Venting low slope roof
I am building shed roof design in zone 6 maine with trusses, double stud design (2×4) blown in cellulose, two story house.
-low slope 1:12 roof
-standing seam metal roof
-high temp synthetic roofing underlayment
-5/8″ APA sheathing, engineered roof truss system
-create a 1.5″+ air gap at top for venting with Accu-vent product
-22″ deep blown in cellulose for R74
-Siga Swiss Majrex 200 at truss bottoms – taped as ceiling air barrier and installed prior to erecting second floor interior partitions
-2 5/8″ (half ripped 2×6) strapping installed on edge at 16″OC installed perpendicular to trusses supporting Majrex200, strapping creates mechanical chase possible Rzr style lighting fixtures
-1/2″ GWB ceilings throughout.
On previous post someone mentioned accuvent not advisable as not permeable. However I am seeing articles by Martin holiday that specify using on site created ventilation space or accuvent. But 2″ gap is preferred.
Question
1. Is accuvent good for my setup since it’s 22″ deep cellulose?
2. Do I need a permeable vent?
3. Is it better to instead not have any ventilation?
4. Do I need dense packed or better to just blow in at medium packed since slope 1:12?
5. Any other recommendations
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Sean,
As opposed to baffles just used at the eaves of attics with insulation on the floor, baffles that extend over the whole of the roof perform a lot better if they are permeable, so any moisture that makes its way up to them can diffuse through them into the vent cavity.
A bigger problem with your assembly is that low sloped roofs don’t vent well. Because there is no mechanism to move the air though them, unless the have very deep ventilation spaces they can experience moisture rated problems. This link explains the issues: https://www.greenbuildingadvisor.com/article/insulating-low-slope-residential-roofs
However, you can't - practically or by code - have an un-vented roof using nothing but permeable insulation like cellulose.
Hi
So those were interesting reads.
Am I too assume if kept the 1:12 vented roof with the 22" of trusses, that according to the articles I need 6" of ventilation space, some sort of baffle (you mentioned something permable), and 16" of Celluolose, an unknow number of capulas for more ventilation in order to meet the 1sqt foot net free ventialtion per 300 sqfeet of roof?
So it sounds like an unvented would be better? Is this true for the 1:12 in maine zone 6. If so, per article it sounds like 4.5" of closed cell for R 29.5 (article said R25 but im falling on the cautious side), then 8-9" of cellulose which is R 30? I believe article referenced a minimum Rvalue of roof must be 51% foam? So I cant go higher then a total of R60, I can just add more cellulose to get more than R60 because it would exceed the the 49% of cellulose?
Sean,
With a 6" vent space you wouldn't need baffles, but rather something to contain the cellulose. Depending on the shape of the roof (how far from the edges the middle is) you might be able to get away without doghouse vents and simply vent all four sides at the soffits. To decrease the risk you can always specify deeper trusses.
The problem with going un-vented is that the foam needs to be against the underside of the sheathing, and the cellulose against the foam - but with 22" trusses that leaves you too deep a cavity to completely fill with cellulose and still maintain your ratios. Hopefully some other posters will have an idea for how best to deal with this.
Thank you again for your response. If I did un-vented I would have the engineers decrease the mono trusses down to the 14", assuming that it could be done and honestly would be a huge win for me as I can increase the hieght of the ceiling on the second floor by 8" (22"vs14"). Building with height restrictions so a thinner roof helps.
I would think it would not be problem for insulators to blow foam first...then return to do cellulose for roof and walls? Or am I confused...or is there some sort of issues with cellulose against foam that I am not aware of?
So with all this in mind....I guess I am wondering...is unvented better in a zone 6 maine with a 1:12 slope roof? Sounds like it has less chance off errors as well per article also
Also, am I wrong, but it appears according to R806.4 Unvented Attic assemblies..... that with un-vented, with 4.5 closed cell and 8-9" Cellulose, that no vapor barrier between bottom of trusses and drywall is needed? This was stated: "No interior vapor retarders are installed on the ceiling side (attic floor) of the unvented attic assembly."
Sean,
I do think that an un-vented low-slope roof is safer - and that would be a win if you can reduce the height of your trusses.
The sequencing you suggest for the two insulation types is pretty common.
You don't want an interior vapour-retarder so it can dry to the inside, but you do want an air-barrier there - which can be the drywall.
HI
Have you or anyone heard closed cell pulling away from the roof sheathing when applied on the underside over time if I chose a flash and batt with 4-5" of closed cell and 8-9" of cellulose?
Sean,
I avoid both spray foam and low sloped roofs, so I have no direct experience with them.
Can I ask why you do not like and ever use spray foam? Is it environmental impact? Cost? Or another reason?
Sean,
In our temperate but wet PNW climate there are a few things I design out of all the projects I do. Buildings with no overhangs, flat or low-slope roofs, and decks over conditioned spaces. Doing that also allows me to use simple, inexpensive, vented-roof assemblies I know won’t experience problems.
I don't use spray foam for a variety of reasons.
- Although problems with curing are relatively rare, when they occur they can be irremediable.
- Spray foam makes future work or even maintenance difficult, and the affected materials - both the foam and wood based - can’t be reused or recycled.
- Cost
- It’s still a high carbon insulation.
Low slope roofs seem to have issues in certain climates. I'm in the north edge of zone 5 with hot/muggy summer and vented low slop roofs are very common. They are definitely not failing and most are improperly vented.
I have built and would have no qualms about building a vented low slope roof. Increase the went gap a bit, you do want more than 1.5", air seal the ceiling (which sounds like you definitely have figured out) and install code required vents on the soffits.
