Hello, first time poster here.
I recently bought a small 1920s bungalow and am in the process of renovating it. Before move in i gutted the front portion of the house and brought down a couple partition walls, and raised the ceiling by adding new 2×12 collar ties to the roof and using those as the ceiling. i then insulated those joist/collar ties with r-38 craft faced insulation. my choice of fiberglass was based on budget and time constraints-ie. i could do it myself at night and still hit our move in date, plus it matches the way the rest of the house is done. Anyway, im now looking at reroofing the house with architectural asphalt shingles, but id like to add some more insulation into the mix….
my initial thinking is to put 1′ rigid on top of the roof sheathing with the roofing over that. as the fiberglass isnt really a vapor barrier even with the craft face i would think this would be a drying-to-the-inside type of setup and be fine even if the insulation is partly above the cold attic and partly below it, but it still seems like a dodgy idea. im sure no one would recommend this to a client(maybe im wrong about that) but it doesnt seem like there is really an issue here….would love someone who has more knowledge or even better- experience than i to give some advice. thanks
Replies
An important detail is missing: Where is this house located??
ah right. its just outside of boston. climate zone 5., i should also add. the attic is vented by a single gable vent on the front of the house. no vented soffits. this makes me think that simply closing the gable vent and venting between th roofing and the rigid would be a good option, utilizing cobra vent or something.
Hey there,
My advice for you would be to do your very best to airseal the ceiling. If there are recessed lights, ductwork, or other penetrations they should be sealed with caulk, sealant, can foam etc. from the attic. This will mean moving your fiberglass aside etc. If you stapled the Kraft paper to the edge of the new ceiling joists and you don't have a lot of holes to deal with, then I would leave the fberglass in place. Then simply pile on the cellulose. Two feet deep if you want. Just make sure the attic vent is allowing outside air in without letting in the weather, i.e. driving rain etc. This is a good bang for the buck strategy that should not cause any new issues, will definitely add insulation value and is easy to do, without changing your roof trim and details. It is a worthwhile improvement to a house like yours that will never be "tight" or super insulated by modern standards.
Roof question......
" as the fiberglass isnt really a vapor barrier even with the craft face"....... Yes! it is a vapor barrier!
Which way is the craft paper facing, up or down?
"my initial thinking is to put 1' rigid on top of the roof sheathing with the roofing over that " ......... NOT a good idea, you really
should apply another layer of plywood sheathing over the rigid, otherwise you run a serious risk of wind blow-off
of the shingles nailed through the rigid into the original sheathing boards. Even if you use 2 1/2" roofers to get to the existing sheathing, it's a little "iffy".
A second layer of ply screwed through the 1" rigid, into the original sheathing would be recommended, then apply the shingles to the second layer of ply using roofers.
Don't forget about your wider Fascia/ Rake trim that will be needed at the soffit/eave locations as well.
Geoff
P.S. I'm in the Boston burb's.
Hey Geoff thanks for the response.
So a couple things;
I didnt really explain all the factors involved with my decision. so here are a couple other details to help you understand exactly what im thinking and why(you may still think im not doing the right thing after, but thats why im here, right?)
1. the roof that im talking about was framing with 2x8s
2. when i gutted it, i raised the ceiling height (which was previously sistered to the rafter tails and sitting on top of the exterior wall plate) of 8' to 9'3" by adding some 18' doug fir 2x12 collar ties. so effectively a 20" portion of the ceiling on the two bearing exterior walls is cathedraled- i did fir that cathedral section down but not all the way to 11', i just compressed the r-39 a little bit.
3. theres also a tiny room in the back second floor of the house where the roof was actually framed with 2x6 same story there except it is like 60% cathedraled.
4. i thought of doing new roof sheathing over the rigid, but wouldnt i then be trapping that sheathing between two vapor barriers? with the rigid and the roof which i will prob do 48" up from eaves with ice and water.
5. youre totally right about all the trim, but it needs new trim anyway so thats fine. was thinking of using this stuff Boral, which ive only used once but was pretty impressed, you have much experience with that? one of the carpenters i work with said he soaked it in a bucket for two months and pulled it out and it was the same as when he put it in.
6. i figure worst worst case if im seeing problems with condensation or somehting, i pull the fiberglass out and have cellulose blown into the cathdral sections and against the remaining roof bays then i have true cold roof that still dries to the inside.
what do you think?
again, thanks for the response. Mike
Geoffrey,
Kraft paper is not a vapor barrier. It is a vapor retarder with a perm rating of 1. This is about the same as 1/2" ply.
http://www.insulate.org/tech6_3.html
KK
Vapor retarder
Hi coonass,
Thanks for the link, you made my point. Kraft faced insulation has a perm rating of 1, 6-mil poly has a perm
rating of .06, 1/4" ply has a perm rating of 0.7. The higher the perm rating the more permeable the material,
the lower the number the LESS permeable, i.e. a barrier to moisture pass through. Mineral wool has a perm
rating of 30! 3/8" Gypsum board has a perm rating of 50! So kraft paper IS a vapor barrier and would block
moisture from escaping if applied in an inappriate fashion.
Geoff
Geoffrey, I think you missed the point. Kraft paper is less of a vapor barrier than 1/4" plywood. It's about equal to ordinary latex paint. The plywood roof sheathing is a better vapor barrier.
if you vent that roof at all ...
If you put foam ontop of the heathing and then vent teh attic in anyway you have wasted teh foam!
You're talking about spending a lot of money for very little return, I'd pop a few pieces of decking off and blow in as much more fiberglass as you want then airseal all the penetrations from the bottom. You'd get a lot more insulation in a lot less time for a lot less monry.