Rafter Insulation Experts?

[ToolsRCool]

New build coming soon, I'd like to invite the band to beat this one up due to my lack of knowledge on such:

1. Roof peak may end up being a ridge beam vs ridge board. Beam may be 5" or so wide LVL, glulam, etc......, 24' span. If so, and I want to use a ridge vent, can it (both) be done? I could attached the rafter boards so their top peak is 1" or so above the beam, but I am not sure that is code compliant? Looks a little goofy, but would work. Or, leave the roof sheathing 1" or so down from the ridge beam edge, but then that would require a ridge vent that is wider than traditional, not sure such is available? Found same topic posted in 19', but his was already built top-flush.

2. Looks like the latest craze is to use rafter baffles on the underside to provide an isolated vent channel from the eaves to the peak (ridge vent), along the underside of the sheathing, a natural flow concept. You can then install fiberglass insulation or what you want over this. If I do this, is there any way to also vent the warm air out of the general area underside of the roof peak through the ridge vent as well? Will have a 2nd floor, so predicting warm up there in the summer. Could it make sense to alternate what runs to the ridge vent, per cavity? Meaning one cavity vents ambient warm room air freely to the ridge vent, next one is the channel baffle vent from the eave, then repeat? It will probably end up all rafter baffles, and then with just gable end vents and opposite end gable end fans for the general 2nd story area, or vents in both gable ends with a decently powered roof vent fan up on the shingles.

Open to ideas and what not to do as well. Not a spray foam guy, like to always be able to make easy changes in wiring, etc......so likely just kraft faced fiberglass rolls. Poly sheet vapor barrier on the underside of the rafters as well as the side walls?

 

[PCustoms]

Maybe I'm missing something, but if the attic/2nd floor is part of the conditioned space you don't want and air exchange from inside to outside.

 

[ToolsRCool]

I did not do well explaining the layout. 2nd floor will be a small wood workshop for my daughter. Mostly just storage, with occasional few hour use. Interior of garage heated, no A/C. Wanting to be able to vent 2nd story down to ambient outside temps, rather than 30 degrees hotter on a warm sunny July day, or at least as close as can reasonably be.

 

[PCustoms]

I did not do well explaining the layout. 2nd floor will be a small wood workshop for my daughter. Mostly just storage, with occasional few hour use. Interior of garage heated, no A/C. Wanting to be able to vent 2nd story down to ambient outside temps, rather than 30 degrees hotter on a warm sunny July day, or at least as close as can reasonably be.

Gable windows?

You're either insulating the place, or not. Doing it halfway for "venting" doesn't make sense.

 

[billconner]

Agree. If you want ability to exhaust attic room put a fan in the gable that can be controlled and have an insulated cover.

Your ridge beam question is interesting. I don't have an answer.

 

[Hank11]

No insulation upstairs will give a miserable experience during summer time. The space will be unusable.

I have 7” of open cell spray foam under the roof deck. Its just enough to make it ok on a 98* day. The knee walls are not insulated. Downstairs is 2x6 walls with fiberglass batts. My entire building is super tight. I have a fan that circulates upstairs air back downstairs. A duct fan is built in a box duct in the knee wall with an AC return grill and filter. It works great.

The roof is not vented. I had planned on staple-in channels under the deck, but my insulation guy told me not to. His take on roof decks is that open cell foam lets you see any leak’s and its easy to dig out by hand to make repairs. I think closed cell may be preferred up north for this job. Look for a sharp insulation guy with experience who is not just looking for a quick payday.

 

[ToolsRCool]

Gable windows?

You're either insulating the place, or not. Doing it halfway for "venting" doesn't make sense.

Correct. Gable vents and fans uncovered for summer, covered for winter. Apology didn't mention that, but you figured it out.

 

[ToolsRCool]

Agree. If you want ability to exhaust attic room put a fan in the gable that can be controlled and have an insulated cover.

Your ridge beam question is interesting. I don't have an answer.

Yep. I think that is where this is heading.

 

[ToolsRCool]

No insulation upstairs will give a miserable experience during summer time. The space will be unusable.

