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bobbotron
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# Posted: 21 Aug 2012 09:42am
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So, we have our metal roof on order for our cabin. It's a 12/12 roof with a cathedral ceiling. I was wondering what I should do about ventilation. Eventually it will be insulated, but for now it wont be. The roof is strapped, with plywood at the top. After I ordered a normal ridge cap, I found out vented ridge caps also exist.... There's a window on the loft side, and I was planning on putting a vent in on the other gable end, but I'm not sure how this works with insulation with regards to ventilation.
Also, for the last strip of metal and the ridge cap, what are good tricks to get them on? Lay a ladder onto the roof, and secure it at the bottom?
Here's a photo of where it's at:
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VTweekender
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# Posted: 21 Aug 2012 10:10am - Edited by: VTweekender
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Just my opinion, others may have a different outlook.
Using the purlins method will cause problems in the long run with putting insulation underneath, as the metal will sweat and fill the insulation with moisture, leaks and mold problems eventually.. OK for open rafter barns and garages ....
I would stop right there and take off the purlins and plywood or OSB the roof then add 1" sheets of insulation board, screw the metal roof flush to that...there will be minimum moisture able to build up underneath if any at all, and any that does will find the groove in the metal roof and run down.
Tie the top of your ladder with a rope, throw the rope over the roof to the other side and tie it to the bumper of your truck., pull truck forward until the ladder is up to the top of the roof.
Your cabin looks great!! Congrats!!
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TheCabinCalls
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# Posted: 21 Aug 2012 10:13am
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If you use spray foam or rigid foam you may not need to vent... Depending on your area and the required depth.
Someone here might see a simple and cheap way to vent it now with the ability to add insulation later.
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bobbotron
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# Posted: 21 Aug 2012 10:37am
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Ugh, I'd really rather not take off the strapping, but something to consider for sure. If I could make this work somehow, that would be way preferable.
I've been reading around on this stuff, insulating a cathedral ceiling is a real pain!
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MtnDon
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# Posted: 21 Aug 2012 10:43am
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It would have been better to ask this before starting the roof. Metal over purlins will cause condensation issues as was mentioned. Sheathing with the weather resistant barrier and then the metal eliminates that issue. Sheathing the rafters with OSB or plywood also makes for a much stronger roof. If you do insist on metal over purlins there are 2 sure fire methods of avoiding condensation. One is to have foam insulation sprayed on the metal underside. It needs to be thick enough to keep the inside face of the foam warm enough to prevent condensing. Spray foam is also very good for cathedral ceilings as quite often the rafter size does not permit proper insulation. This method eliminates ventilation altogether.
The second way is to apply foam sheets to the topside of the sheathing, as was mentioned. That too should be thick enough to properly insulate. That varies with climate. Here too, the roof will be earm enough on the inside to be able to be unvented.
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bobbotron
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# Posted: 21 Aug 2012 11:10am
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How about putting two vapour barriers down, one above the insulation, and one below? I've read about this in a fine homebuilding article on cathedral ceilings.
I've also read that spray foam is a good way to go, we might go that route.
I'm not worried about reaching a high R value - it's a very small building, and will heat easily with a wood stove. The idea of keeping it uninsulated for some time to avoid these insulation headaches is quite tempting.
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GomerPile
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# Posted: 21 Aug 2012 11:24am - Edited by: GomerPile
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Be super careful with internet advice on vapor barriers. There is a *ton* of really bad advice out there.
Vapor barriers should only be on one side of insulation to let any accumulated moisture escape. Failure to do this means you start a mold farm.
Your best bet is a few inches of spray foam on the inside and/or foam sheeting on the outside. You could do a hybrid approach with an inch or so of foam inside to seal up the underside of the roof and then apply a couple inches of foam outside. Spray foam is $$$ so the hybrid idea might save you a bit.
You want to use enough foam so that the dew point is within the foam...this almost eliminated condensation issues.
EDIT: Just to be clear. There should be only one impermiable vapor barrier. Most people install a semi-permiable vapor barrier on the interior which allows moisture to escape. Many people are using a special paint applied to gymsum board.
Quoting: bobbotron How about putting two vapour barriers down, one above the insulation, and one below? I've read about this in a fine homebuilding article on cathedral ceilings. I've also read that spray foam is a good way to go, we might go that route. I'm not worried about reaching a high R value - it's a very small building, and will heat easily with a wood stove. The idea of keeping it uninsulated for some time to avoid these insulation headaches is quite tempting.
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bobbotron
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# Posted: 21 Aug 2012 12:15pm
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That's fair. The nice thing is, the roof is pretty small, so even the $$$ spray foam solution wouldn't be too much cash.
