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WILL1E
Moderator
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# Posted: 2 Feb 2021 03:58pm
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Since my other post have been focused on getting the foundation questions answered, figured i'd jump to the top side and get this dialog going on roof insulation.
Current plan is a 16x20-28' cabin with a shed style roof. Will likely have 1-2 lofts and a 3/12 pitch roof w/ metal roofing. Location is northern WI. Based on calculations i should be able to use 2x10's w/ 24" OC.
Based on my reading here, a common recommendation is spray foam...seems to be the answer for everything but comes at a cost. I've seen more recent users who've built this style cabin use batt insulation. So i'm curious what options do most go with?
Do you install the trays between the rafters, cut some holes in your blocking (not sure this is the right term) on either wall and then install batts and then vapor barrier?
Are there requirements/guidelines/calculators for what the R value needs or should be in the roof?
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rpe
Member
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# Posted: 2 Feb 2021 04:27pm - Edited by: rpe
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I built a 14x24 shed-style building this fall. I used the same 3/12 pitch, with rough-cut 2x8's on 12" centers. as 16" centers were undersized given the 14 ft span and significant snow load. You're good at 2x10's on 24" centers?
I'm working on getting it insulated now - 3 walls are done at this point. The 12" centers make the batt insulation a pain, as does the rough-cut lumber, which is 1-7/8" to 2" wide, rather than the dressed 1-1/2". I'm going with R24 Rockwool batts cut to fit, and inserted from below. I'm not sure how much of a pain this cutting and fitting is going to be. Talk to me again in a couple weeks!
As for venting, regular aluminum vented soffit panels were used at the top and bottom, and solid soffit panels for the sloped side overhangs only. The rough-cut 2x8 rafters are the full 8" in height, so shoving the R24 batts (5-1/2" thickness) in, leaves lots of venting space. I have no plans to put those foam venting trays in, as they don't fit in a 12" center bay, and wouldn't do much in my case anyways given the extra clearance I have available.
Note that no building inspections apply in my area, and ceiling insulation is well below code. This building is primarily workshop/mancave with the ability to support overflow hangout/sleeping quarters for guys on snowmobile weekends. Heat source is not yet finalized. Likely wood stove, or possibly pellet stove.
Here's a link to some build pics.
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gcrank1
Member
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# Posted: 2 Feb 2021 05:11pm
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My prev 12x24 with 12x12 stand up loft, 432ish sq ft, heated only with an antique upright wood stove had no insulation in the roof/no ceiling for the 37ish years we used it. We stayed warm and the snow load disappeared off the fairly steep roof too. Small tightly built cabins with little insul arent hard to heat, they just cool off faster once the stove burns low. Once we stopped using it in deep winter that wasnt much of an issue. Perhaps the bigger one was heat stacking in the hot, muggy summer.
I had considered cutting blue styro to fit flush leaving an air gap to the plywood roof sheathing (but I had no venting) with boxcar siding nailed up as a ceiling. Never got around to it (it would have been a miserable overhead job anyway).
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Irrigation Guy
Member
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# Posted: 2 Feb 2021 05:12pm - Edited by: Irrigation Guy
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This is where I made a mistake. I had my cabin shell built and shipped to my site and I have been finishing it out. I had them install the metal roof over the purlins with no sheathing. I insulated it with batts and those baffle things and it is toasty warm but condensation would form on the underside of the roof panels. I have remedied this by cutting in vents and adding a vapor barrier. I probably should have had 2†of foam shot in.
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Brettny
Member
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# Posted: 2 Feb 2021 05:51pm
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There are many ways to insulate a roof but it having a metal roof makes things a bit more difficult because you can get bad condensation under the metal if not insulated or vented right.
You can put foam board ontop of your rafters then the perlins and metal. I would still insulate more between the rafters.
You can build it like a normal metal roof with rafters, outlines then metal roof underlayment then spray foam.
I would not solely use batts unless you have alot of ventilation between them and the roofing.
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rpe
Member
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# Posted: 2 Feb 2021 07:08pm
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I put a layer of Tyvek down under the roofing steel, on the advice of a local guy that said it would rain on me if I didn't. Maybe that was un-necessary, given the venting, insulation, and vapour barrier? Not sure, but I had a part-roll kicking around, so I used it.
I'll take some interior pics next time I'm there, working on the shed roof insulation. It's great to get the thoughts of others on these things.
