I'm about to replace a roof on 60 year old cape cod style house, long island, NY area. There are two"dog house" dormers and knee walls in front and full dormer with shallow slope in the back, very small attic with two gable vents, no soffit vents. Attached to the side an extra room (use to be breezeway) with its own separate very very small attic with no ventilation at all. Both attics are inaccessible.

 Rafter size 2x6 on main house, on addition either 2x8 or 2x6. I managed to get in rafter bay area through sheetrock and insulation is fiberglass of probably
R13

I was trying to educate myself on matter of roofing and then ventilation and insulation as they come along, had a few roofers and a couple of insulation contractors
in my house. What I learned and what they propose don't seem to match well. Roofers say they would put up a ridge vent and combining with existing gable vents it would be good without soffit vents. For a smaller attic - either leave it alone or install a ridge vent
Insulation guys suggest to pack rafter bays, ceiling, gable walls, dog houses and flats behind knee walls with cellulose and go ventilation route proposed by roofers. Everybody but one contractor strayed away from considering rigid foam on a roof top and he did not call back

I would like to know what would be a practical and economical way to deal with the issue

Thanks

Greetings,

Although ridge vents are the most efficient method, using them without soffit vents can lead to moisture being drawn in thru the ridge vent, plus seriously affecting the draw of moisture out of the attic. Ck with cor-a-vent.com. They are the experts when it comes to venting. Generally speaking, it has been my experience that roofers screw up venting much more than they ever help it.

QUOTE> Insulation guys suggest to pack rafter bays, ceiling, gable walls, dog houses and flats behind knee walls with cellulose and go ventilation route proposed by roofers.

Excellent way to rot out the roof sheathing. If you are going to use this space the best way is to install a Type 3 Radiant barrier insulation. It can be installed in the middle of the rafter leaving plenty of air space above for RV air flow. Install a 3rd RB sheet to the bottom of the rafter, like a VB, and install furring strips across the rafter face and then dry wall.

If this is an up grade and you have at least 6" bulk insulation then laying a PERFORATED RB sheet on top will give you the best results. If there is no insulation and you can get to the joists easily then you can install Type 3 from above, between the joists. CK with fifoil.com

Posted By mik on 12 Oct 2011 02:21 PM
I'm about to replace a roof on 60 year old cape cod style house, long island, NY area. There are two"dog house" dormers and knee walls in front and full dormer with shallow slope in the back, very small attic with two gable vents, no soffit vents. Attached to the side an extra room (use to be breezeway) with its own separate very very small attic with no ventilation at all. Both attics are inaccessible.

 Rafter size 2x6 on main house, on addition either 2x8 or 2x6. I managed to get in rafter bay area through sheetrock and insulation is fiberglass of probably
R13

I was trying to educate myself on matter of roofing and then ventilation and insulation as they come along, had a few roofers and a couple of insulation contractors
in my house. What I learned and what they propose don't seem to match well. Roofers say they would put up a ridge vent and combining with existing gable vents it would be good without soffit vents. For a smaller attic - either leave it alone or install a ridge vent
Insulation guys suggest to pack rafter bays, ceiling, gable walls, dog houses and flats behind knee walls with cellulose and go ventilation route proposed by roofers. Everybody but one contractor strayed away from considering rigid foam on a roof top and he did not call back

I would like to know what would be a practical and economical way to deal with the issue

Thanks

Rigid foam above the roof deck is still the better approach, but you have to go pretty thick to hit code-min that way.  A combination of 3" of exterior iso and 2" of closed cell foam sprayed from the interior would get you there, as would 3" iso above the roof deck with a 1" flash foam (closed cell) followed by dense-packed or wet sprayed cellulose.  Air sealing kneewalls and cut up dormers are nearly impossible with any of the other combinations.

Dense packed cellulose alone against the roof deck would be code violation and puts the roof deck at risk, but with 3" of exterior iso would be code-legal & low risk, but not nearly as air-tight as a flash'n'fiber. (With exterior foam you can still dense-pack the areas that aren't accessible for an interior spray foam seal. You can usually also dense-pack over the existing batts if they're too difficult to remove.)  Flash'n'fiber without the exterior foam would also be moisture-safe in a L.I. climate, but wouldn't meet code-min for R value.   See:

_{http://www.buildingscience...file}

Reflective insulation (even multi-layer versions) A: wouldn't meet code requirement for wintertime heat loss and B: wouldn't be air-tight.  It would be well ventilated and wouldn't rot, but would leave a large whole-house stack effect to drive wintertime infiltration rates. 

Making them tight as possible is as (or even more) important than R value in these old capes & bungalows from a total energy use point of view.  The experience of having worked on tightening up a number of them points me toward the flash'n'fill + exterior rigid approach over the others.