I have 7” of open cell spray foam under the roof deck. Its just enough to make it ok on a 98* day. The knee walls are not insulated. Downstairs is 2x6 walls with fiberglass batts. My entire building is super tight. I have a fan that circulates upstairs air back downstairs. A duct fan is built in a box duct in the knee wall with an AC return grill and filter. It works great.

The roof is not vented. I had planned on staple-in channels under the deck, but my insulation guy told me not to. His take on roof decks is that open cell foam lets you see any leak’s and its easy to dig out by hand to make repairs. I think closed cell may be preferred up north for this job. Look for a sharp insulation guy with experience who is not just looking for a quick payday.

Thanks for posting your config and results.

 

[mike93lx]

Your roof pitch will dictate how much of the beam is in the way of venting the ridge. I'd have no problem knocking the top corners off to increase area for flow

Here's one way to do it

Timothy Uhler on Instagram: "Setting some 2x12 rafters. Block at the ridge is notched to provide ventilation and the block itself for edge nailing the sheathing per engineering.#framersareadyingbreed #roofframing #framing #huberpartner"

1,118 likes, 20 comments - awesomeframers on December 18, 2024: "Setting some 2x12 rafters. Block at the ridge is notched to provide ventilation and the block itself for edge nailing the sheathing per engineering.#framersareadyingbreed #roofframing #framing #huberpartner".

www.instagram.com

 

[ToolsRCool]

Your roof pitch will dictate how much of the beam is in the way of venting the ridge. I'd have no problem knocking the top corners off to increase area for flow

Here's one way to do it

Timothy Uhler on Instagram: "Setting some 2x12 rafters. Block at the ridge is notched to provide ventilation and the block itself for edge nailing the sheathing per engineering.#framersareadyingbreed #roofframing #framing #huberpartner"

1,118 likes, 20 comments - awesomeframers on December 18, 2024: "Setting some 2x12 rafters. Block at the ridge is notched to provide ventilation and the block itself for edge nailing the sheathing per engineering.#framersareadyingbreed #roofframing #framing #huberpartner".

www.instagram.com

Interesting take, thanks. Probably need to do that after inspection. Primary roof pitch will be 11/12. Dormer roof pitch 2/12 or maybe 4/12, need to see what different ones look like on the sketch.

 

[billconner]

I agree with Mike on this but code is pretty specific on the ridge beam or board covering the entire plumb cut of the rafter. I'd estimate a lot, perhaps majority, of inspectors would give you a pass but some would not. Read of on on a building code officials bulletin board of requiring adding a piece to a 2x12 ridge board with 2x12 rafters. Goofy IMHO but worth being aware of.

Good luck.

 

[ToolsRCool]

I agree with Mike on this but code is pretty specific on the ridge beam or board covering the entire plumb cut of the rafter. I'd estimate a lot, perhaps majority, of inspectors would give you a pass but some would not. Read of on on a building code officials bulletin board of requiring adding a piece to a 2x12 ridge board with 2x12 rafters. Goofy IMHO but worth being aware of.

Good luck.

Yep, that was exactly my concern as well. Thought the rafter board needs to totally meet the ridge top to bottom, because I remember reading where they do specify that the ridge beam needs to be at least as deep as the rafter boards attaching to it. My LVL or glulam will probably end up being about 16" deep or so, so no issue there.

I'll probably just end with a list of such questions to run by my city's building inspectors. But even then, what if they send the "other guy" out that wasn't working the counter when I asked and got a verbal answer? ;/

Looking more and more that it may be better to just shim the sheathing up at the top when installing it with some sort of shims that would be 4' long, and taper from 0" to 1" thick along its length.

 

[billconner]

Found one answer I suspected and another one I would not have thought of - both for after being turned down by inspectors. (1). an inch deeper lvl than required structurally and chamfer too edges between beams and inch. (2). I believe it's permissable for a ridge beam to be dropped and rafters bear on top of it. May be awful appearance if that is a factor. I could see a birds mouth cut at bottom of rafter and it sits on lvl. I'd probably but in a block across rafters stop lvl. May also be a metal bracket (Simpson String tie) that allows this. But best is getting permission to drop it or font ask and practice asking for forgiveness.

