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kvjeff73
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# Posted: 3 Jan 2022 10:23am
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I have a 14' x 40' Premier lofted barn cabin up here in MN. I have the walls insulated/finished, and started on the ceiling by installing fiberglass batts.
I've since came across info that is making me worry about ventilation and moisture in the ceiling. I did not install baffles between the roof deck and the insulation. My cabin-neighbor tells not to insulate all the way up to the peak and that will be fine as far as air flow/moisture escaping.
Before I go any further, should I start over and install baffles? Or will the ribs on the metal roof provide enough cooling that I don't need to worry about it? I've been getting conflicting info from different sources.
It has 2x4 studs/rafters. Metal roof. I'm more worried about moisture/rotting than heating efficiency. Any help would be appriciated.
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travellerw
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# Posted: 3 Jan 2022 11:35am - Edited by: travellerw
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I don't think you will get a definitive answer! I can only offer the advice as if it was my cabin...
1. Roofs need to breath.
2. Air should not pass through insulation
So in my opinion, you need to ensure you have a vent channel that is separated from your insulation! Now since you only have 2X4 studs that doesn't leave a lot of room to make a channel. There is a youtube video (link below) of a couple with a similar build that didn't do any venting. They lived in the cabin full-time though. After 1 year they had mould issues and had to take everything down. They opted to just use "Bubble-Foil" to create the channels. However, they decided not to add more insulation below the bubble foil and instead rely just on its insulating properties. If you read the comments, you will see that didn't work out well.
How to Vent a Gambrel Roof Using Reflectix
So if it was me. I would probably extend the Rafters with 2x2s, then create a 1.5' channel using bubble material (ensure you have a ridge vent). On top of that I would install standard batts for 2X4s. It wouldn't add that much work or expense and would allow you to sleep better at night.
(I would probably just use standard bubble wrap packaging material as "Bubble-Foil" is expensive and has debatable benefits).
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Brettny
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# Posted: 3 Jan 2022 11:39am
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Rafter height? Roof type? Sheething type? Many questions to awnser before anyone can say.
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hinterlander
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# Posted: 30 Oct 2023 09:16am
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Im in the same boat.. Im doing baffles and possibly cutting small 1"x4" slits along the length of the roof under the very small over-hang. I can feel air coming through without the slit though, so Im still deciding if its even necessary. I am a bit concerned about snow drifting in..
How is your situation going now that almost a year has passed since your post?
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hinterlander
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# Posted: 30 Oct 2023 09:18am
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Hey Brett,
12' at the peak, 8' walls. Gambrel roof. Plywood sheething.
Im in the same situation as kvjeff73, so any tips would be great man. Thanks
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gcrank1
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# Posted: 30 Oct 2023 10:49am - Edited by: gcrank1
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Im no expert and made at least my share of mistakes, so I try to pay attention and do better.
To prevent condensation the underside of the roof has to be the same temp as the outside. Imo the min. space from the metal roofing to a sheathed And 'weather-shielded' decking is furring strips.
Air cant freely flow out the ridge, or eve, vents if there isnt somewhat equal air inlet on the low end. There are formula's, probably even online calculators. Ime going 'more square inches' is better than the bare min.
Its easier to do it all as you build than retrofit (btdt  )
I also like the 'rainwall concept' for exterior walls; similar concept; ie, let it breath.
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Brettny
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# Posted: 30 Oct 2023 11:58am
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2x4 rafters just dont leave enough room for everything that needs to fit above it. Even 2x8 wont work. fiberglass alone is just a poor choice for this type of ceiling. It has far to many air leaks and dosnt have a great R value per inch. You should really be useing foam.
I have a gambrel roof made from 2x10s with proper metal roof underlayment. We are going to do 3-4in of poly iso foam and spray foam all edges. Then use R21 fiberglass to fill up the rest of the 2x10 depth.
Technically with 3in of foam and 5.5in of fiberglass il have a 1in air gap above it but il be foaming that gap also. I dont have a ridge cap that's vented and have had poor luck with ridge vents. For instance they dont work with snow on them.
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Cedar Fever
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# Posted: 30 Oct 2023 12:24pm
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I took the roof off and plywood/ISO foamboard/ plywood and put the roof back on. Has worked great.
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travellerw
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# Posted: 30 Oct 2023 11:50pm
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IDK.. Our Gambrel is 2X8 with a 1" air channel made with 3/4 foam (1.75" total depth), followed by pink fiberglass. We have open soffits and a ridge vent.
We are entering winter number 3 and its amazing. We heat with our heat pump down to about -15C about use the wood stove below that. In the summer we run the heat pump in AC mode and easily keep the interior at 20C with our solar installation (at +35C). The wood stove easily keeps the interior at +20 when its -40C outside (based on at least 5 weekends at that temp).
We have never had an issue with the ridge vent being blocked by snow (and we get tons). The snow seemed to slump away and let enough air through. However, I think the venting is more for summer to keep the roof deck from overheating and dry rotting (no snow issues then).