Code-min for a cathedralized ceiling on L.I. is still R30, IIRC.  Fiber-filling a 2x6 bay delivers about R20, but merely R10 on the roof isn't sufficient R to protect the roof deck from interior moisture drives.  You'd need a minimum of R15 to be OK with an R20 cavity fill, and R20 is even better.   A 3" layer of iso runs about R19. With 1" of cc foam on the interior the wood can still dry toward the interior just fine, an the condensing surface is now the foam, which doesn't wick the moisture toward the wood.

Thanks for the detailed description,
1) I understand I have to go with rigid foam on top R15-R20

2)Flash'n'fill - cool weather plus inaccessibility of attic makes it very hard and delayed until the next spring (my roof leaks), even if I  managed to get inside the attic I could only spray foam on ~1/3 of inside the roof deck (peak area) due to the attic being very small so moisture could probably travel through the remaining 2/3 of
unsprayed roof. Also spraying job volume would be low, spray foam contractors I talked to did not like the idea of tight spaces and low volume jobs

3)If I go with cellulose under roof deck how safe is it, how does BIB compare application and cost wise, is BIB diy doable ? Would I need vapor retarder ?

4)If using RF on top - should I close off gable vents and remove insulation from attic floor ?

5)If using Hunter Cool-Vent does it take care of new deck ventilation only or of original roof deck in some way ?

Greetings,
QUOTE> Reflective insulation (even multi-layer versions) A: wouldn't meet code requirement for wintertime heat loss and B: wouldn't be air-tight. It would be well ventilated and wouldn't rot, but would leave a large whole-house stack effect to drive wintertime infiltration rates.

RB do not have "R" factors, however they do perform better than bulk insulation.

As far as air infiltration comment - BS

You should be looking at insulating above the drywall/ plaster not the rafters unless you want to use that space.

Unless your area inspects rehab you can use what ever you deem best.

Posted By mik on 14 Oct 2011 11:16 AM
Thanks for the detailed description,
1) I understand I have to go with rigid foam on top R15-R20

2)Flash'n'fill - cool weather plus inaccessibility of attic makes it very hard and delayed until the next spring (my roof leaks), even if I  managed to get inside the attic I could only spray foam on ~1/3 of inside the roof deck (peak area) due to the attic being very small so moisture could probably travel through the remaining 2/3 of
unsprayed roof. Also spraying job volume would be low, spray foam contractors I talked to did not like the idea of tight spaces and low volume jobs

3)If I go with cellulose under roof deck how safe is it, how does BIB compare application and cost wise, is BIB diy doable ? Would I need vapor retarder ?

4)If using RF on top - should I close off gable vents and remove insulation from attic floor ?

5)If using Hunter Cool-Vent does it take care of new deck ventilation only or of original roof deck in some way ?

Regarding #2:  It's not rocket-science to use the 600-board-foot (or smaller) closed-cell foam kits for a flash'n' fill + air-sealing. Even if it's uneven, the primary protection factors still work. is While it's somewhat vapor-permeable, it's both air-impermeable and won't wick moisture toward the roof deck during hours when the deck is below the dew point of the interior air, but it can still dry toward the interior via vapor permeation.  If you can't foam-seal it, dense-packed cellulose is "good enough" air retardency from a thermal point of view, but not from a air-transported moisture point of view.  If you can't flash-foam it, you MUST put on sufficient exterior-R to avoid wintertime moisture accumulation in the roof deck.

Regarding #3: Without exterior foam or a flash-foam on the interior putting cellulose against the roof deck has some moisture risk- it would need a vent cavity between the cellulose & roof deck.  Cellulose can buffer quite a bit of moisture, but you'd still want a 2-5 perm interior vapor retarder such as standard latex on gypsum (or Certainteed MemBrain) unless you did the flash-foam as well.   Dry-blown BIB can be  a DIY project, but to meet ignition barrier code on flash foam you either need to then put up half inch gypsum or OSB/ply on the interior, wheras with wet-sprayed the cellulose itself qualifies as an ignition barrier.  (With exterior foam dry blown BIB is fine without the gypsum, but you'd want to put up MemBrain.) Under no circumstances should you put up poly or foil sheeting, since that would prevent an unvented roof from drying toward the interior, which needs to be it's primary drying direction.

Regarding #4:  With R19 above the roof deck, yes close off the gable vents, but leave the insulation on the attic floor- it's doing you at least SOME good.  You should also insulate the gable ends. If you go with only R15 above the roof deck you could theoretically have a small risk of moisture issues the deck, depending on the effective-R on the attic floor & knee walls, etc., (which is probably performing to R10 or less after thermal bridging & air leaks are factored in.)  See the table in section 5.3- you are in climate zone 4A, and the presumed total R is R38 in that chart.  With R19 on the exterior you can crank the total-R up to R50-ish  and be just fine. I think you have some margin here, especially if you use fiber-faced iso or unfaced EPS combined with a vented nailer deck (either a pre-made vented panel or a DIY with furring.)  Also, since you probably have plank decking, it's absorption rate at condensing temps is much lower than OSB, which (a presumed possible material in the IRC), so it's less susceptible to mold & rot issues than the code presumes- you could probably push it out to R60 and be fine in most cases.