Don't use accuvent, need to use something permeable for the vent channel.
Yes I hate low slope roofs. But I have a height restriction so my only option for a higher sloped roof is to eliminate the second floor al together or suppose redesign and loose the back 1/3 of the second floor as usable space to have rooms/walk around.
If vented was used.....what would be the minimum gap for zone 6 using dense packed cellulose in the ceiling? I need at least 14" deep trusses for R49. 6" per article is challenging as it means I need to decrease ceiling height.
Also, what exactly is used for permeable venting solution if accuvent can not be used? Trying to keep things simple tried and true and bullet proof as possible.
Code min vent gap here is 2 1/4". Depending on the length of the roof (say about 20'), I would not have issues with something around there. If you don't want to get very tall trusses, you can strap the top with horizontal 2x2s or 2x4s on flat, this will allow for airflow in all directions and you can use the top strapping to form the overhang.
For the baffles you can use any permeable sheet good. If working with cellulose, probably the simplest is insulweb stapled to the side of the top chord. Other options is to nail up 2x2 to the side of the truss and nail thin OSB/CDX/Fiberboard/house wrap to it.
Also since it is low slope, you don't need to dense pack. You can even loose fill but it will settle a bit. If you semi dense pack (somewhere around 2.5 to 3) it won't settle and won't need as much pressure as dense pack.
Hi....question and thank you for the information. If deciding to use a low slope unvented roof system using flash (closed cell) and batt (cellulose), would you still use semi dense cellulose versus dense packed? If so why? Less insulation overall so cheaper? I understand normal attics are loose packed as dont have to worry about gravity shifting the insulation, however wouldn't dense packed be preferred with a flash and batt unvented roof in order to limit any moisture making its way to the spray foam from the house and preventing any gaps occurring between cellulose and foam? Just curious...I always like to know reasons behind things.
You dense pack when you don't want the insulation to settle. In case of hybrid assembly, the fluffy needs to be tight against the spray foam.
The issue isn't amount limiting moisture. When you have a gap in a hybrid assembly, you create this in-between space that is neither inside or outside. The RH there could get very high and might create issues especially in a roof.
So vented roof, unless steep, settling doesn't matte, so you don't need to dense pack.
Unvented roof, always densepack.
Sean, I design homes and renovations in Maine. For the reasons Malcom listed, I avoid foam whenever possible, and do my best to design without low-slope roofs so foam isn't necessary. But sometimes I have no choice but to use a low-slope roof and in those cases I either use a flash-and-fill system, with at least 50% of the R-value in the foam layer (the long-long-term aged value, about R-5.5/in, since this is a long-term moisture issue and not an arbitrary thermal requirement.) In some cases I have used rigid foam above the roof sheathing but that can further complicate things, depending on the situation.
I have read Martin's article many times and have recommended that system on a few occasions, but the builders have always ended up using foam, due to the extra height and complication required.
I would not go with less ventilation than Martin recommends in his article. Vapor drive in our climate zone is more strongly to the exterior than in CZ5, and there are a lot of variables involved. Most assemblies will last 10+ years with no apparent issues, but after that they can degrade rapidly. Most of my work is renovations and I have seen plenty of things that should in theory have failed but didn't, and I've also seen many homes with major rot due to poor or risky detailing. I design homes to last 50+ years with minimal risk of mold or rot. To create a 6" gap using dense-packed insulation you would need to install netting 6"+ below the sheathing.
If this a new build and not permitted yet, as of next month we need R-60 insulation in roofs. That can be reduced a bit using one of two U-factor methods. If you're in a small town, code enforcement is not required but the code is still in effect.
Thank you Micheal for all your information, especially the 5.5" closed cell due to long term value and minimum code change coming up. Sounds like I need to stick with a low slope unvented ceiling and the flash and batt is what is needed. Could you tell me what you would recommend for a minimum R-value with this type of roof? I want to make sure I gear the engineer toward the depth needed with the trusses taking into consideration keeping the depth smaller due to ceiling height but also deep enough for good insulated house. Thanks
I recommend R-60 total, so at least 5.5" of closed cell foam and R-30 batts. Or, because we have a lot of skilled cellulose installers here in Maine, and we produce the only wood fiber insulation in North America, I spec one of those to products, dense-packed. Both insulate at around R-3.6/in when dense-packed so I would use 8". Or R-30 batts of your choice. 7 1/4", locally-made wood fiber batts are also available: https://www.timberhp.com/products/timberbatt.
Is it correct to assume that in order to meet the 1 square foot net free ventilation per 300 square feet of roof, I will need 6 inches of ventilation space, a baffle (you mentioned something permeable), 16 inches of cellulose, and an unknown number of capulas if I maintain the 1:12 vented roof with 22 inches of trusses?
You don't need a baffle in that situation, just something permeable to contain dense-packed fibrous insulation if that's what you choose to use. Someone mentioned loose-blowing the insulation; that would only work if you can do it before the roof sheathing is installed, as you can't accurately loose-blow a space that you can't see. Batts would not require baffles, but are also the most prone to gaps during installation, which short-circuits their thermal performance.
In such case, a baffle is unnecessary; all you need is a porous material to contain dense-packed fiber insulation, should you so desire. You can't precisely loose-blower an area that you can't see, therefore doing it before the roof sheathing is finished is necessary if you want to loose-blower the insulation, as someone else said. Batts are the most likely to have gaps when installed https://doodle-jump.co , which reduces their thermal effectiveness; however, they do not need baffles.