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ALwoodsman
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# Posted: 14 May 2015 13:20
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I am looking for some input on insulating a roof. My plans are to have a metal roof. I will be using 3/4" pine boards for the ceiling. I have seen this done with the boards on top of the rafters and foam insulation used between the board decking and the metal. I would like to use this method so that the rafters will show on the inside of the cabin for a more rustic look but I am worried about the amount of insulation that I would get from the foam. My other option was to use the boards on the bottom of the rafters and use fiberglass insulation. Any suggestions?
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creeky
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# Posted: 14 May 2015 13:56
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ALw, You can get whatever level of insulation on top of the roof you desire. If you go big: specialty fastener stores will have the long screws you will need.
Having done the cool roof both ways. Putting insulation on top of your roof and then strapping. Then the metal roof. This is far far faster and simpler than cutting between rafters.
The thing in Alabama you will like is the cooling factor. You need a 3/4" gap between insulation and metal with a vented ridge.
See my washroom post on small-cabin. There is a schematic there.
For even more detail see my blog. Both the washroom and a later build the chalet use that roofing style. Search chalet or washroom for the relevant posts.
This roof has become very popular. You can google "texas cool roof" to see those imitating my builds. 
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bldginsp
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# Posted: 14 May 2015 14:44
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If you use 'polyiso' rigid foam insulation board, you get R-6 per inch of thickness of the insulation. Whether you have the rafters exposed or not, if you don't have a vented attic space above your insulation you run the risk of getting condensation above the insulation which then causes mold. Warm air inside the cabin holds more moisture than cold air, when the warm moist air works it's way above the insulation it condenses on the cold roofing above. That's why its best to have a 1" air space above the insulation and ridge vent as creeky said, so cross ventilation can get the moisture out. Sometimes people try to prevent the moisture getting past the insulation with a 'moisture barrier' (sheet of plastic) but in practice it sometimes doesn't work. There's no substitute for good old basic ventilation.
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ALwoodsman
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# Posted: 14 May 2015 15:35
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Creeky, I noticed in your drawing that it is showing 1" insulation. Is this enough or do I need to go with 2"? I know 2" will probably be better but it is pretty expensive.
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toyota_mdt_tech
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# Posted: 14 May 2015 16:03 - Edited by: toyota_mdt_tech
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Can you lay your exposed rafters, then install your T&G as planned and then lay rafters again on top of that? IE 2X8 or whatever you need to get the R value plus air movement, then sheet over the top, felt and them tin roof?
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creeky
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# Posted: 14 May 2015 19:49
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1" worked fine for cooling / removing the heat of the sun off the roof. Use the 3/4" pine as your roof sheathing. Put down a layer of roofing felt. Put your insulation down. Put 3/4" strapping over your insulation. Metal roof. Vented ridge. You want to seal the sides to encourage robust air flow. If you have a lot of critters (bats, bees, bugs) you may want to use vented foam or whatever strips at the ridge and at the eave to prevent entry.
For my strapping, I went on 24" centers screwed through the insulation into the rafters. but I live in an area where 60 mile an hour winds are about peak and maybe once a year, plus my building is tucked into a tree line. So I don't have real wind issues.
The tinfoil coated 1" polyiso has been shown to work the best. If you're going more that's all good. More would mean you would be more efficient using a/c or a heater in the winter (do you even get winter?). You could even use 1" xps or something cheaper and then just put the 1" tinfoil polyiso as the top layer.
I don't know how you're using your building, or what you're trying to achieve. Are you cooling with an air conditioner? Or are you just trying to remove the suns heat.
I can say, that with the 1" alone the hottest my washroom gets (and the main wall and 6 windows face south) is 1 degree above ambient (ie. outside temp). I don't try to air condition tho. So I don't know how it would work at keeping the cold in. I only know it keeps the hot out.
Let me tell you, I have put my hand up by the vent and hot air moves robustly out the ridge vent.
Basically with the cool roof you have indoor space. It's a vented area. Exposed beams and wood. Then you have a moisture control layer. Obviously you don't want indoor moisture escaping nor do you want rain water penetrating the living space. Then the insulation. Then vented area above with metal roofing. There shouldn't be any condensation issues. Condensation that forms on the metal roof should be quickly evaporated and removed by the sun heated air.
Ditto inside. Any moisture condensing on the ceiling should be able to evaporate back into the room. I've never seen this happen and we have relatively hot and very humid summers. But hey? Alabama is hotter and I imagine just as humid.
The guys at buildingscience dot com do a better job of explaining this than I can.
It's a great roof tho. easy to build. and more efficient than previous roofs.