Perhaps I'll invest in a large tarp while I decide what to do...
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TheCabinCalls
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# Posted: 21 Aug 2012 12:50pm
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We used spray foam and didn't vent the roof. We couldn't be happier. The temp of building has never been over 80 degrees and we don't have A/C.
We sprayed 3" and filled the rest of the cavity with batts. The temp of upstairs is the exact temp as downstairs. Unlike our house built in the 60's which is 10 degrees warmer.
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bobbotron
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# Posted: 21 Aug 2012 01:10pm
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Quoting: TheCabinCalls We used spray foam and didn't vent the roof. We couldn't be happier. The temp of building has never been over 80 degrees and we don't have A/C. We sprayed 3" and filled the rest of the cavity with batts. The temp of upstairs is the exact temp as downstairs. Unlike our house built in the 60's which is 10 degrees warmer.
Very interesting. Did you use sheathing or purlins for your roof? Did you do the spray yourselves? How long has the roof been up?
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bobbotron
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# Posted: 21 Aug 2012 03:14pm
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I guess I should mention, the reason we originally went with purlins was for safety while working up there. They make a great ladder to "walk" up and down. What a pain to find out we might have to redo it. Better planning next time, hopefully someone else will read this thread in the planning stages and not repeat my mistake!... 
I think we're probably going to go with uninsulated for now, on purlins and plan to spray at a later date when we're feeling spendy. Replacing all of the purlins with plywood/OSB sounds like quite the effort, and we have unfortunately run out of vacation time for the main build effort.
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HopefulHomemaker
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# Posted: 4 Sep 2012 07:31pm
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Ok, so let me get this straight (same issue, so not a hijack, I promise). First OSB, then blue board, then attach the corrugated metal roofing straight onto that? No Tyvek or anything? And there won't be any mold or rot issues with this method?
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Malamute
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# Posted: 4 Sep 2012 09:54pm
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Quoting: bobbotron ...I guess I should mention, the reason we originally went with purlins was for safety while working up there. They make a great ladder to "walk" up and down. What a pain to find out we might have to redo it....
You can also sheath it with sheet stock (plywood/OSB)and nail or screw cleats to it to stand on while you work above, going up.
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MtnDon
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# Posted: 4 Sep 2012 10:00pm - Edited by: MtnDon
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Quoting: HopefulHomemaker First OSB, then blue board, then attach the corrugated metal roofing straight onto that? No Tyvek or anything?
Not quite so quickly with that.... First you need another layer of OSB (usual stuff used) on top of the rigid foam board. (...and the rigid foam should be in 2 layers with overlapping seams for best insulation results. That is held in place with a few long screws into the first OSB layer. Nylon disks are used to help keep the foam down tight and not have the screw heads pull through. A second upper layer of OSB is usually used to provide a surface to secure the roofing material, metal, shingles, whatever. That OSB is secured to the rafters with screws long enough to penetrate. The hard part is lining them up and driving them straight to hit the studs. Then a layer of roofing paper or synthetic underlay if you prefer. Then the final roofing material. Takes time and patience and completing a lot more details (like a fascia for the eve edge of all that foam, etc. It works but is not as simple as building a "traditional" style roof.
Have a look at buildingscience.com and search their site for something like "foam insulation on roof". Thay have lots of info if you look for it. Many different approaches too and they are different for different climate zones. Lots of PDF files with drawings.
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bobbotron
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# Posted: 5 Sep 2012 02:04pm
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Quoting: HopefulHomemaker Ok, so let me get this straight (same issue, so not a hijack, I promise). First OSB, then blue board, then attach the corrugated metal roofing straight onto that? No Tyvek or anything? And there won't be any mold or rot issues with this method?
Are you building from scratch, or insulating a new roof? There are some good options for insulating listed here.
http://www.finehomebuilding.com/how-to/articles/insulating-a-cathedral-ceiling.aspx?a c=fp
http://www.greenbuildingadvisor.com/blogs/dept/musings/creating-conditioned-attic
To be honest, I'm probably never building another cathedral ceiling as long as I live. The look is nice, but working that high up is awful. If I did it over again, I'd either do a proper two story, or something one story with a much lower pitched roof, normally insulated ceiling and possibly 10 foot ceilings to get some of that nice grand room feeling.
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cabinbiscuits
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# Posted: 6 Sep 2012 10:46am - Edited by: cabinbiscuits
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This problem did not occur to me until i read this and my roof plan is the same- purlins with a metal roof and then I was going to use unfaced Fiberglas between the roof trusses underneath the metal between the trusses.
