner foam is generally better than one thick layer, since the foam expands & contracts with temperature & moisture cycles, and staggering the seams puts thermal breaks over the potential gaps.

A nailer deck of 5/8" OSB runs about 10 cents per square foot, 5-5.5" timber screws at 16 per 4x8 sheet adds another 50-60 cents per square foot, R15 fiberglass less than a buck a square foot, so using reclaimed foam it should come in around $2.25-$2.50 per square foot before labor. On simple roof lines without a lot of hips & valleys the labor isn't much. If there are bunch of gabled dormers or something it would require adjustments to the dormer siding and kick flashing and compound angle cuts to the foam, which starts to add up.

By comparison, R15 closed cell spray polyurethane would run about $2.50-$3.00 per square foot in my area, and 9"/R34 of open cell would run another $3.00 per square foot or more, something like $5.50-6.00, installed. For really complicated roof lines this is often the cheaper way to go, despite the higher environmental expense of the virgin-stock polymer and the HFC blowing agents for the closed call foam. There are more environmentally benign HFO blown closed cell foams out there, but 15-20% more expensive per R than the HFC245fa blown stuff in my area.

All of this makes sense. I trying to figure out the best way to not have to put insulation on the floor of my attic if I don't have to since the quotes I got for doing it this way range from $6500 all the way up to $11000. We have a rebate in my area from my electric company for up to $7500 to do improvements to save energy so I figured if I could get spray foam it would be a better investment in the long run.

 

[Reach4]

I presume that you noted to remove any vents, including soffit vents. I think you also want to remove any effective vapor barrier under old insulation.

 

[Geran]

I presume that you noted to remove any vents, including soffit vents. I think you also want to remove any effective vapor barrier under old insulation.

Yes all vents would be removed. Another reason why I'm trying to figure out which way to go because I need to replace/repair the soffit vents and if I go with spray foam I don't want to install vented soffits and then have to replace them later.

 

[Dana]

My current roof isn't even 10yrs old so it will probably be another few years. How is it safer exactly?

Putting insulation on the exterior keeps the structural roof deck warmer and drier and allows the interior side to be much more vapor open, and thus more moisture-safe. R15 closed cell foam on the interior side is a Class-II vapor retarder (it would qualify as a "vapour barrier" under Canada's building codes). That protects a roof deck from accumulating adsorbed moisture of interior air in winter but it severely reduces the drying rates toward the interior compared to vapor permeable fiber insulation.

Roof decks dry primarily toward the interior- a #30 felt + asphalt shingle layup is only about 1/5 as vapor permeable as R15 closed cell foam, and when it wetted by rain/snow/dew there is exactly zero drying toward the exterior going on.

Added up, keeping the roof deck above the dew point of the wintertime indoor air and allowing it to be very vapor open toward that indoor air reduces the mold & rot risk by quite a bit.

If going with R34 damp sprayed fiber for the rest rather than open cell foam, note that even at the same steady-state R-value the cellulose will outperform fiberglass due to it's much higher thermal mass/lower thermal diffusivity, as briefly explained in this bit of marketing fluff from a German fiberboard insulation company. Cellulose might have twice the thermal diffusivity of fiberboard, but it's less than half the diffusivity of open cell foam or fiberglass. Cellulose is also better able to manage wintertime moisture accumulation issues during an extended cold snap than fiberglass, since it can buffer substantial amounts of moisture as adsorb without damage or loss of function. That performance benefit occurs primarily during the shoulder seasons and summer, not so much in the dead of winter, but it's measurable even in climates cooler than yours.

 

[Geran]

Putting insulation on the exterior keeps the structural roof deck warmer and drier and allows the interior side to be much more vapor open, and thus more moisture-safe. R15 closed cell foam on the interior side is a Class-II vapor retarder (it would qualify as a "vapour barrier" under Canada's building codes). That protects a roof deck from accumulating adsorbed moisture of interior air in winter but it severely reduces the drying rates toward the interior compared to vapor permeable fiber insulation.

Roof decks dry primarily toward the interior- a #30 felt + asphalt shingle layup is only about 1/5 as vapor permeable as R15 closed-cell foam, and when it wetted by rain/snow/dew there is exactly zero drying toward the exterior going on.

Added up, keeping the roof deck above the dew point of the wintertime indoor air and allowing it to very vapor open toward that indoor air reduces the mold & rot risk by quite a bit.

If going with R34 damp sprayed fiber for the rest rather than open-cell foam, note that even at the same steady-state R-value the cellulose will outperform fiberglass due to it's much higher thermal mass/lower thermal diffusivity, as briefly explained in this bit of marketing fluff from a German fiberboard insulation company. Cellulose might have twice the thermal diffusivity of fiberboard, but it's less than half the diffusivity of open-cell foam or fiberglass. Cellulose is also better able to manage wintertime moisture accumulation issues during an extended cold snap than fiberglass since it can buffer substantial amounts of moisture as adsorb without damage or loss of function. That performance benefit occurs primarily during the shoulder seasons and summer, not so much in the dead of winter, but it's measurable even in climates cooler than yours.

If it wasn't so long that I would need a new roof, I would definitely be interested in doing the insulation on top. It doesn't help either that I have solar panels installed too so getting a new roof is going to be interesting.

 

[Dana]

I'm feeling your pain, given that I have plenty of under-insulated cathedralized ceiling in much of my house, and a decade or more to go on most of the roof, and in my case, insufficient rafter depth to do it all from below.

Sounds like 2-3" of HFO-blown closed cell with the remainder as damp sprayed cellulose blown behind a stout smart vapor retarder such as Intello Plus would be your best option thermal performance/moisture-safety/verditude-wise, but 2-3" of closed cell with the remainder being half pound open cell is probably cheaper (and still pretty expensive.)