 

[ToolsRCool]

Found one answer I suspected and another one I would not have thought of - both for after being turned down by inspectors. (1). an inch deeper lvl than required structurally and chamfer too edges between beams and inch. (2). I believe it's permissable for a ridge beam to be dropped and rafters bear on top of it. May be awful appearance if that is a factor. I could see a birds mouth cut at bottom of rafter and it sits on lvl. I'd probably but in a block across rafters stop lvl. May also be a metal bracket (Simpson String tie) that allows this. But best is getting permission to drop it or font ask and practice asking for forgiveness.

Interesting take, I value creative thinking. Thanks.

 

[C-S-H]

I assume you need a cathedral roof assembly for your build. Have you considered a ventilated roof deck with exterior insulation? It is a premium system (and more costly), and is a big step up in performance from the ventilated/insulated rafter cavity assembly.

From inside to outside:
1. Latex paint
2. Drywall
3. Rafter cavity with mineral wool insulation, max 60% of assembly R-value
4. Roof shear plate sheathing (plywood)
5. Water and ice peel and stick barrier (air, water, and water vapor barrier)
6. Polyiso with foil side up and joints taped, min 40% of assembly R-value
7. 2x4 pressure treated strapping run up the rafter lines and properly screwed to them
8. Roof deck sheathing (plywood or OSB)
9. Roofing substrate
10. Roofing

 

[ToolsRCool]

I assume you need a cathedral roof assembly for your build. Have you considered a ventilated roof deck with exterior insulation? It is a premium system (and more costly), and is a big step up in performance from the ventilated/insulated rafter cavity assembly.

From inside to outside:
1. Latex paint
2. Drywall
3. Rafter cavity with mineral wool insulation, max 60% of assembly R-value
4. Roof shear plate sheathing (plywood)
5. Water and ice peel and stick barrier (air, water, and water vapor barrier)
6. Polyiso with foil side up and joints taped, min 40% of assembly R-value
7. 2x4 pressure treated strapping run up the rafter lines and properly screwed to them
8. Roof deck sheathing (plywood or OSB)
9. Roofing substrate
10. Roofing

I have not heard of this. Lost me at #7, but it sounds like you are making a foam sandwich on the "outside" of the primary roofing sheathing?

 

[C-S-H]

Yes. The foam insulation goes above the structural sheathing. The 2x4's provide the 1.5" spacing for the ventilation cavity underneath the roof deck sheathing. There are 2 fascia boards at the eave, with air flowing up between them to a ridge vent.

 

[billconner]

Yes. The foam insulation goes above the structural sheathing. The 2x4's provide the 1.5" spacing for the ventilation cavity underneath the roof deck sheathing. There are 2 fascia boards at the eave, with air flowing up between them to a ridge vent.

Not what I would recommend for a heating climate. Sounds good for cooling climates.

 

[C-S-H]

The assembly works in all climates. The minimum exterior insulation R-value % varies by climate zone:

Zones 1, 2, 3 >= 10%
Zone 4C >= 20%
Zone 4A,B >= 30%
Zone 5 >= 40%
Zone 6 >= 50%
Zone 7 >= 60%
Zone 8 >= 70%

The drawbacks are that the exterior insulation % results in really thick foam board in northern climates, but they build them anyway. Also, the ceiling/roof plane is now water vapor impermeable, so that puts more demand on HVAC if vapor semi-permeable wall assemblies can't keep relative humidity under 60%.

 

[billconner]

Yes it would work at cost of a lot of foam and some issues to solve around edges. It's basically a hot roof built differently. I didn't understand in my post above how much foam was on top.

Simpler perhaps, in cold climates at least and I'm in zone 6, would just be all foam on top of roof deck. I'm thinking about a "cabin" or house build doing just that. From insude to outside: rafters and sheathing; ice and water shield - the best I can find; at least 8 if not 10" of foam; 2x4 nailers attached through foam to rafters; metal roofing. Part of that is using rough sawn rafters and rough sawn board sheathing, and looking at something like 6x12 rafters on 5' centers with two-by sheathing or similar. I may go to one-by sheathing and see if I can get to centers greater than 2'.