I was wondering, does it make any difference at all that the vast majority of the time the building will not have heat or air conditioning on like a house would?
Also would it be helpful to run a plastic membrane across the bottom of the trusses (to stop air and moisture infiltration into the insulation batts and to the inside bottom surface of the metal roofing) prior to putting up the interior finish panels (be they wood or drywall). Or would the plastic sheet just trap moisture and cause more problems like people did in their basements years ago?
I can see the foam being the way to go, but man that stuff is expensive
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MtnDon
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# Posted: 6 Sep 2012 01:01pm
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Quoting: cabinbiscuits I was wondering, does it make any difference at all that the vast majority of the time the building will not have heat or air conditioning on like a house would?
The barn outside has a metal roof on purlins. It is not heated or cooled. Moisture condenses and drips on the underside at times.
Applying plastic or roofing paper or a membrane of some kind under the metal can help with that, until the water finds a hole. Life seems to be full of things that go wrong. I have enough tasks to do without using techniques that may cause me problems later. That's me and my outlook; might not be everyone's.
Quoting: cabinbiscuits plastic membrane across the bottom of the trusses (to stop air and moisture infiltration into the insulation batts
In most climates this is a good idea. May not be a good idea in hot and moist climates, especially with A/C in the rooms underneath.
Quoting: cabinbiscuits roof plan is the same- purlins with a metal roof
If it is still just in the planning stage it might be simpler and give you a roof with more resistance to wind forces to skip the purlins and sheath with OSB, then the weather resistant barrier and the metal roofing. You still need to sort out the best insulation method but at least the roof won't be collecting condensate. Just a thought.
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cabinbiscuits
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# Posted: 6 Sep 2012 01:30pm
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Unfortunately I just bought a skid of 36 2x4x10's at a building material auction this past weekend to use for my purlins. And I got a great deal on clearance rolls of unfaced R-19 insulation at $7 and change a roll a few weeks back. While I can return the insulation if I wanted, I'm stuck with the wood.
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beachman
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# Posted: 6 Sep 2012 01:30pm
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Just a thought - I built a fairly steep cathedral ceiling on our cabin and attached boards to the top of the 2 x 8 rafters. On each side of the inside top of each rafter, I put 1 1/2" strapping then nailed 1/2" sheathing strips - about 14 1/2" wide up each rafter to form an air space of about 1 1/2" on top. The steel roofing was attached directly to the boards above and each rafter space was insulated with fibreglass batts and then vapor barrier applied on the inside. Then, pine T&G boards were nailed over the vapor barrier on the inside up the length of the roof. I have insulation and a free airspace above the insulation so no condensation problems. A ton of work but worth it in the long run. Instead of the strapping and 1/2" sheathing, you could opt for those styrofoam spacers.
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cabinbiscuits
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# Posted: 6 Sep 2012 02:40pm
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5/8 OSB ok for trusses on 24 centers? What about if I switched to asphalt shingles instead of metal, would that solve the problem and allow me to put the Fiberglas right up against the bottom side of the OSB?
bobbotron- Sorry I'm not intending to hijack your thread, but I plan on heading up to camp to set trusses tomorrow evening and I apparently need to change some of my plans and materials quickly.
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bobbotron
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# Posted: 6 Sep 2012 02:49pm
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Quoting: cabinbiscuits bobbotron- Sorry I'm not intending to hijack your thread, but I plan on heading up to camp to set trusses tomorrow evening and I apparently need to change some of my plans and materials quickly.
It's all good, I'm quite a thread jacker and it is on topic, so it's doubly good. 
Check out those links I put up above. I can't comment too much on what the best material is for decking, with strapping vs OSB etc. But from what I've read (in those links above and elsewhere, you will mostly likely want a ventilation channel from soffit to ridge if you're doing fiberglass.
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cabinbiscuits
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# Posted: 6 Sep 2012 03:33pm - Edited by: cabinbiscuits
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I tried the two links last night. I read the second one, and the first one seemed to be for members only.
I agree on the ventilation channel, but unfortunately in my case I don't have a soffit to vent from. I'm doing a gambrel roof and the bottom of the roof will be right over the gutters.
I talked to an engineer from one of the metal roofing companies who agreed with my idea to use plastic on the bottom of my 2x6's to help reduce the amount of moisture that makes it into the Fiberglas in the first place.
At this point since I can still make some changes I'd like to get more input from these guys and girls on here before I commit to going that way though.
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bobbotron
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# Posted: 6 Sep 2012 04:13pm
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Maybe plan on using a good few inches of spray foam? The stuff is crazy expensive, but would solve your ventilation problems.
