Tiny cabin cathedral roof insulation–vented or not vented?
Setting: I’m building a 12’x12′ off-grid cabin with a metal roof in texas in insulation zone 2 near houston. it would seem like texas code calls for r-30 (not r-34 as mentioned on this website for some reason?). and r-25 if rigid insulation on top of roof alone. fyi, I’m not building in an area that enforces building code, so I have some leeway as to what I can come up with.
the 12:12 roof pitch will help accomodate a livable attic for sleeping, and so the insulation needs to go in between or above the rafters.
My criteria are
1. quick enough installation
2. drywall ceiling on the underside of rafters to hide them (mostly because on a cabin this small I might bump my head on one if they were exposed).
3. relatively cheap.
4. very probably no dehumidifier, because space is at a premium, and it’s too loud for sleeping. it’s also a serious drain on solar panel power.
With these criteria in mind,
what insulation assembly should I go with?
here are some ideas:
1. exterior rigid insulation + interior fiberglass insulation per these instruction (however, doing so I was wondering about how or the necessity of vapour diffusion ports and whether it would be more convoluted–cutting through the rigid foam to make them.): https://www.greenbuildingadvisor.com/article/installing-rigid-foam-above-roof-sheathing
the other advantage is less wide rafters, easier to carry.
2. fiberglass insulation inside rafters alone, and I would assume in this case they would HAVE to be vented with the baffles and ridge vent although I’m not SURE. 2×10 rafters in this case, unless I wanted to go below code. quite heavy to install. Also, wondering if it’s “acceptable” to allow for humid Houston air underneath the sheathing to create so-called ventilation. I’m not from Houston originally, so I don’t know how it’s historically done.
3. only r-25 rigid insulation on top of roof alone, and allows for empty rafter cavities, and a bit easier electrical light box wiring since I could probably just bore holes into the rafters and not do strapping. on a space that small, strapping seems like an unnecessary waste of time, also because remodeling is probably never gonna happen in that loft. but maybe I could still do the strapping. the main benefits are just no fiberglass insulation mess–and no breathing in of some of that dust, and how all the wires will still remain visible until putting on the drywall.
4. seal the fiberglass entirely, by sealing the soffits and no ridge vents like this off-grid guy SEEM to have done (although he lives in a colder state that gets snow so I’m not sure): https://youtu.be/umjXExg0lcM
just use human-powered effort to manually open windows when it gets too hot in the livable attic, and also install a constantly running passive wind turbine (no idea what he does in the winter though… probably seals it). (youtube timestamp: ~8:39)
basically, his house becomes a big ventilated “carport” and a hostel for the wind gods in the summer.
it’s also not the worst idea because he “mostly” uses evaporative coolers and might need a way to get that extra water out of the house.
that said, I’ll be near houston: not on the coast but about 2 hours north. maybe it’ll work, maybe it won’t because the air is already too humid. I think it might work a little.
I’m more interested by whether the sheathing in that case might rot, or if the passive wind turbine (and occasional window openings) is enough to suck out the hotter air and humidity before it even gets into his seamingly sealed fiberglass roof assembly.
5. seal fiberglass assembly’s soffits but put a vapour diffusion system on top.
PS. I just read the GBA article on vapour diffusion ports (https://www.greenbuildingadvisor.com/article/vapor-diffusion-ports). It says it’s possible to do fiberglass inside and rigid foam outside, as long as the rigid foam does not cover the ports (duh). But now what if there were only outside rigid insulation?–can or should I have still have vapour diffusion ports given my circumstances? hm…
6. yet even another option seems to be to put the rigid insulation AND fiberglass inside, but I have gotten mixed advice as to whether it should be rigid insulation first or fiberglass first under there. I suppose doing so would allow rafters to still be narrower and maybe make the vapour diffusion ports easier to cut from the outside, since there is only one layer of sheathing.
The right choices and advice are needed!
Thanks y’all.
