Cold climate, hip roof, venting the rafters, insulating the rafters
House will have wood heat (drying out the building). Current framing includes 2×6 studs, 2×6 ceiling to attic, hip roof (5/12), 2×8 rafters, open soffit venting, two-foot overhang. Large variation from one end of the 42×28 building to the other — enormous Sun on south side, full shade on north end. (42-foot length is north-south).
— Hip roof — I’ll drill large holes to open one creeper to another creating air movement up the hip to keep the sheathing dry and cold in Winter.
Question 1: To separate the rafters from the attic, can I place a layer of foam, say one-inch, inside rafters and below the rafter holes?
Question 2: A “vapor barrier” such as mil-plastic covering the foam, providing complete air separation from the attic to rafters — is this of any benefit?
— The attic is simply storage but would not necessarily be leak proof from second floor ceiling.
— Second floor ceiling — does it need a vapor barrier to prevent humidity from getting to the attic?
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This roof design sounds like trouble to me, hip roofs are more suited for blown insulation in the attic space. This is the space between the standard 8' ceiling height and the roof framing. Vent at both the eave and ridge at 1 sf per 300 sf of living space.
Doug -- in a hip roof, the "creepers" creep up the hip to the ridge. They are closed to the ridge. The methods of venting the creepers are 1) furring strips underneath, 2) drilling holes through the creepers, or furring strips above the rafters under the sheathing. Merely venting at the ridge does not move air around creepers. My questions pertain to isolating the rafters from the attic, is there any benefit. I'm not creating a cathedral ceiling in the attic. It will remain an attic. I intend on insulating the attic to the eaves. I have excellent soffit venting. --- I'd be glad for help from someone who has worked with a hip roof.
Wood heat doesn't dry out a building by itself, it increases air exchange by bringing in cold exterior air warming it up, reducing it's relative humidity, and sending it up the chimney. If you have an air tight house with a sealed combustion wood stove with outside air intake you can actually have humidity problems in the winter usually seen as condensation on windows and interior side of cold exterior wall and roof sheathings. This is why we say "build it tight, ventilate right" you want to control the ventilation, not just let it happen when the wind is blowing or the air handler is running. I would recommend that with a hip roof and attic storage you would want to use spray foam on the underside of the roof deck and build an air tight, sheet rocked storage area in the attic to isolate the stored materials and humidity from bulk air leakage at the access door from the spray foam and roof deck.
If you already have generous soffit venting you can notch the tops of the rafters or fur the roof deck up from the rafters with 1x4's 24" OC easier than drilling all those holes so close to the tops of the rafters. With a 14' span to the ridge on a 28' roof you probably can carry the load with 2x6's and since the rafters you are considering drilling will be on the hips they will all be significantly shorter than 14' in span so notching the tops looks like the way to go to me. Not sure where you are building but on the east coast what you are calling a "creeper" is called a "jack rafter." so you may be in a place with more snow load than we have here in North Carolina. I'm guessing that either way you will need more than 5 1/2" of fiberglass and one inch of foam board in a 2x8 space with one inch of venting on the top.
I think your least expensive option is to build a well insulated 12' x 16' storage room with pull-down stairs in the center of your attic space (away from the hips) with the ceiling furred down to allow R38 insulation and 1" of conventional insulation baffles at the roof deck. I would seal the drywall to the attic floor and seal the attic floor with blocking to the ceiling of the rooms below. then I would blow R-48 cellulose on the rest of the flat ceilings not under that storage room. there are some details to address, you need to have exterior sheathing on the storage room walls if you are using fiberglass batts to prevent air-washing and you will need access to the attic to blow the cellulose which I would recommend that you seal at completion with drywall or a gasketed door screwed tightly in place to keep future occupants from leaving it open. the entire ceiling plane, including light fixtures wall intersections and plumbing stacks needs to be caulked or gasketed as usual.
