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harrymanimus
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# Posted: 18 Feb 2018 10:04pm - Edited by: harrymanimus
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I'd like to insulate my cabin roof. Rafter beams are 2x4, distance between is 2'. I don't see how I'd do regular wall insulation due to the width. I looked at the spray foam kits, just not too sure I want risk that and there is the cost.
So I was thinking 2" Foamular board. From what I've seen, you are supposed to glue it? Are there other ways?
I should note I want to stay within the rafters there is a loft and I don't want to give any head room away.
Edit: I did just find this which looks like the right size. So this is an option.
23" Insulation for 2x4
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toyota_mdt_tech
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# Posted: 18 Feb 2018 11:35pm
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You can get 24" wide insulation in rolls for ceilings. Its made for trusses which are usually spaced at 24" OC
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Steve_S
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# Posted: 19 Feb 2018 05:50am
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I would suggest Roxul 24" wide batts which are only a tad more than fiberglass but far outperform it. The advantage in ceilings is that it's more dense and expands out so it's a tight fit between rafters, not like fibreglass that lays there and sags. *Vapour barrier required* I cannot & will not recommend fiberglass batts for anything.
Ideally spray foam is the best, yes it's pricey but you really do get what you pay for. TEMPS are critical for applying it though, it will not setup if too cool and you really have to follow the instructions to the letter. Commercial application is more but there are some advantages to that, including foam density so a bit more R value. *no vapour barrier required, foam is a barrier*
Board Foam like XPS, EPS work good BUT (I used sheet foam) it's a LOT of work and results in some Blue Air. You have to cut each one for each cavity, (wood will be slightly twisted, some cupping / warps making it more fiddly) fit as tight as possible, whack it in patiently (use a 2x4 on flat or breaks) and once in and tight, then spray can foam to seal any / all gaps, if tight enough the foam sheets won't fall, the spray foam also glues it into place and seals them in. BTW: Cutting foam is messy, far more than anyone imagines so you have to consider that when setting up to cut. *No Vapour Barrier Required*
PolyISO Foam is much lighter & easier to handle and cut (use electric carving knife or hand saw) has a higher R value, Typically R6 per inch, same deal as other board, much cutting and fitting as tight as possible, + spray foams in seams / cracks to completely seal. Again no Vapour Barrier required. *This foam breaths a bit but is hydrophobic so it does not pass water / moisture, better for the building overall & you too.
On my sheets (cathedral ceilings with 4" & 3.5" ISO) I used cedar shims once pressed & "whacked" in place to keep them up, made it easier installing second layer which was then spray foamed in place. Same method for using other foams.
PS: Foam Insulation in sheets is $$ at Builder Store, I discovered that commercial roofers have to take off such insulation and send it for recycle when they redo commercial roofs. It costs them $ to ship it out by truckload. I bought Good undamaged "Take Off" foam for my project, which was far cheaper. 4'x8'x4" (R20) PolyISO was $8 a sheet & $7 a sheet for 3.5" (R17) thick. My 2'x4'x4" XPS (R20)[blue foam] (used under slab foundation) was $8 a sheet. The HD-EPS 4'x4'x5-1/2" (R33) was $10 a sheet
Again, Board / Sheet foam in an structure is great for insulation ! but a royal PITA to do and required heaps of patience. Roxul would be the fastest and likely most cost effective and least likely to cause Blue Air.
REF: from HomeDepot Canada but available in USA too.
https://www.homedepot.ca/en/home/p.comfortbatt-r14-24-inch-oc-for-2x4-wood-studs.1000 123042.html
Hope it helps.
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harrymanimus
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# Posted: 19 Feb 2018 11:07am
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That is a super lot of good info! I'll have to digest.
I do not actually see where I can find the Roxul in that size near me after searching around. Not available for shipping even. I might have to contact the company.
My roof is metal over plywood with felt paper in between. Would I still need a vapor barrier in this case?
I will reconsider spray foam. Any recommends on brand? Also I'll see if I can find any used PolyISO.
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xtolekbananx
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# Posted: 19 Feb 2018 12:13pm
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I had the same problem like you. I have 2x4 rafters 24'' OC and first I insulated my roof with pink insulation. At the time ACE hardware was carrying 23'' wide insulation. The problem was that the roof was not vented and I had problem with condensation under that insulation. I have the same roof, metal, tar paper and OSB. What I did is I cut a ridge vent and drilled holes in the soffit, the I nailed 2x2 pieces to each rafter (1.5x1.5) and then I used a ridgid foam board from Home Depot, it is 2'' thick and rated R-13. I cut those boards to fit between the rafters very tight and than taped it with aluminum tape. That gave me 1.5'' air space between my insulation and roof sheeting and proper venting takes care of the rest.
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xtolekbananx
Member
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# Posted: 19 Feb 2018 12:17pm
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here are a few photos
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xtolekbananx
Member
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# Posted: 19 Feb 2018 12:24pm
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and finished
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Steve_S
Member
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# Posted: 19 Feb 2018 02:29pm
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@harrymanimus Roxul is a "Brand Name" but others make Rockwool as well, so you may possibly find another brand but Roxul is one of the oldest making such a product. It is also not liked by bugs nor critters and hydrophobic as well, will not absorb or retain water.
xtolekbananx points out a very critical point which depends on what you use and how to insulate your roof. IF spray or non-breathing foam, then your going for a hot roof design, no options as air & moisture won't go through it. IF using BATTS either type, then you need to vent it.
Vapour barrier is only used internally and that is to prevent moisture (condensation etc) from getting into the insulation and that depends on insulation type being used. Your sheathing, breathes, so does the felt to an extent. The felt just helps keep moisture / damp from getting directly on the sheathing and affecting the structure... helps water run off downslope. Hopefully there is airspace between the tin & the roof sheathing that allows for some limited air circulation to keep it dry under there.
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harrymanimus
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# Posted: 20 Feb 2018 01:43pm - Edited by: harrymanimus
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Quoting: xtolekbananx I had the same problem like you. I have 2x4 rafters 24'' OC and first I insulated my roof with pink insulation. At the time ACE hardware was carrying 23'' wide insulation. The problem was that the roof was not vented and I had problem with condensation under that insulation. I have the same roof, metal, tar paper and OSB. What I did is I cut a ridge vent and drilled holes in the soffit, the I nailed 2x2 pieces to each rafter (1.5x1.5) and then I used a ridgid foam board from Home Depot, it is 2'' thick and rated R-13. I cut those boards to fit between the rafters very tight and than taped it with aluminum tape. That gave me 1.5'' air space between my insulation and roof sheeting and proper venting takes care of the rest.
So you did both venting AND foam? Do you need to do that? I was thinking I'd want to skip as much of the extra work as possible if going with tight fitting rigid foam or spray on.
Also I found this technique of cutting XPS. Doesn't look too bad.
https://www.youtube.com/watch?v=1cv46GuniIY
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harrymanimus
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# Posted: 20 Feb 2018 01:47pm
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Quoting: Steve_S @harrymanimus Roxul is a "Brand Name" but others make Rockwool as well, so you may possibly find another brand but Roxul is one of the oldest making such a product. It is also not liked by bugs nor critters and hydrophobic as well, will not absorb or retain water.
I did find another brand, Thanks!
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