Insulating a T&G Roof Deck
Hello- (sorry this is long) will clarify in subsequent posts. 1970’s home in mountains of NC climate zone 4 I think. I am remote located family member trying to get reroof done properly with some insulation added – to get it better than it was but it won’t be up to modern R/efficiency code. House has inefficient electric baseboard – and is usually empty year round – except for family visits and downstairs is a tenant occupied level (insulated between floors). shingles have been torn off and roof is weather tight but needs final roof this fall. in helping my family get it re-roofed – I discovered ceiling is the roof deck – tongue and groove boards – with exposed 4×4 clear pine rafter beams inside on 48″ centers on the interior – there is no attic space no vent space, no where to insulate inside – interior wood is attractive ceiling – pine log cabin look not air or vapor tight. roof was leaking from bad vent boots and cheap shingles job from the 90’s – pneumatics through shingles onto fiberboard ‘deck’ was not good nailing base (1970’s shingles hand nailed through fiberboard into 1.5″ t&g roof deck/ceiling). In tearing it off 1″ fiberboard was discovered on top of the roof deck – old tar paper nailed onto it (wow old 30 lb felt was the real deal vs. newer “#30 felt” is not equivalent/so thin)- so no hard sheathing layer for shingle install on top. Much of fiberboard had soaked up moisture/keeping moisture in contact onto pine deck – though felt paper had done good job amazing it hadn’t rotted through more. Lovely pine Ceiling intact. 1″ fiberboard was 1970’s roof insulation – only insulation on the ceiling – in a place that gets pretty cold – or used to before climate change. Primarily concerned with future winter use, keeping heat inside – and preventing problems during winter from warm heated air/moisture vapor moving up into roof sandwich and condensing to form ice or to keep sheathing/decking wet leading to rot, etc. trying to get it re-roofed and put back some amount of rigid foam on top of the roof deck to replace where the fiberboard was removed and to better insulate the interior ceiling/rooms. (AC is not present – it is very damp and cool there in summer – and assume 70’s family spent a lot of money on elec heat with this scant ceiling insulation.). Need to keep it simple with available local crews to get this done in next 2 months – using conventional roof/framing/metal crews I can find nearby. will not be able to meet new code requirements for R value but would like to do better than the old fiberboard. Very concerned with winter moisture migration up into roof/vapor and moisture barrier strategy for foam board/protect ceiling/roof deck and new plywood deck I assume I’ll put on top of foam. Was thinking 3″ of rigid foam board – topped by new plywood sheathing – 5/8″ or 1/2″ 4 ply? Onto purlins? (I know this is but a fraction of required R value – but will be better than 1″ fiberboard it had for last 45 years. roof deck is low-ish slope 3 or 4 in 12 I think – so metal would be ideal if I could afford. 1. Really need to nail down the moisture and condensation possibilities here – it is likely not used in forseeable future much in winter – so indoor heat and cooking moisture isn’t an issue – but suppose it could be sold or used differently in next couple decades. How do I make sure any interior winter moisture does not migrate up into the roof and condense where it can wet the roof boards unseen – and lead to ice dams, or unseen wet plywood/deck/rot in the roof sandwich on the exterior side. I need an air/moisture vapor barier applied to the top side of roof deck to stop any moisture from moving through and at that point I need it to be insulated enough so it doesn’t condense there? Is this just peel and stick membrane? Can titanium nail on-non peel and stick if taped and sealed work for this barrier? 30lb felt? I can’t put anything on the interior – and the t&g is not vapor/air tight. 2a. how to most economically apply 3-4″ of foam board to the exterior side of the roof deck (which is the ceiling) so I can then put a shingle roof over it – assume I want to put rigid foam down with spaced out purlins which are screweed down into the rafter beams below? (can’t attach to the t&g roof deck as it is 1.5″ and penetrations would be inside the visible ceiling surface) Also ease of construction working with local crews, whre to get it, (salvage foam boards?) Then put new plywood sheathing on top of this – and peel and stick – and then shingle or metal roof (budget depending)? 2b. Do I need thermal break for purlins – if I run purlins on roof deck screwed down into rafters and put foam between them I’ll have thermal break through wood to wood running out to exterior. Do I need to divide foam into 2 layers so no wood to wood thermal bridges are present? 2c. Can I skip the purlins alltogether and lay foam and then screw plywood deck down through foam into rafter beams on 48″ centers inside? would need 6-8″ screws to do this – and new deck would just be on top of the foam. 3. can foam panels be directly glued or screwed to roof deck (being careful not to penetrate 1.5″ thick t&g deck which is the interior ceiling – and or hit rafter beams on 48″ spacing? Then cover with plywood laid on top of foam board and screwed down through onto rafters at 48″ centers down below? (any penetration of 1.5″ roof deck/is the ceiilng and will be seen in the finished space – so only 1-1.25″ penetrations allowed and/or only on the 48″ rafter beam centers. 4. can plywood deck sit on the foam? 5. can i do a metal roof with foam and purlins and skip the plywood deck? – but walking the metal roof would be walking on the foam under the metal. 6. local rigid foam – lowe’s, HD, bulders supply, salvage? THanks for any input/thoughts/referalls/links/Western NC suppliers/contractors, etc. some photos attached to give general layout/interior ceiling etc.
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