However, with all that said, our cabin technically doesn't meet local code as we don't have enough R value in the roof. R49 is required here as minimum for constant dwellings. Its not a constant dwelling and we heat with wood in the coldest temps so its not a problem (we have 160 acres of bush, so unlimited wood).
TLDR: An air channel is required from the soffit to the ridge (spray foam may be viable, but I know nothing about it). 2X8 construction is minimum for an air channel and enough space for insulation. Follow local code!
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Brettny
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# Posted: 31 Oct 2023 06:19am
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Have you ever looked for mold at the peak?
Adding more heat for poor insulation does help the comfort factor but it dosnt help air leaks and air leaks can cause mold.
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hinterlander
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# Posted: 31 Oct 2023 08:19am
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Ive got 2x6 rafters. I dont really think its necessary to extend them with 1x2's or 2x2's, but I guess there is a chance the batts could compress my baffles a bit.. I'll cross that bridge when it comes. Whats your opinion?
Also, how specifically did you do your soffits??
Thanks!
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gcrank1
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# Posted: 31 Oct 2023 11:01am
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For a cabin/cottage Im inclined to go for adequate, even more than just adequate ventilation beneath the metal then use whatever the best R-Value insul will fit in the rest of the depth. Iirc loose, fluffy insul has less R when compressed, thus less $ value too.
Playing with the numbers Id look at the option of adding some 2x2s to see if that is cost effective to increase RV.
Note: for 30+ years at our old 380ish sf cabin we had No insul in the cath ceiling, just a small antique wood stove and a woods full of dead/dying oaks. Whenever we had the inclination we would use it in the winter even if we had to snowshoe and manhaul in. That stove fired quickly and we stayed plenty warm, down to t-shirts often even in below 0*f.
Had a little condensation dripping off the exposed tips of the roofing nails through the 1/2" CDX though.
Those were 2x6 rafters that I had intended to insul between, then lost my 'roundtuit'.....
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Brettny
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# Posted: 31 Oct 2023 01:02pm
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Your actualy better off with no insulation vs leaky not enough insulation. With no insulation things can dry.
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geobuild
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# Posted: 31 Oct 2023 02:57pm
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Take a look at the smart vapor barriers such as Intello Plus. It was recommended to me by an architect who was using it in his own home he’s building near me in the northern Adirondacks.
My situation is an Amish-built 14x24 shed/cabin (gable ends) with 2x6 walls and rafters. There’s a metal roof over ¾†plywood. The walls are 7’6†with collar ties at 9’ and 11’6†peak. I insulated walls and ceiling with Rockwool up the catherdral ceiling and across the collar ties.
Planned use is 10-12 weeks from April to December and a couple of weeks Jan/Feb/Mar.
To vent the ceiling and get in 5.5†of Rockwool insulation meant firring strips on the underside of the rafters, soffit vents and air channels. Using the Intello Plus, I was told I would not have to vent the roof.
We’re heading into our second winter and so far, I’ve very happy with results. Only time will tell.
Has anyone else heard about Intello and/or used it?
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snobdds
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# Posted: 31 Oct 2023 05:07pm
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I just insulated my cathedral celling this summer. He's what I did.
I have 2x10 rafters.
I drilled 3 1 inch holes in the blocking between the rafters above the top plate. Put in wire mesh to keep the bugs out.
Put in 1x2 strapping on the sides of the rafters to create a gap for air to travel from eve to peak.
Fit in 2 inch foam, under the air gap.
Fit in prodex insulation the entire length of the rafter bay to create a air tight channel. Extended it 2 inches down each side of the rafter.
I can go outside and hold a feather by one of the holes in the eaves and you can see the feather bend towards the hole, thus confirming air is moving through the roof.
It was a pain and it took forever to do. Very tedious. However during hunting season when it snowed, the roof was still covered in snow, showing heat did not make it's way to the roof. It worked.
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travellerw
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# Posted: 31 Oct 2023 05:09pm
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Quoting: Brettny Have you ever looked for mold at the peak? Adding more heat for poor insulation does help the comfort factor but it dosnt help air leaks and air leaks can cause mold.
It would be difficult to check as I would need to go up top and actually take the ridge cap off, then remove the snow filters to look down the hole.
However, I'm pretty confident in how it was built. I was super anal with the vapor barrier (even taped the staples). Any penetration was done with a poly hat and accoustical sealant was used for wire penetrations.
Quoting: hinterlander Also, how specifically did you do your soffits??
They weren't difficult, I just made little triangles out of 2X4 and then nailed them to the rafters. Then just used standard soffit material from HD to close them in . I tried to find a better pic, but this is all I could find. It does give the idea though.
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travellerw
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# Posted: 31 Oct 2023 05:13pm - Edited by: travellerw
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Wait.. I found some better one that show it really well.
IMG_20210912_1248199.jpg
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IMG_20210912_1248339.jpg
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Brettny
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# Posted: 31 Oct 2023 09:22pm
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Quoting: snobdds drilled 3 1 inch holes in the blocking between the rafters above the top plate. Put in wire mesh to keep the bugs out. Put in 1x2 strapping
Theres a spec on this ventilation opening. It's very roof specific but I dont think 3 1in openings would be enough. I did 2 1.25in openings in 7ft of cathedral with baffles and it didnt work. This isnt even with a metal roof. Its shingles with plywood.
