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lburners
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# Posted: 23 Sep 2020 22:37
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Im not there yet but just trying to plan ahead.
Lets say I am putting finished 3/4" ply on top of my roof rafters-ice and water shield-foam board insulation-metal roofing.
Questions do I need some sort of purlins between the foam and roofing? Will 1/2" penetration into the 3/4" ply be adequate? Would there be issues without insulation as I think Winter visits would be extremely rare although there will be a woodstove. Roof pitch probably 4:12.
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darz5150
Member
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# Posted: 23 Sep 2020 22:54
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Can you put on the plywood, then the sheet metal roofing. And put insulation between the rafters underneath. And finish the ceiling inside to hide the insulation if it's open living space?
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toyota_mdt_tech
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# Posted: 24 Sep 2020 00:01
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I put metal roofing on my stuff, the roofing mfg said purlins or I could use a solid underlayment.
They said sheeting, 30# felt then screw. The roofing I used had the ability to breath, had chutes. Make sure roofing screws goes all the way through sheeting, and get the fatter ones. Follow roof mfg screwing schedul to a "T"
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Brettny
Member
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# Posted: 24 Sep 2020 06:07
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1/2in penetration into plywood isnt enough. 1/2in out the back may be.
How much insulation and what heating zone are u in?
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lburners
Member
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# Posted: 24 Sep 2020 08:22
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I figured 1/2" wouldn't cut it. The end goal was exposed rafters with the finished ply sheathing showing. Without fasteners poking through. I am in heating zone cold af new england. The insulation would be more to minimize condensation from heat transfer than to keep me warm. Its just a 12x16 so Im thinking whatever stove I get in there is gonna roast.
I was thinking I could make the following combo work with the purlins fastened to the joists.
From bottom to top
Rafters
3/4 ply finished side exposed
ice and water shield
2" foam board insulation
purlins
metal roofing.
This would allow me get a longer screw through the purlins without affecting the 3/4" exposed ply.
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Brettny
Member
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# Posted: 24 Sep 2020 08:40
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2in foam board is no where near enough. Heating the Inside will allow heat to escape out the roof and hit the cold metal roofing and condense. Allowing for mold or rot to happen.
Screw the perlins/strapping to the studs through the foam and plywood.
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Steve_S
Member
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# Posted: 24 Sep 2020 08:55
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You are 3/4' of the way to building a Cool Roof system so you mighjt as well do it all at once and get the full benefit instead of being only part way to it.
I built a Cool-Roof on my place and it is amazingly good at keeping the heat out in summer & the warmth in during winter. I did one thing slightly different, I ran a 2-1/2" slot above the soffit (5" in from the edge, I have 24" soffits) to allow for better airflow and that resulted in very good cnvecional flow summer & winter. I installed 2x4's on the FLAT along the Rafters and then 1x6 crossing which has the Tin attached to it. This provide easy convenctional air movement up through the Ridge Vent and prevents any "moisture" catches and with a bit of cross moveent, the air is always moving.
WORD OF ADVICE - LESSONS LEARNED:
- If putting a vent slot, do install Wire Mesh with 1/4" spacing to ensure critters don't get up there.
- If a chimney will pass through the roof, be sure to block around the opening's properly, install snow guard / deflector for chimney and be certain that there is support under the tin for all the attachment points. TBH, much better to NOT go through the roof ! wish I hadn't, what a PITA to sort it out.
Below is one of the best explanations and they have several videos explaining it as well. This method is being added to our Canadian Building Code Revision for 2021.
REFERENCE:
https://www.houstoncoolmetalroofs.com/
Hope it helps, Good Luck
Steve
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lburners
Member
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# Posted: 24 Sep 2020 09:00 - Edited by: lburners
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The rot from condensation was my main concern.
Think I have seen a few projects with 2 layers of 2". Is that a pretty safe bet you think.
Appreciate the input.
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lburners
Member
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# Posted: 24 Sep 2020 09:04 - Edited by: lburners
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Alright Steve I will do a little cool roof search on here.
A little torn with its a cabin in the woods and lets do it right.
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Brettny
Member
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# Posted: 24 Sep 2020 11:26 - Edited by: Brettny
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The north east requires r38-r60 for a ceiling. I believe 3 layers would be the minimum depending on the foam. Exposed rafters and plywood really start to add up dollar wise. Have you priced out just 1 DIY spray foam kit and useing fiberglass for the rest?
This way would be roof rafters, strapping, roof underlayment then roofing. You could spray directly on the underside of the strapping and underlayment. No long screws needed, no 3/4 plywood and no foam board. It also gives you the ability to quickly dry in your building and insulate later.
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Steve_S
Member
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# Posted: 24 Sep 2020 15:22
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@ lburners
My build is documented on here, although the whole completion is not there as I stopped updating it. When looking at advice, look at peoples "About my Cabin / My Cabin Build Page" to see what they have actually done and are using.
If using a BATT type insulation, use ONLY Rockwool as rodents & bus do NOT tolerate it all. It is also hydrophobic, meaning it will not hold or transfer moisture and will not wick it. Fiberglass batts are homes for critters & bugs to nest in. Just open up any wall/roof stuffed with fiberglass and you'lll see.
I was fortunate to connect with Commercial Roofers who provided me with all of my Foam Insulation. Walls have 5-1/2" of HighDensity EPS-II between the studs. Came in 4'x4'x5-1/2" sheets for $10 a piece. My Cathedral Ceilings used 4'x8'x3.5" and 4'x8'x4.0" between the rafters (3.5" cost $7 and 4.0" cost $8 per sheet) paper faced. My FPSF (Frost Protected Slab Foundation) used 4"x2'x4' HD XPS Foam under an up the sides (code requires 2") which cost me $5 per sheet. This is all "Take Off" insulation that must be removed for reroofing commerial structures and sent to recycle, the companies will separate good from bad and sell the good if they can. Sure beats the hell out of Big Box Stores prices and you cannot buy 3.5" or 4" ISO let alone XPS or EPS in retail land.
Tin Roof IMO is the best bang per buck and also safest from cabins and cottages (think fire embers etc). Lesson Learned Suggestion: IF buying Tin for your roof, get it with CondenStop (that a tradename) which is a coating on the underside, that prevents condensation on the metal and also acts as a sound dampener, as a Cool Roof system will echo the raindrops ! That was something I should have done, woudl have if I had known in advance just how loud it can get. Side effect of the airspace.
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toyota_mdt_tech
Member
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# Posted: 24 Sep 2020 15:43
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Quoting: lburners I figured 1/2" wouldn't cut it
1/2 plywood will cut it, just make sure screws go well past it. But you said you are using 3/4", so even better.
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