Insulating the roof....yes again

[peternap]

I need ideas.

This roof will be a post and plank and the sheathing will also be the exposed ceiling inside the room.



I realize I will need to lay the insulation over the sheathing before the roofing....but what to use??? hmm

Any and all ideas are appreciated.

[MountainDon]

So this roof is over living space, barn or shop?
How much R-value do you want?

[glenn kangiser]

Commercial Buildings I work on commonly use foam board - I think Polyisocyanurate Roof Insulation.

http://www.benchmark-inc.com/articles/Perspective%20Articles/issue37a.html

I don't know much more about it or suitability.

[peternap]

It's over living space Don. This will be on the two side wings for the barn/house


I want as much R value as possible although the center section is higher and will have an open space I can pack full of blown insulation.

Foam board was what I was thinking also Glenn but I don't know much about it.

[MountainDon]

Yep, foam board sounds about right. Don't know much about it myself either...
Approx R5 per inch.
Pink or Blue.

[glenn kangiser]

I think the Polyisocyanurate is more rigid.

[Redoverfarm]

Not that familiar but what I have been told is that a exterior frame board is made around the exterior edges out of 2X6 which form the cavity for the ridig foam. Most around here use two layers of 2".  I have even heard of laying tin directly over the ridig and with the use of long screw directly into the ceiling boards through the insulation board and into the ceiling generally 2X6 TG(1-1/2 X 5-1/2) without perlins. 

Personally would rather use sheeting and screw metal roof onto it. But if you were going with a shingle roof sheeting is a muss.  If you have someone in the area that builds log homes they could p[robably give you the straight scoop if someone on the board doesn't.

[ScottA]

Peter this is what I'm planning.
3/4" planks over framing-
#30 felt-
1/2" foam board-
2- 2x4" layed flat over rafters and and around edges,screwed in place-
2" foam board between the 2x4" + 1" foam board over that(joints staggered) -
7/16 OSB decking-
#30 felt and shingles.

Total R= 20.2
3/4 wood= .7
1/2" foam=2.5
2" foam=10
1" foam=5
7/16 OSB=.6
shingles=.4
Boundry layer = 1

This just gets me into code compliance (R19 min) for this area with all other aspects of our house construction taken into account. YMMV

Scott

[Redoverfarm]

I think if it were mine I would opt for 5/8 sheeting even with 16"OC. 7/16 is a little thin and once it heats up it will probably start to dip between.  I have seen roofs where 7/16 was used and over a short period of time the roof looks wavy. The cost difference is very minimal in comparison.

[ScottA]

Redoverfarm I asked the guy at the lumber yard about what you said. He told me pretty much every house built around here has 7/16 OSB for roof decking even on 2' centers with clips. I have 2 outbuildings both built with 7/16 OSB decking on the roof. They both look fine. maybe the one you saw they didn't stagger the seams and use clips? Anyone else had this problem?

Scott

[MountainDon]

23 yr old roof here. 24" OC trusses. 7/16" OSB. It has the prescribed clips. Never saw any waviness in the original asphalt shingle roof. When we replaced it last year we stripped it down to bare OSB and checked it over. No problems. Then we had roof wrap; Titanium brand textured synthetic applied. That was nice to walk on. 26 gauge ribbed metal roof screwed on top of that.

[Redoverfarm]

Maybe it is just prone to the area but it does exist. Not sure with the type of construction.  I don't know if 2-3' of snow sitting on them for a month at a time would have some bearing on it.  Last year in the higher elevation of the county they got 115" for the winter.