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Brettny
Member
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# Posted: 2 Feb 2021 07:14pm
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Tyvex isnt a vapor barrier nor is it a metal roof underlayment.
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rpe
Member
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# Posted: 2 Feb 2021 08:20pm
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Quoting: Brettny Tyvex isnt a vapor barrier nor is it a metal roof underlayment.
My present plan is (top down...): Roofing steel then Tyvek, plurlins, 2x8 rafters (lower 5-1/2" rockwool), vapour barrier, then ceiling finishing of some type - plywood, or perhaps corrugated steel sheeting. The insulation and vapour barrier are not installed yet, but hopefully in the next few weeks. There should be a good 2-1/2" clearance between the insulation and the underside of the purlins given the full-size 2x8's used, and another inch to the underside of the steel, so lots of ventilation space. I agree the Tyvek is likely of little if any value where I used it. It was thrown in last-minute just prior to the steel at the suggestion of a local. Any thoughts or suggestions on the insulation/vapour barrier side of things would be appreciated. Walls are mostly done with R14 and vapour barrier, but underside of rafters not yet started.
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ICC
Member
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# Posted: 2 Feb 2021 09:31pm
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Tyvek and its many imitators is not a vapor barrier, that is true. It is vapor permeable while it is also an air and liquid water barrier.
There is a special variety of Tyvek for roofing as well as other brands that are specific to roofs. A very noticeable difference between the roof and wall versions is the surface texture of the roof type is safer to walk on.
Roof underlayment is quick to apply and offers several months of time to get the finish roofing installed. It is a layer of protection against water and air infiltration.
Many DIY'ers don't sheath the roof before installing metal roofing, opting for the purlin method instead. IMO, that is a mistake. The sheets of sheathing (OSB, plywood, zip panels, all make the roof rafter or truss assembly much more rigid. The sheathing is a structural component. Most metal roofing panels, especially the standard BigBox store type are not rated as a structural panel. Purlins laid at ninety degrees across rafters does little to strengthen a roof.
FWIW the WI energy code calls for R49 in a ceiling with some special less stringent requirements for insulation that is applied on top of the roof deck and the finished roof treatment on top of that.
https://www.polyiso.org/page/StateEnergyCodeResources1#:~:text=The%20New%20Mexico%20E nergy%20Conservation,is%20effective%20September%2025%2C%202020.
https://www.energycodes.gov/sites/default/files/becu/2018_IECC_residential.pdf
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Ontario lakeside
Member
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# Posted: 3 Feb 2021 12:22am
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I used 2X10" with 1" air space 1" rigid foam board (sealed with can foam) next 6" rockwool batt then vapor barrier. vented at both ends.
No issues after 5 years.
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rpe
Member
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# Posted: 3 Feb 2021 06:58am
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Lakeside, what was the reasoning for the rigid foam board? If it's well sealed to the rafters, any moisture that does get past the vapour barrier will be trapped in between. I suppose for intermittent winter use there's lots of opportunity for drying time in between visits.
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WILL1E
Moderator
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# Posted: 3 Feb 2021 08:10am
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Wow, thanks for all the insight fellas!! This is why i love this place, people answer quickly and honestly regardless if the subject has been covered a million times!
Quoting: rpe You're good at 2x10's on 24" centers
Quoting: ICC The sheathing is a structural component.
My thought from top down was metal panels, roof underlayment, sheathing, foam trays, batt insulation and vapor barrier...some ceiling finish at some point later on. The tray's may not be necessary with 2x10's as R30 roxwool is only 7.25 thick.
If i do need something higher than R30...what's the consequences of being below that R value? I'm leaning towards pellet as primary heat source with radiant propane as backup.
I'll have to send an email to the BI and see if she is going to "require" specific insulation values or not.
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Nate R
Member
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# Posted: 3 Feb 2021 08:19am
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Quoting: ICC FWIW the WI energy code calls for R49 in a ceiling with some special less stringent requirements for insulation that is applied on top of the roof deck and the finished roof treatment on top of that.
FWIW Wisconsin is still under the 2009 IECC, and allows for REScheck Total UA as an alternative. So one can use less than R49 in the ceiling IF they go through the ResCheck software calciulations and show total UA (insulation value) for the entire building would meet code.
So you can use say R30 in the ceiling IF you make up for it elsewhere in the walls, floor, better windows, etc. This is what I did in my project.
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Nate R
Member
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# Posted: 3 Feb 2021 08:23am
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Quoting: WILL1E I'll have to send an email to the BI and see if she is going to "require" specific insulation values or not.