Regarding #5: Hunter Cool Vent allows the nailer deck to dry, but provides only  a modest drying capacity for the structural deck below. They specify sub-1 perm facers on the iso, and I wouldn't assume it's more than 0.5 perms, but it's probably not much less than 0.5 perms either.  OTOH with a vented nailer deck version there will be at least SOME drying toward the exterior even during winter, even with a snow-load- it's pretty good from that respect.  The original roof deck sans foam, sans exterior venting had quite a bit of saturated condtions with snow or rain on top, and zero ability to dry toward the exterior.  With foil faced iso it's similarly blocked, but it would also block moisture drives from the exterior.  With fiber faced iso and a vent gap above it's quite a bit better, even with only a very modest drying capacity.

rbsys1- It's your BS, not mine. It's remarkably difficult to adequately air seal these stoop-attic capes & bungalows with rooflines cut up by dormers by concentrating only at the ceiling & kneewalls without doing a full-gut treatment. (Been there, done that, multiple times.) Dense-packing them works, as does foam, and the reduction in heating & cooling energy use exceeds that which is accounted for by R-value alone. The efficacy of dense-packed cellulose & foam at reducing infiltration is well established, confirmed via blower door testing.

RB (even multilayer) comes nowhere close to performing as well as the rigid-foam exterior/fiber interior stackup, and doesn't provide then necessary air-retardency, leaving many thermal bypass channels. Even with a full-gut to detailing 3-layer RB as an air barrier, it still wouldn't perform even as well as a flash'n'dense-pack sub-minimum solution.

Dana, thanks very much for your thorough analysis. But would it make sense to use spray foam if I could cover only 1/3  of the hole roof getting inside the attic
and the rest would be packed with just cellulose

Greetings,

Been doing it for thirty + yrs, don't you think I'm smart enough to solve such problems?

Didn't says anything about 3 layers being better than 2 or 1 layer.

You are amateurish when it comes to RB.

If you can get to the ceil'g insulation and cover it with a perforated RB that is the best way to go.
Since the area is not going to be used the expense of insulating the roof and gable cannot be justified.

Posted By mik on 14 Oct 2011 05:10 PM
Dana, thanks very much for your thorough analysis. But would it make sense to use spray foam if I could cover only 1/3  of the hole roof getting inside the attic
and the rest would be packed with just cellulose

As long as you put the requisite R in rigid-foam on the exterior for moisture control and seal it well at he edges & seams, you can get the air-tightness you need for thermal performance with densepacked cellulose (or dense-packed, Optima, Spider, etc.)

Unfortunately I'm yet to find a roofer/contractor who would give me not even a quote but just discuss exterior RF on a roof.
Meanwhile how much advantage a cool-vent (or diy version of it) has over exterior plywood/OSB on top of RF with no air gap ?

The advantage of the vented version is primarily the longevity of the nailer deck, followed by longevity of the shingles. Without the venting below the nailer deck can only dry toward the exterior, and even relatively minor leaks (even wind-driven rain, in some locations) can create rot conditions in the nailer deck. With the venting it stays dry and the peak shingle temps will be a handful of degrees cooler in summer. If you were roofing with purlin-mounted metal roofing there's not much advantage at all, since the nailer deck would then be inherently ventilated.

The complexity of the roof makes a difference on how easy it is to install. A hipped roof cut up with a bunch of peaked dormers with lots of valleys, and maybe a few skylights can be a real PITA to install exterior foam onto compared to a simple gable or shed roof. With simpler lines it can even be a DIY 2 person project (1, if you're desperate.) But lots of compound angles and other detailing to contend with can quadruple the labor cost, and increases the scrap rate. In some instances pre-installing the furring for the nailer deck as depth guides (using blocks of XPS for spacers) and spraying ~3" of closed cell foam would be easier & cheaper (at ~$3 per square foot) since the odd shapes hardly matter in a sprayed application. There's no scrap fraction, and it goes on pretty quick, and since it's waterproof & air-tight you end up with very good performance as well. Spray foam is ~1.5-1.8x the material cost per square foot of doing it with iso, but it's and installed price, as opposed to FOB the distributors loading dock, and installations with a very small number of full-sheets the labor costs of doing it with iso goes way up. If going the pre-installed furring + SPF approach, use 2x4s rather than 1x or the flatness of the roof may be negatively impacted.