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hinterlander
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# Posted: 1 Nov 2023 08:18am
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Quoting: geobuild Using the Intello Plus, I was told I would not have to vent the roof.
Did the Architect tell you that? What happens to all the ambient moisture with no venting?
Quoting: geobuild Has anyone else heard about Intello and/or used it?
Yes! I intend to use that myself but I was told it wasn't needed on the ceiling/roof, only walls haha.. Depends who you ask eh?
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hinterlander
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# Posted: 1 Nov 2023 08:26am
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Quoting: snobdds I just insulated my cathedral celling this summer.
So do you have the "Gambrel" roof with the barn look then? How much over-hang is there at your angle where you did your soffit?
Quoting: snobdds I drilled 3 1 inch holes in the blocking between the rafters above the top plate. Put in wire mesh to keep the bugs out.
Could you explain this further please? Not sure what you mean by "blocking".
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hinterlander
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# Posted: 1 Nov 2023 08:31am
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Quoting: travellerw Wait.. I found some better one that show it really well.
That looks fantastic.. mine came already built, so I dont have that at the bottom edge of my roof. It just goes straight down the exterior wall, basically (with a tiny bit of overhang to a rain gutter). What is your opinion on how to soffit vent a roof like mine?
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gcrank1
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# Posted: 1 Nov 2023 08:59am
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You dont really have a soffit on a build like that.
My pre-built is similar and has only a strip of angle cut maybe 2x4 up under there. Perhaps drilling A LOT of holes between the rafter tails would get some flow going, pulling that out would for sure. No idea how you would 'screen' that....
Thing is that those structures are not made to vent other than the puny little gable end vents mine has (size is a joke).
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travellerw
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# Posted: 1 Nov 2023 12:26pm
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Quoting: hinterlander That looks fantastic.. mine came already built, so I dont have that at the bottom edge of my roof. It just goes straight down the exterior wall, basically (with a tiny bit of overhang to a rain gutter). What is your opinion on how to soffit vent a roof like mine?
Unfortunately the right answer would be to modify the building so it has the kickouts and soffits. I know its a ton of work, but it would be the proper way. It has other advantages too as it kicks rain and snow away from the foundation.
Failing that, I'm not sure. You could try what Gcrank1 suggested but I think it would be pretty limited venting.
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ICC
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# Posted: 1 Nov 2023 02:52pm - Edited by: ICC
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FYI, code based on the IRC specifies 1 sq. ft. of vent space per 150 sq. ft. of floor space. That is meant to be divided in two; one half upper (ridge or gable end) and one half lower (soffits). The venting should be spread around as evenly as possible.
So, a 14x40 building = 560 sq ft but is actually less inside (maybe 525 sq ft?). Divide 525 by 150 = 3.5 sq ft total vent space. 144 sq in x 3.5 = 504 sq in. of venting total or 252 sq in for soffits.
A 1" dia hole has an area of only 0.785 sq inches. A 1-1/4" hole has an area of 1.227 sq in.
252 ÷ 1.25 = 201.6. Is there room for about 200 of the 2-1/4" holes along the eves? And remember that screening reduces the open area. Inspectors/code account for that.
-izzy
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gcrank1
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# Posted: 1 Nov 2023 03:14pm
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Amazing stuff there Izzy 
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snobdds
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# Posted: 3 Nov 2023 12:58pm
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Quoting: Brettny Theres a spec on this ventilation opening. It's very roof specific but I dont think 3 1in openings would be enough. I did 2 1.25in openings in 7ft of cathedral with baffles and it didnt work. This isnt even with a metal roof. Its shingles with plywood.
That is for each rafter bay. 3 one inch holes is 20% of a 14.5 inch bay.
It's plenty.
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snobdds
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# Posted: 3 Nov 2023 01:43pm
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Quoting: hinterlander Could you explain this further please? Not sure what you mean by "blocking".
The blocking is the piece between top of the top plate and the bottom of the roof sheathing.
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ICC
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# Posted: 3 Nov 2023 02:40pm - Edited by: ICC
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Quoting: snobdds 3 one inch holes is 20% of a 14.5 inch
Measured linearly that is correct.
However, is 20% of a straight line meaningful when one is considering ventilation area, a 2 dimensional measurement?
-izzy
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snobdds
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# Posted: 3 Nov 2023 04:10pm - Edited by: snobdds
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Quoting: ICC However, is 20% of a straight line meaningful when one is considering ventilation area, a 2 dimensional measurement?
Yes.
Don't confuse venting an entire attic with venting a roof deck of a cathedral ceiling 
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gcrank1
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# Posted: 3 Nov 2023 07:09pm - Edited by: gcrank1
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What we are venting is only the sq.ft. of air space beneath the roof and above 'deck' below, whether it be a 1" gap or 3", etc.
So we do the calc's for that space and the air in/air out sizes.
Right?
And there is no downside to having more than the calc min?
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