PS. if anyone here understands how Matt Risinger did his vapor diffusion ports (timestamp: 19:14) (https://youtu.be/zhkGcklWB_Q), I would love to know. cause in the video he seemed to have had cut the slots/slits (not right on the ridge but a little below, which is smart because the membrane is less likely to sag and collect water I think), but 1. something was blocking the slots underneath the sheathing in the video, and 2. seems like he covered the top with strapping too, in SEEMINGLY the same location as the ports.
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Replies
the rosehermit,
Re your PS: I don't know much about vapour-diffusion ports but his installation does raise some questions for me.
- I wonder what the advantage is of a wide piece of very permeable underlayment when the slit in the sheathing is only about 1" wide?
- How you do determine the necessary width of the port? With roof ventilation we have definite guidelines. The ones I've seen designed by Joe L. look to have much larger open area.
- If the problem is moisture accumulation at the ridge, how far down from the ridge can the port be and still be effective?
Maybe someone with more experience with them can comment.
hey there Malcom. fun questions. let's see if I can answer them! 1. probably because it's faster sticking it on in one go and also to respect overlap. 2. "How wide is the slot?
In his “Venting Vapor” article, Lstiburek wrote that builders who retrofit a vapor diffusion vent on a SIP roof need to remove 12 inches of OSB from each side of the ridge. But later in the same article, he recommends that for most roofs, narrower slots are just fine: “How big should the vent area be? Turns out the same vent area we have been using for airflow vented roofs —1:600 of the ceiling area.” Also Matt's got a dehumidifier in his attic I believe, which might further reduce his needs for big ports. 3. good question. I'ma guess, relatively close, for a lack of science 😆, maybe someone else can answer better indeed.
With a metal roof I'd definitely be thinking ventilated and batt insulation. With high-density fiberglass you can achieve R-30 with 8-1/4" of thickness. I'd do a 2x8 rafter padded out with 1" rips of 2x material. Something like this: https://www.homedepot.com/p/Knauf-Insulation-R-30-EcoBatt-Kraft-Faced-High-Density-Fiberglass-Insulation-Batt-8-1-4-in-x-23-in-x-48-in-8-Bags-691002/313646773
The purpose of venting or continuous insulation is to prevent moisture from condensing on the sheathing and framing and rotting it out. It's not to cool the attic, to do that you use a combination of insulation, ventilation in the inhabited area (aka windows) and air conditioning.
What is under the roofing? Sometimes with metal roofing there isn't sheathing, just horizontal purlins on the rafters, that's pretty easy to ventilate.
1. do you mean you use the "rips" as baffles?
2. yeah but the way he uses the passive wind turbine is to cool the livable attic. as a bonus that he might not know, he might have avoided disaster because it "might" have kept his sheathing from rotting out at the same time!
3. I'm putting sheathing and very probably that high temp ice and water shield down there, which unfortunately I don't think they sell in "permeable" fashion, hence the even bigger need for vapour diffusion ports. PS. it's not a shed, and I do not come from a third world country; I'm putting down sheathing 😆.
The rips are to increase the depth of the rafters from 7-1/4" to the 8-1/4" you need to get R-30.
Ventilating the living space is a completely different problem from ventilating the roof. There must be an air barrier between the living space and the insulation, the more air-tight that barrier the better.
If you have sheathing you're going to need a ventilation channel between the insulation and the sheathing. The simplest way is to get premade channels like this: https://www.homedepot.com/p/ADO-Products-14-in-x-48-in-Rafter-Vent-10-Per-Carton-UPV1448010/202541345 . That will add another inch which will mean you need 9-1/4" which means 2x10 rafters.
What some people do is put a 1" spacer between the sheathing and a sheet of polyiso foam board to create a channel. The polyiso has a higher r-value per inch and allows you to have a thinner roof.
1" polyiso is R-6.5 and 2x6 high-density fiberglass is r-23 so together they are r-29.5. Total thickness would be 1" of vent, 1" of polyiso and 5.5" of fiberglass for 7.5" total. So a 2x8 with slight padding would hold it.
