Energy efficient and durable roof design
Hey everyone,
I am planning to build a 1 1/2 story in zone 6A in deep snow country (ground snow load is 60 psf). The roof design is a simple gable with one doghouse dormer. I would prefer not to do a hot roof. Because of the snow, I’ll have to vent the roof sheathing regardless, so I’d prefer to go with what I believe is the most straightforward option. Thus, my original plan was to vent the roof and fur out the bottoms of the rafters to provide lots of room for insulation (R50-R60) and use a high-quality smart vapor/air barrier. My only concern is the inability to vent the roof from the eaves above the dormer (obviously the ridge vent is still at the ridge) . Should I install spray foam in this specific area instead? Is this relatively small area worth losing sleep over? How do people usually deal with this detail?
I attached a drawing below Green=Ventilation Red=What I perceive to be inadequate ventilation
Thanks in advance for your expertise and wisdom!
Derek
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Replies
I'm not in as big of a snow country, but areas like that are usually left with just a ridge vent.
If you want to be safe, SPF is a good option. You only need 50% of your roof R value in SPF, the rest can be kept as fluffy insulation. You can probably do the whole thing with one of the two part spray foam kits.
You'll need the SPF kit anyways to seal up the 1/2 story floor joist /rafter intersection area anyways, this area is impossible to air seal any other way.
P.S. Another option is to build a mini attic near the ridge and use gable vents. In heavy snow area, this might be a better option as they won't get covered by snow. The bonus of the mini attic is that you can now loose fill the rafter area as any settling there will be take up by the extra insulation in the mini attic.
I say ditch the story and a half design by the time you find ways air seal the first floor to second floor connection and find a way to get good insulation crammed into the rafter with some venting you will cost less to build a 2 story with the same number of square feet you will get less downstairs and more upstairs. You will get a big attic that is easy to vent and fill with R60 cellulose.
Did I mention I hate half story buildings?
Walta
Broadening Derek's question (with a focus on the durable aspect), what are best practices for creating a serviceable and inspect-able roof assembly (that is, for a vented cathedralized ceiling*)? Would Akos' mini-attic suggestion allow for a useful degree of service access? (at least enough access to be able to snake an inspect camera down the rafter bays to inspect for leaks, etc?) Robert Riversong has an old article briefly describing an exterior hayloft door for service access (albeit on a 2-story building, with larger attic): https://www.greenbuildingadvisor.com/green-homes/a-thick-cocoon-of-cellulose-protects-this-superinsulated-house
* To Walta's point, the gold standard for a serviceable/inspect-able roof is probably just a full vented attic.. Still a possibility in new construction, but not for my 1.5 story 1800s farmhouse..
> Should I install spray foam in this specific area instead?
Safest to do both - use spray foam at the right ratio and leave the vent to the ridge vent in place.
In my mind the use spray foam in new construction is red flag, showing a desperate attempt to cover up poor planning in a very un green way.
Walta
Derek, spray foam can be a problem-solver but it comes with a large-to-huge carbon footprint and is best avoided when possible if you care about our future. The area you show is not huge; ridge vents alone should be enough to keep those areas safe, but not really best practice.
Best practice is to eliminate dormers, largely for this reason.
Next best is to use one of many options for unconventional (but safe) construction techniques to allow full venting. My favorite is to install a high quality WRB (watertight and vapor-open) over the rafters, then add 2x furring over that, aligned with each rafters; then either sheathing and roof shingles or purlins and metal roofing. That allows the roof to dry and vent easily to the exterior. Those who have not tried this system think that it's too difficult to install the membrane without sheathing, but everyone I know who has done it (including me) says it's not very hard.
If you want to stick with more conventional methods, you can "drop the valleys," meaning keep the tops of them 1 1/2" below the tops of the rafters. Technically it doesn't meet IRC code requirements for full support but most ridge boards are framed like this and most inspectors understand that it doesn't reduce the strength enough to matter. If they did care, you could notch or drill holes in the valleys instead.
(I'm in a 70 psf snow zone.)
Derek,
I second Akos' suggestion of a mini attic over the area you identified in red. I would couple that with 2"x4" horizontal strapping on the dormer to aid air-flow.
Another option is to convert the traditional dormer to a shed dormer and vent as normal.
A 60# ground snow load isn't too bad. If you can get a bomber air tight ceiling, lots of insulation, and get a ridge vent in, then you will probably be okay. Attic venting is meant to wash away any warm air that makes its way through. It doesn't 'cure' the underlying cause, it merely 'treats the symptoms'.
Eliminate the warm air from hitting the sheathing and attic venting isn't necessary. (Except in really deep snow... 60 PSF is borderline). Much easier said than done though.
Rick,
A shed dormer makes a lot of sense and would look good on that house.
Derek,
May I offer a contrary-ish suggestion?
I am in a 60psf snow zone and have all "hot" roofs, if by that you mean unvented. Aside from the losses from long screws holding down my 7.5" of EPS nail board, the snow pretty much sits until the sun get a toehold on the metal. My roof pitches are quite low, which is part of the reason I chose not to vent. I may be tempting the fates, but if the nail base sheathing is encapsulated on the roofing side with ice and water shield and the other side is protected by EPS , it should stay dry and not need to alter its moisture level. My primary roof deck is covered with a synthetic roof paper in case a failure allowed water to make it past all the foam. Batts between the joists finishes the layup. The overall roof is calculated to be about R54.
Although your roof pitch is very steep by comparison, could you have times when the snow is covering the roof ridge vent. Once the vent is covered any errant heat would remain trapped. I have helped shovel a cabin roof in NH that had 5' of snow on the roof, which showed no change of depth as we worked up to the ridge. Obviously, for maybe two thirds of the year, the venting value to keep the roof sheathing vapor open is valid. However, even with plans to go R50-60 between rafters, you will still have the rafters moving heat to the roof deck. Why not solve both the rafter losses and the dormer venting puzzle in one go with insulation on the top with no venting?
Using nail base on top of the primary sheathing will keep the sheathing warmer and less prone to absorbing moisture to dangerous levels. The batts on the underside between the rafters allow vapor transport to the interior, although you shouldn't expect much moisture build up in the now warmer deck. And the dormer detailing becomes moot with insulation on the outside.
If by chance the secondary sheathing just under the roofing does fail, then the EPS foam should still be okay.
If the sheathing on your plan fails from water intrusion the insulation under may well take a hit that requires interior work. It might also be worth looking at the change in heat losses during the winter months to assess the hit you are taking on your whole roof R rating. It is after all subject to the same rules as walls for final functional R values.