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Don_P
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# Posted: 15 May 2015 06:18
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This is another similar way. There are two 2" and one 1" thick layers of foil faced polyiso, seams offset over the tarpapered roof deck. Then 2x strapping screwed thru with long screws, ply, tarpaper, and this one got shingles. I prefer this for metal as well.
biltuproof3.jpg
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biltuproof1.jpg
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creeky
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# Posted: 15 May 2015 08:17
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Going with metal saves you the extra sheathing layer ($), gives you better venting and should make for a longer lasting roof.
Instead of the vertical strapping, run horizontal and metal over top.
It's a great roof. and look how purty inside.
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ALwoodsman
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# Posted: 15 May 2015 09:07
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Yep, we get winter here. Not as bad as most. Lows can get in the teens. We don't get much snow. Summer gets in the upper 90s. I will be running a window ac unit on a generator and heating with a wood stove.
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creeky
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# Posted: 16 May 2015 08:28
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you'll do well with more insulation if you're air conditioning.
Don I see the biltuproof as kind of old school. with the sheathing and tar paper, shingles on the outside like that. how'z it holding up? I'd think with the cooler roof the shingles should last longer?
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Don_P
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# Posted: 16 May 2015 14:31 - Edited by: Don_P
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That's new school to me  . Dad was kind of on the bleeding edge in the early 70's. We would build a post and beam roof system decked with 2-1/4" thick T&G, tarpaper, 2" foil faced polyiso, then shingles nailed directly over that with 3" nails. The shingles on those failed prematurely, the foil cooked them from the backside as well as the sun on the front. This was the retrofit and then the method I prefer, it seems to work fine. I prefer to tarpaper the ply deck and apply metal to that solid deck if it is a metal roof. The metal is gasketed down, no airflow on the underside of it, rather venting through the channels you see with vertical strapping on the underside of the ply sheathing. Metal, non porous, sweats immediately if the surface drops below dew point making liquid water to drip and cause trouble. By running the vent air over the foam and under the ply there is a more stable deck temperature made of an excellent hygric buffer. If it hits the dew point it can absorb some of the vapor before it sweats and drips, then release it as vapor when drying conditions return. Similar to an exposed metal porch roof often sweats at dawn but you rarely if ever see a wood porch ceiling sweat.
The same basic technique can also make a rainscreen wall, say on a post frame building.
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creeky
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# Posted: 17 May 2015 08:51
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You're going to have to help me on the dew point sweating.
So in the "texas cool roof" or creeky's confabulation yer wrapped and then sheathed with polyiso. There's no moisture coming up to the metal from the interior of the cabin. A common source of moisture problems with roofs?
Then if you have vented metal, there is a 1" gap for the venting. Then the outside air.
Okay. So I can understand that on those dew point days the top side of the roof which has lots and lots of square meters of warmer humid air on top of it will attract moisture. But under the metal in that 1" space you don't have a lot of air or moisture to condense.
I could see an exposed porch roof sweating, again, lots of air to carry the water to the roof. But on a vented roof I don't see where the water moisture is coming from.
I'll be watching my metal roof on the washroom this summer to see if I can see any sign of dew point moisture. I know the roof can sweat heavily but I've always assumed the moisture is coming from the top.
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Don_P
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# Posted: 18 May 2015 23:11
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The way I look at it is both surfaces are exosed to the same humid outdoor air. There is the same square footage of exposed metal in the vented bay as outside, the temperature of both faces is the same. There is probably an active draft pulling air over the inside face. I'm making that inside face out of something that will hit the same dewpoint, but it is basically thirsty, dry wood that can absorb a good bit of that condensation, probably a good bit while it is still vapor, and then when the sun heats the roof the moisture in the wood is vaporized and whisked away on the draft.
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creeky
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# Posted: 19 May 2015 10:27
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My first observations on my washroom metal roof with ventilation are: (obs: rh 88%, 6:30 a.m.)
* metal is clearly wet top side with dew
* enough moisture has fallen from the roof to create wet lines on the grass / decking below the roof edge (6" width)
* no sign of moisture on the underside of the metal roofing
I would imagine tho, that a second plywood deck would have structural benefit in larger buildings? While I wonder about durability with the heat load from that metal.
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Don_P
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# Posted: 19 May 2015 23:30
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One more variable needed in your morning observations, tape the outdoor temperature probe to the metal to take its' temperature. Then there a number of dewpoint calculators on the net. See how you react in comparison to the calcs. If you're right the observable condensation is going to happen way below the calculated dew point.
As far as helping with the diaphragm action of the roof I doubt that layer, basically attached to sleepers through foam is contributing much. It is structurally helping by making a continuous flat bed to support the metal, you'll see a plywood spec for some metal guages and patterns, most of the ribbed metals are fine over either.