"Ventilating the living space is a completely different problem from ventilating the roof."... did you not read what I wrote?? either agree or disagree that the passive wind turbine might have served both functions of cooling the livable attic AND prevented sheathing rot. don't just repeat what you just wrote and act like a robot.
"put a 1" spacer between the sheathing and a sheet of polyiso foam board to create a channel." smart. got a diy link?
"Total thickness would be 1" of vent, 1" of polyiso and 5.5" of fiberglass for 7.5"
nice, but no padding if you mean the 1/4" lacking, the fiberglass will just have to be crushed a bit 😆.
No need to be snippy, I'm trying to help.
Did you get to the part where I said "There must be an air barrier between the living space and the insulation, the more air-tight that barrier the better."
So no, I don't think the turbine will serve both purposes.
Generally mechanical ventilation is a bad idea, it ends up drawing more air from the living space to the underside of the roof and actually increases the moisture content.
https://www.greenbuildingadvisor.com/article/site-built-ventilation-baffles-for-roofs
Thanks for the link. However it doesn't mention the type of fasteners used to attach the rigid foam to the strips I think. Sad.
I still think you are wrong. In my mind the turbine definitely serves both purpose since it's similar to a vented roof only it does it before the vapor has a chance to get through the drywall, then into the fiberglass etc.. I believe the constant motion of air current in a certain direction is like a riverbed--directing the river away constantly so that most of it doesn't go beyond the bank. What you said about airtightness in this case, assuming you are talking about the drywall, actually helps my case, because the airtight drywall will just direct that vapor even better due to the drywall guiding the air current and vapor it carries towards the exit (turbine).
PS. is the "or" here a typo? "the purpose of venting or continuous insulation " I might have misunderstood you.
A roof either needs to have ventilation between the sheathing and the insulation, or it has to have continuous insulation. The continuous insulation can be foam board above the sheathing, or it can be spray insulation below it.
The air inside a house is warm and humid. If the sheathing is cold and warm and humid air comes in contact with it, the moisture condenses out. If allowed to accumulate it causes the sheathing and roof framing to rot.
The three methods approach the problem in three different ways:
If the roof is ventilated the moisture won't accumulate but will dissipate.
If there is continuous insulation above the sheathing the sheathing isn't cold.
If there is spray foam below the sheathing the humid air can't reach the cold sheathing.
DC,
"A roof either needs to have ventilation between the sheathing and the insulation, or it has to have continuous insulation."
Over the years I remember a lot of discussion of roofs where the vent channel was above the roof sheathing. Is that not allowed under the IRC?
I don't know.
It always seemed an odd arrangement to me as any moisture making its way up through the insulation had to diffuse through the sheathing before it could be exhausted.
venting above the sheathing is mostly for safeguarding the roofing assembly integrity I believe not insulation.
the rosehermit,
Sorry I don't understand the distinction you are making.
I wasn't familiar with the term continuous insulation. thanks for that.
"If the roof is ventilated the moisture won't accumulate but will dissipate."
I wouldn't say dissipate but more like always moving/circulating with the houston air no? is the humid houston air damaging still? is there a better setup?
"If there is continuous insulation above the sheathing the sheathing isn't cold."
not cold, but does that REALLY mean 100% no condensation? if the heat and humidity inside reaches a certain point with that rigid insulation acting as a vapor barrier almost, maybe the sheathing doesn't HAVE to be cold. and in my case, I can't really use a dehumidifier. hence why a defusion port might be interesting in that scenario.
"If there is spray foam below the sheathing the humid air can't reach the cold sheathing."
I get it but you should say reaches the sheathing slower and in sparse amount; it'll still travel through the rafters at a slower pace towards the sheathing. also spray foam is expensive unless you are just trying to air seal for some reason.
Unless you're running air conditioning or dehumidification the air inside the house will always have higher moisture content than the air outside. They start at the same moisture content and the people inside add about five pints a day of moisture per person. Moist air is less dense and tends to rise and that moisture ends up on the underside of the sheathing unless steps are taken to prevent it.