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cabinbiscuits
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# Posted: 6 Sep 2012 05:51pm
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The thing about the foam is the expense. If I am figuring right I have about 600 sqft of roof space which equates to $550 or so per inch to put foam on the bottom of the roof myself. That's really high for me to swallow. Maybe I could do like TheCabinCalls, and put Fiberglas over foam but there's no way I can afford to put three inches of foam on there. Not sure if it would work the same way with only one inch of foam.
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HopefulHomemaker
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# Posted: 13 Sep 2012 02:16pm
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As I'm reading the articles I've been directed to, I keep reading that certain kinds of vents are not compatible with certain kinds of insulation. Our cabin (which we are heading up to build starting this weekend) has ridge and soffit vents. So will the method described by MtnDon a few posts back still work for us? The OSB-foam board-OSB-wrap-roof method?
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MtnDon
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# Posted: 13 Sep 2012 11:02pm
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If you use several inches (climate dependent) on top of the base roof sheathing you then need no venting under that roof sheathing and insulation. Have a look at buildingscience.com and do a search on their site for insulation and roofs. They have lots of info and they look at it differently for different climate zones. Unlike many parts of constructing a building the insulating and use of vapor barriers changes with climate.
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HopefulHomemaker
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# Posted: 14 Sep 2012 10:10am
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The trouble is, I'm using a set of plans that I'd rather not deviate from much. Being inexperienced builders, I feel it's better for us to stick to the plans as much as possible. We need to insulate the roof more than the plans say because they're for a recreational cabin, not a full time residence. But I don't want to add or take away anything if I don't absolutely have to, because we might mess something up if we do.
So, speaking to me like a building pre-schooler (because I am), can we use the board/foam/board/wrap/roof method with ridge and soffit vents, or will we mess things up somehow if we do? I would absolutely go searching on that website on my own if a) I could find a simple yes or no answer to the question, b) or I could find an article saying "do exactly this:...", or c) I was not already at my building site today, staring at a pile of lumber that I have to start playing cabin Tetris with this very day. I really need help. We might not even get to the roof this week, but we're hoping to, and I want to make sure we don't screw up our house.
My husband should have been doing this research starting months ago but, alas, he did not. So here I am.
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MtnDon
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# Posted: 14 Sep 2012 01:06pm - Edited by: MtnDon
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You are correct in being cautious about deviating from a plan. Some deviations from a plan are simple and do not affect much else in any serious way. Others changes have impacts that are not desirable.
If you place all the insulation on top of the roof sheathing as described and if soffit and ridge vents are installed there will be unintended consequences. I assume you intend to put some sort of interior skin (drywall, T&G boards, or whatever) on the underside of the rafters as in a typical cathedral ceiling. That will result in exterior air, hot or cold, circulating through the now empty uninsulated spaces between the rafters. That negates the value of all that added roof insulation. So in that case I would not install the roof venting.
It's difficult to say what the best approach is without knowing more about the plans and what size materials are called for for the rafters, for example. The local climate also influences insulation requirements as well as the vapor barrier. These two items change as the climate zone location changes. What's good for ND is bad for MS.
Off hand I can not say what is best for you. That would require more details about everything. You could start a discussion topic specific to your project and go through it all step by step before lifting a shovel or hammer. That would be best in view of your self stated inexperience. However it sounds like you are up against a time crunch. I don't know what else to say at this point. I will say it is easier to make the changes to the plans now than try to accommodate changes/wants/needs after the construction has begun. I'm willing to pass along whatever knowledge I can but I'm not here full time and usually disappear for three day weekends.
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Greyhawke
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# Posted: 22 Sep 2012 04:21pm
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botbotron, I have read all of the replies and agree except for one thing. If you take off the purloins and install 1/2 in. plywood sheathing, then put down either 15 lb. or 30 lb. felt which acts as a vapor barrier, then then reinstall the purloins and put the metal roofing on, you will be ok. I did that years ago and have had no problems, and I live in Central Fla. High humidity, heat, etc. Or you can leave the purloins (furring strips) on and put the plywood on them and then put on the metal roofing on new purloins. Later, you can put insulation on the inside of the cabin. This will buy you time and cost a heck of a lot less. I speak with experience. It works.
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MtnDon
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# Posted: 22 Sep 2012 05:09pm
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Why the purlins when there is sheathing and weather resistant barrier? Makes no sense. Horizontal purlins block any venting and draining of any water if that's the idea. And the roof metal is still susceptible to damage from too heavy a roofer walking on it and stepping between purlins. Plus hail damage is more noticable than with roof metal flat on the decking / weather resistant barrier. ??? They simply are not needed when the roof deck is sheathed.
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