Look into the RESCheck software. It's free from the gov't, and can be used to calculate your total building energy use. You have to do heat loss calculations anyway for the WI building permit, so you might as well. This allows you to figure out if the whole building meets code... ie: The R30 in the ceiling (being a bit less than prescriptive code) is made up for the R20 in the floor, etc.
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WILL1E
Moderator
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# Posted: 3 Feb 2021 08:30am
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Quoting: Nate R You have to do heat loss calculations
This is why i emailed her...in my early discussions with her she said because of size and that it was going to be a dry cabin that i wouldn't have to do this. So that's why i'm reconfirming (in writing).
Regardless, i'm going to check out the calculator/software.
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WILL1E
Moderator
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# Posted: 3 Feb 2021 08:37am
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Just in from the BI...no insulation minimums or calculations required!! Man i need to get this cabin built before she retires 
Her only caution (in her 1 sentence reply) was to think of the future if i decided to add plumbing.
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Nate R
Member
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# Posted: 3 Feb 2021 09:49am
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Quoting: WILL1E Man i need to get this cabin built before she retires
Nice! Yeah, good on no calcs!
This was a concern I had too, esp since my original building permit expires next month...I didn't want to start over with a new BI.
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gcrank1
Member
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# Posted: 3 Feb 2021 10:46am
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Youve got a gem of a BI there!
Back about 7 years ago in the fall my '83 first build needed reroofing. We ripped off the cheap 25yr asphalt shingles and had to replace 2 sheets of cxd roof deck; that was a surprise and the long trip there and back to the yard then the install messed up our timing....storm coming. The friend roofer guy had a roll of some kind of weatherproof sheeting that he said they used in Florida that holds up to wind and all for months so we stapled it down well (didnt use any wood stripping on the edges which is advisable) and figured to be back in a week. Didnt happen...and that week turned into a month and winter came early. That product held up All Winter Long and I only had one corner pull up in wind but caught it to retack before more wind could possibly get under and rip it up. I was impressed by the product.
Doing a metal roof now I am strongly leaning toward:
1)plywood roof deck over rafters (insul between rafters later).
If I wanted a nice looking exposed ceiling (no insul between rafters) Id use boxcar siding applied from above as the deck with rigid styro over that. I Do Not like working overhead! Btw, if doing exposed make sure you like the look from below of the rafters you use.
2) apply my above mentioned product atop the deck
3) 1x2 nailers up each rafter eve to peak
4) horizontal perlins
5) metal roofing
6) venting ridge cap
No.4-6 are to provide that passive convection cooling air flow in summer.
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willywilly2020
Member
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# Posted: 3 Feb 2021 12:36pm
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Depends on how beefy your rafters are.
I have shed roof, 4/12 pitch, 2x12 rafters. I used batt insulation only (with vent holes at both ends) and there's plenty of ventilation space between insulation and ply.
Some folks use less insulation if their cabins are tiny, but frankly, I think it's less about staying warm in the winter and more about staying cool in the summer.
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WILL1E
Moderator
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# Posted: 26 May 2021 01:54pm
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I assume this method same method works for a gable style roof with the exception that you put in a ridge vent?
Outside to inside: metal panels, roof underlayment, sheathing, foam trays, batt insulation and vapor barrier...some ceiling finish at some point later on.
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Brettny
Member
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# Posted: 26 May 2021 02:07pm
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Gambrel style roof? Should be about the same.
In my own house I have found that a ridge vent really dosnt work so well with a cathedral style ceiling. If I was to do it over(and I may be forced to) il be spray foaming the bottom of the sheething. Air leaks are really a worst nightmare when you have a metal roof. Useing plywood sheething can help this a bit but how much are you saving over perlins and spray foam? Your also left with a superior insulation that wont have air leaking issues.
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Upper
Member
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# Posted: 9 May 2022 06:03pm
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To bring up am old thread and comfirm,I plan on 2x8 rafters,one inch spray on closed cell foam,and fil the rest with pink craft faced batts. 12on 12 pitch straight gable, no eve soffit.
What do you all think,This will be in central Idaho at 6k feet.
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gcrank1
Member
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# Posted: 9 May 2022 09:47pm
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Ime, batts make fine housing for insects, vermin, mice and the Devil Red Pine Squirrels that I have declared war on.
It also gets soggy wet if there is a leak and probably loses insulation value.
I like dense cell foam but havent bought any recently; maybe I cant afford it?
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