Michael -- thank you -- yes, jack rafters, I'm building this with 2x8s as all the rafters, in New Hampshire. I had not intended on fully insulating the rafters and expected to keep the "storage area" cold. My question -- as an attic -- do I get any benefit by giving the rafters a little separation from the attic by adding a 1-inch layer of foam board thereby helping to contain the air flow up the rafters to the holes/notches (as you've recommended). I'm simply trying to assist the flow of air along the rafters. My second question -- is it any benefit to even seal off the rafters with plastic? You have definitely recommended completely sealing the attic. Ok. I'm wondering if adding the extra rafter "sealing" is worth it.
-- I see the notes on the woodstove. Yes, it will have outside air intake.
I think you should resist the temptation to store "stuff" in your attic.
If so.....you have a chance to build .....
Lstiburek: "A vented attic, where insulation is placed on an air-sealed attic floor, is one of the most underappreciated building assemblies that we have in the history of building science."
The attic headroom with a 5/12 roof pitch is going to be pretty lousy anyway.
I also think the "attic bump-up-storage" concept will be fussy to build properly...
will degrade the Overall "UA" of your shell....
add multiple nodes for possible air barrier failure....
and consume extra energy for the life of the house.
Since you don't care if your "stuff" is climate controlled....
why not purge some of your "stuff" and spend your money on a very nice,small garden shed?
(Inside Joke)...and add a small closet to the conditioned house in order to store several buckets of premixed drywall compound.
Lewis, by "storage" do you mean just stuff that you want under cover (eg: Christmas decorations) but otherwise don't need protection from cold and summer humidity (conditioned space)? You certainly DO want the attic space isolated ("leak proof") from the rooms below with respect to air leakage, since you don't want interior humidity migrating into the cold attic in winter. Any attic hatchway from the interior also needs to be well gasketed to prevent leakage.
Otherwise, I don't understand your concern about attic ventilation with a hip roof. Your goals should be to have adequate soffit and ridge ventilation (which you will), no air leakage from house into the attic, and piles of insulation on the attic floor to keep heat out of the attic. This will give you a dry attic year round in NH. I also am in NH, have a hip roof with two-foot overhangs, essentially no air leakage of interior air into the attic, and R60 worth of loose cellulose on the attic floor. In winter, the attic stays dry, the snow doesn't melt from below, and thus there are no ice dams or icicles at the roof edges.
John Brooks I do agree with your perspective but I have had several clients who really don't want an out building and have ended up going up into spray foam attics and building storage areas with leaky doors and it's just not all that hard to detail a little storage cube up there and seal it up as part of the plan if the client really needs conditioned storage and doesn't have room for a storage building or extra storage in the house or basement / crawl.
I have concerns with relying on the gaskets on a pull-down staircase and framing up the attic floor to accommodate R-38 or more under the storage deck. I'm a lot more comfortable relying on a small dry walled room with R-19 below and and on the walls and R-38 above. It's really not that difficult to do. But yes, a storage shed in the back yard is better on any number of levels.
well thank you all very much -- I catch the drift -- seal off the attic, stop trying to make some use of the space....but it's hard to look at it and not want to at least store....Christmas decorations. I'm not doing spray foam. I want to ensure I can either see or get to the roof sheathing, at least until I'm sure I don't have any ice-dam issues. I have to build "access" even if just to make sure the attic is ok; not a pull-down stair; it will be a doorway/hatch and I'll make it well sealed. Final concept -- "adequate soffit and ridge ventilation... no air leakage from house into the attic... piles of insulation." Got it. -- thank you all for your help.
I think the ideas presented by Lewis and all are horrendous. I build homes. I would not contemplate this mish mash of construction. Really?
Stick with basics to get half way to where you want to go. John Brooks is right on.
Michael, I am all for you doing your design. But for many, they will not be able to build your idea well enough, and many who come here are on very low budgets. I could build this many ways but waving my hands and posting some tidbits is not going to be enough to pass on the abilities to do Michael Chandler level workmanship.
Dick yaa beat me to it... Another vote for sanity.
One more voice for leaving the underside of your roof sheathing and your rafters open to the attic and available for visual inspection. That allows you to look for future roof leaks, and it also allows the roof sheathing and rafters to dry quickly if they ever get wet.
There really are very few times when it makes sense to encapsulate wood construction materials with polyethylene.