The times I've seen deteriorated roof decks is because of leaks or really humid indoor air condensing on the underside, unvented appliances, bath fans discharging in the attic, that type of thing. Heat doesn't seem to be an issue as far as degrading wood, up to a point. There are strength corrections deducting as the temps get into and above 150 or so. Regular kilns run about 180*F with high temp softwood kilns going above the boiling point. I ran a high frequency panel gluing machine for a bit, that thing could warm up some wood. Working with both conventional and high temp wood side by side you can see the high temp wood is a little toasted, the sugars are just a little bit caramelized.
Now there was a huge batch of an early blend of fire retardant ply for commercial/multifamily type use. The way this stuff worked is it kind of pyrolized the wood at lower than kindling heat and denied the fire the fuel. Except they found out after a few years that attic temps did it, whoops!
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creeky
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# Posted: 20 May 2015 12:16
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so what you're saying is that the extra layer of plywood is extra money spent. no real benefit.
my local weather report actually gives the dew point. but I'm wayyyyyyy to lazy to climb up and tape a thermometer up there.
why first I'd have to put a new battery in it. which I bought. and i believe is sitting in a drawer somewhere. then I'd have to find the remote thermometer.
all this while I could be listening to the birds sing and the wind rustle through the trees.
hey al, are we getting too far off topic yet?
thx for bringing this up. I'm revising my drawings or adding a rev 2.a anyway. to add the roof felt/water/moisture barrier.
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ALwoodsman
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# Posted: 20 May 2015 12:23
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Yeah,I got a little more than I asked for. What about a radiant barrier like Reflectix? Any experience with it?
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MtnDon
Member
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# Posted: 20 May 2015 13:20
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Quoting: ALwoodsman radiant barrier
Google "FTC radiant barrier". Don't look at the paid ads at the top of the results.
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Don_P
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# Posted: 20 May 2015 23:29 - Edited by: Don_P
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Quoting: creeky so what you're saying is that the extra layer of plywood is extra money spent. no real benefit.
Not at all .
I remembered this pic that might help explain why I go the extra step.
This is the underside of a metal roof on a dewy morning.
There are 3 other wood decked roofs with the roofing sealed down to the deck here,the metal is gasketed down to the deck, outdoor air is in contact with metal above and wood below. There is no condensation on the wood under the same conditions.
I mentioned that some types of metal call for a solid deck. Monday before last a crew of 5 of us put 80 sheets on the main roof of a house destined for fairly high end standing seam metal. They didn't even want plywood clips or buttonkaps under the metal, so we used 5/8 T&G osb and roofing nails for the underlayment. Point there is do check manufacturer specs. Skip sheathing that would have been a loss laborwise. We were also working between rains, It's a temporary factor but speed to cover is something else to consider...solo on mine I can swing a board easier than a sheet of ply, but I lay a solid board deck
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creeky
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# Posted: 21 May 2015 13:40
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Quoting: Don_P underside of a metal roof
Not only is there a lot of condensation on that tin look at the rot forming in the wood. still. that is not a cool roof build. the amount of humid air available to condense is very large.
I have read about some mfct calling for wood underlayment. and others make available a more solid coating for the under side. but very few reference to the "texas cool roof".
of interest is building science research
and these guys, who claim to have invented the system. I claim intellectual property theft 
still great pics and the section at the bottom gives details on how to deal with dew point moisture. see it here
ya. Don_P. I like board too. Lot less swearing. Zen question of the day: "What's the sound of one guy building."
My answer: It's not a sound. It's a colour that rhymes with kung fu.
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rockies
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# Posted: 4 Jun 2015 19:32
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This article may help.
http://www.greenbuildingadvisor.com/articles/dept/musings/how-install-rigid-foam-top- roof-sheathing
http://www.greenbuildingadvisor.com/blogs/dept/musings/martin-s-ten-rules-roof-design
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creeky
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# Posted: 6 Jun 2015 10:06 - Edited by: creeky
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boy I am not lovin' that thick roof.
the key advantage of the "cool" roof is that it moves the sun heat away from the building.
also the layer of plywood trapped between the underlayment and the polyiso insulation. didn't most of the early failures with foam insulated roofing come where moisture was trapped between the foam insulation and the shingles?
the drawing of the thick roof also shows pink insulation. pink performs poorly with moisture. but use Roxul and your good.
also there is no mention that the drywall on the wall and ceiling will have to painted with vapour permeable paints to ensure moisture migration.
I used figure 7 from the building science document I reference above. I improved it (smile) by using strapping over the roofing paper instead of osb/ply sheathing underneath. in combination with a metal roof I move the suns heat away from my building. and i avoid the dreaded two vapour barriers problem.
thoughts?
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