They didn't have dehumidifiers commercially available 100 years ago. Somehow I doubt everything just rotted. So there are always solutions for those not willing to jump on the bandwagon of somewhat fragile modernity. I like hybrid solutions that won't send my house rotting right away when dehumidifiers fail me or in my case, might be close to impossible at first because I won't have enough solar panels.
Malcom, I've never seen an assembly where vent channels above roof sheathing were made to help the insulation. So maybe you can elucidate me instead.
What do you mean by "help the insulation?"
exactly.
thx for the high density insulation suggestion btw!
so far, I'm leaning towards high density insulation suggested by DC-Contrarian (without the kraft paper ofc just saying) however have it be unvented--sticking to the base of the sheathing AND have vapour diffusion ports! comments on that welcomed (and whether it's ok).
now let's see if we can come up with a better build!
[replying to #21 since we've hit on limit on nesting.]
The house shown in that YouTube video is a building science disaster. The vent fan is just a hole in the roof. He says it's "super-insulated" but then he says it's R-19 walls and roof, that's not even code minimum. Roof isn't ventilated, the fan doesn't help except that it probably makes the house unlivable in the winter.
He's using interior shades to try and control solar gain. But once the sunlight is through the glass, the energy is already inside the building. The shades are just going to absorb the solar energy and radiate it back into the room. In short, this house isn't a good example of anything.
The thing you don't seem to be getting is that the purpose of venting is not to shed heat. I don't know why I keep responding, because I keep saying it and you keep not believing it. The purpose is to shed humidity. I'm in my mid-fifties, I'm old enough that when I learned about roofs it was believed that heat killed roofs and that venting was needed to protect the roof from heat. It's only in the past twenty years or so that it's become widely known that humidity, not heat is the problem.
If you want to ventilate your living space, that's a perfectly legitimate strategy for cooling your living space. Put in a gable vent or a window fan or even just open a window. But don't confuse that with venting your roof. There needs to be an air-tight barrier between the living space and the roof, you don't want that air mixing. Not to protect the living space, but to protect the roof.
and I keep telling you that most of that humidity probably make its way through that turbine, on a never-ending basis. but it's probably too novel of an idea for you, which is why you keep trying to change the subject and put words in my mouth. do you really think I don't know the difference between water vapor and heat? sad.
"a building science disaster." -says the guy who thinks not putting sheathing under a metal roof is a good way to ventilate at the expense of a decent roof.
Response to #20:
"They didn't have dehumidifiers commercially available 100 years ago. Somehow I doubt everything just rotted. So there are always solutions for those not willing to jump on the bandwagon of somewhat fragile modernity."
A hundred years ago nobody built cathedral ceilings. They believed that heat destroyed roofs, and ventilated them.
They also typically heated by burning either coal or wood in the living space so houses had to be very leaky to provide enough make-up air. And they illuminated with candles and oil lamps. To the extent they insulated at all it was cursory. Have you ever read any Victorian fiction? A standard plot device is a draft blows out a candle and the characters are cast into darkness and chaos ensues. Think of how leaky a house has to be to have drafts capable of snuffing a candle.
"A hundred years ago nobody built cathedral ceilings. " lmao. ever heard of churches?
[reply to #21]
"Thanks for the link. However it doesn't mention the type of fasteners used to attach the rigid foam to the strips I think. Sad."
Why so snippy? We're trying to be helpful here.
The answer is the fasteners don't really matter. The strips rest on the foam, which can be a friction fit, it only has to hold its own weight. And it's supported by the fiberglass batt, which is stapled to the rafters by the facing. And the whole thing is supported by the drywall. The strips only need to be held in place long enough to get the foam up. You could probably use toothpicks if you had to.
Why do you take everything so personally? I recall reading that article before and it didn't mention fasteners, and I needed to know. still do. so yeah; sad.
toothpicks won't hold in a hurricane that's shoddy work mate.