I’m planning to re-roof part of my house soon. Its an old house and this part is a half cape, has (gulp) 2×4 rafters at about 7 in 12 pitch. This room is mostly pitched ceilings with a narrow flat ceiling under the ridge. When I renovated the inside I put 3″ solid foil faced insulation in between the rafters, which should be decent, about r-21, but man this room is cold in february. The new roof is going to be cedar shingles on new 1×3 or 5/4×3 lath. My gut tells me it should be fine to put a new layer of maybe 1″ or 3/4″ solid insulation over the rafters, then maybe 1×2’s (vertically) before the new (horizontal) lath and shingles go on, as long as lath is well nailed to rafters. My concerns are: lot of heat under shingles in summer, and just maybe crazy to put foam panel between lath and rafters . Any opinions ?
, new cedar shingles
over 1×3 lath
Replies
So this is a vented or a hot roof? I don't know what a 1/2 cape is, so maybe this is a dumb Q but are there kneewalls on the outboard sides of these upstairs room(s). If there are kneewalls, where in the insulation in this area. There could likely be major air leaks in this area.
BTW - are you sure the insulation is R-21? You didn't say it was fiberglass - I'm guessing it is - but I've never seen fiberglass insulation designed to fit in 2x4 bays that produced R-21.
3" solid foil faced insulation
I'm betting it's a foamboard product.
Parolee # 53804
Edited 4/5/2007 11:44 pm ET by rez
I bet your right. I didn't compute the word "solid".
In that case, I have to wonder if it was really installed in an air tight fashion, and I still want to know if the roof is vented.
I'm wagering it wasn't vented. 2 Bud lites and a box of milkbones on the table.
Bet he's got something akin to 30" kneewalls and with that flat ceiling at the ridge he's sweating bullet over the headspace which is why he didn't put any insulation covering the rafters on the room facing side as well.
Parolee # 53804
I'm wichya...
But... Bud Lite???? Ya gotta do better than that... How about something in a green bottle? I like cheap imports :-) Or, if that is all you have to offer, all milkbones. I've got a lab with a 2 track mind - food and paying... We buy Milkbones in the "economy" box. :-) Actually I'm not betting - my guess is that you are right and I'm wondering about big dead airspaces behind kneewalls.
I'm wondering if he'll be back.
Parolee # 53804
The rafters I can't and don't need to change. They sure are undersized, but keep in mind its only like 8' or 9' from the kneewall to the to the ridge. they got doubled, then the 3" foam ("Tuff-R" value approx. r 7 per inch) This room all newly sheetrocked and painted, etc... complete inside. All the old asphalt, two layers, old cedar and lath is going in the dumpster. So from outside we'll have just the rafters with foil faced panels fit in between. Thinking of 3/4" foam over this, then 1x3 or 5/4x3 lath and cedar shingles
Yes, lots of old buildings like that and still doing well after all the years.
You haven't said what the length of the room is but if it's around 20ft or so those uninsulated doubled 2x rafters are going to add up to a significant area.
You are going to have to play detective and thoroughly check out the area around your kneewalls as there is a great potential for an influx of outside air making little current tunnels and finding it's way into your living space.
In some ways this is more important to correct than adding additional insulation on the roof as kneewalls have a history of being problematic trouble spots in insulation scenarios.
After investigating and correcting the kneewall situation it might serve you well to study the possibility of using structural insulation foamboard panels on top of your roof
as they come ready to go with a nailing board already glued to the surface and in greater thicknesses than the 3/4" you mentioned
which although is better than nothing still seems light for the amount of labor you are involved in to make it worthwhile.
If you scroll up in the upper left corner of your screen there is an advanced search function that will take you to a page of instructions directing you to previous threads dealing with whatever you type in the search bar.
If you type in kneewall insulation or other keywords of the subject matter you'll get a supply of data from those old threads.
I take it the roof is unvented?
Parolee # 53804
Edited 4/6/2007 8:27 pm ET by rez
Thinking of 3/4" foam over this, then 1x3 or 5/4x3 lath and cedar shingles
do it -
it'd be better if you can ventilate the roof, but that gets more complicated -
attention to well thought out details is the key to satisfaction -
"there's enough for everyone"
I didn't comment on venting although I asked some Qs pertaining to it. And, the reason I didn't directly address his Q is because I'm not comfortable with the hot roof (unvented) on the sloped portion, and then the apparently good sized dead air space behind the kneewall. If detailed right, I guess the skip sheathing will provide ventilation just below the roof shingles.
yea thats right, I have a 3 or 4 foot kneewall (insulated), with a small crawl area behind it, then the sloped ceiling starts and goes up to about 7'6", to a small flat ceiling under the ridge. the sloped ceiling has 3" thick foil-faced solid foam insulation (=r value is "7 per inch" ) cut to fit tight between the rafters(not fiberglass, I just can't spell the name of it-poly-isocanurate???) Any way the idea is another layer of this "tuff-R" panel maybe 3/4", right over the rafters, then 1x2s' for an air space,1x3 lath horizontally, then the cedar shingles
I too would recommend looking closely at the knee walls. What kind of insulation is there? The backside being open on most kneewalls is the reason they tend to be problematic. Also, what is the wall and ceiling covering?
Regarding >> the sloped ceiling has 3" thick foil-faced solid foam insulation (=r value is "7 per inch" ) cut to fit tight between the rafters(not fiberglass, I just can't spell the name of it-poly-isocanurate???) << Yes, polyisocyanurate is some of the best performing rigid foam insulation. I still wonder if it was really cut tight enough. It actually has to have an air tight fit against the rafters and at the peak to work properly. If not it can be foamed in using spray foam.
You say that it is rather cold up there - that seems odd, assuming that you have adequate HVAC, and the R-21 of rigid foam. Actually the R-21 of rigid foam should theoretically preform perhaps as well as say, R-30 of fiberglass. Significant air leaks are a probable cause.
I'm not going to comment on the rafter sizing, or your proposed methods of adding height to them. They are undoubtedly undersized and over spanned. I would consult with an engineer.
I would not insulate between the 1x3 lath to which you are attaching your shingles. This will severely limit the ability of your shingles to dry and shorten shingle life significantly. (I would guess at least by half.)
With all that money ( insulation board x2 and cedar shakes) Don't you think you should address the rafters? I hope you plan on removing the old shingles 1st.
as far as your plan. If you had better rafters and where thinking of laying insulation board on top of sheathing then installing lath and shakes that would be fine as the shakes would have room to dry with the air flow between the lath as long as you take the right approach at the rake boards to allow the air to come and go.
i would also not use foil face again on the roof as this would create a second vapor barrier.
You can add some strength to your rafters and vent space (or insulation space) if you want it. After you strip your old roof you can attach 2x4s on the flat to your 2x4 "rafters" using PL glue and screws. This will make a "T" beam in cross section, adding some strength and rigidity to your roof, and giving you a 1 1/2" gap on top to be used for more insulation or ventilation. I don't know if the extra height and detailing would cause problems in your situation.
Or add SIP panels above for insulation and strength...
Billy
Edited 4/7/2007 10:11 am ET by Billy
Actually I can't add too much extra height, maybe 1 " or so, because I have a sidewall with a window sill very close to the roof (anybody from Vermont?) we've been calling it a "coffin window" having seen a few up there, its installed on an angle, giving light to an upstairs hallway- not to worry, its all flashed correctly. Thanks for the ideas, I think I'm gonna go with a layer of foam, maybe 3/4", then the 1x2s' vertical, and horizontal lath, all well nailed to existing rafters, then wood shingles. The new panels will keep the fresh air out of the kneewall space and give plenty of venting to the shingles. I would consider just plywood and then that cedar breather stuff, but don't think it actually gives much air under the cedar