Nits & grits redux
SPECIFICATIONS:
Climate zone 3A (Atlanta, GA)
3 levels with basement walk-out to rear
Bonfiglioli wall
2×6 studs
Batt insulation in 7.25″ cavities, type TBD
Plywood sheathing
Seams taped, type TBD
Housewrap lapped, type TBD
No exterior insulation
Air gap to majority brick exterior wall veneer
Rainscreen to minority fiber cement siding
*VENTED UNCONDITIONED ATTIC*
Fluffy insulation above upper ceiling, type TBD
Site-built roof frame, 5:12 hipped upper roof with some 3:12 lower level roofs
Plywood sheathing roof deck *NO INSULATION*
Peel & stick lapped, type TBD
No battens
Clay roof tiles
Target <2 ACH 50 Based on previous feedback and further research, we are pursuing design change to eliminate surface area use of spray foam by venting attic. This has introduced some challenges, and I would appreciate any feedback on if my understanding of a few affected considerations. VENTING AREA: The wide eaves offer plenty of easy opportunity for venting but ridge venting is more challenging, both because the geometry of hip roof has limited ridge lengths but also because ridge venting clay roof tiles is challenging. On a 5:12 hip roof, what risks would be introduced by venting only at eaves? My initial estimate is that at most, clay roof tile venting could accommodate 25% of venting area required by code. HVAC LOCATION: Initial design calls for 2nd HVAC system servicing upper level to be in attic space along with ducts connecting to registers in or near ceiling on interior space below. Placing system in floor space on upper level would create major floor plan issue so we are considering keeping system & ducts in same location but building small mechanical room in attic space that would be air sealed and conditioned with living space below. Because of low clearance in attic space, the system would have to be horizontal and the clearance between room and roof rafters might be only a few inches at top edges of mech room, but central location would keep it clear of the jack rafters. My rough idea is that room would be framed with insulation to exterior with taped plywood to interior, and that duct runs would be kept centralized to floor plan and within the room. Except for tight working clearances and expense, any considerations that I am missing? VAULTED CEILING: Master bedroom is located under rear hip of roof, and current plan calls for vaulted (cathedral) ceiling. I can squeeze down vaulted ceiling and get increasing amounts of open space between ceiling joists and roof rafters as well as encourage some cross venting with https://dciproducts.com/valleyvent/ between jack rafters and roof deck. Because the roof itself is so low slope, as I make the open vented space above much bigger than 6″, the vaulted ceiling regresses quickly to standard flat. Any advice on how much vented space is enough if fully inside of a hip? Thank you again. I recognize that some of these design choices are suboptimal by compromise with aesthetic and zoning requirements. However, I really appreciate any help in finding creative solutions to accommodate multiple competing requirements while staying away from anything that could lead to catastrophic problems.
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Replies
P.J. Clem,
As you probably know, GBA has long advocated that all mechanical equipment and all ductwork should be located in the home's conditioned space. We have also long advocated that mechanical rooms should be oversized, to allow for equipment changes and easy maintenance.
In short, I'm not in favor of your plan to build a low-ceilinged mechanical room in your attic. It would be far preferable to convert your vented unconditioned attic to an unvented conditioned attic.
Concerning your problem with venting a hipped roof: If you are talking about a cathedral ceiling, you can't use the vented approach with a hipped roof (because every single rafter bay needs to be vented).
If you are talking about an attic with a hipped roof, it's fine from a building science point of view if you reduce the amount of ridge venting below the code requirements -- especially if your roofing consists of clay tiles. Clay tiles dry readily to the exterior, as long as you remember to choose a vapor-permeable roofing underlayment.
If you go that route, your only hurdle is convincing your building inspector to accept your plan.
Thank you for the reply. I appreciate the need for proper access to undertake repair & maintenance, and I believe that I understand the ventilation impediments a hipped roof introduces.
I am willing to undertake a detailed design study to determine if there is space to achieve both a sufficiently-sized, attic-located, conditioned mechanical room as well as sufficient ventilation of an unconditioned attic under hip roof.
My current understanding is that an attic under hipped roof with sufficient intake and exhaust venting will perform well, even though the jack rafter bays are blocked at top end by hip rafters.
Are you aware of any resources that might be helpful in answering:
(1) How much open air space below jack rafters yields air flow venting behavior that is approximately equivalent to the entire attic being open?
(2) Is there any net benefit to cross-ventilating jack rafter bays by drilling holes, making notches, or installing corrugated material above rafter and below roof sheathing (i.e., connecting consecutive bays such that there is clear path to open air space and vents to exterior)?
Martin,
If you recall, PJ was initially planning to build a conditioned, unvented attic, but everyone here piled on him and encouraged him to build a vented attic with the mechanicals inside the conditioned space. He was concerned about spray foam and rigid foam between the rafters for all the reasons commonly discussed here. So, he is trying to figure out how to bring the mechanicals inside the conditioned space without moving them into the top floor, by building a mini third floor surrounded by vented attic.
I agree that building a tight mechanical room is not a great option. I've seen it done several times recently, and never done well. Even if it is well insulated, it has significant surface area that increases energy loss, and air sealing the room is very challenging. I don't find the space constraints much different than if he tried to carve a mechanical room out of his currently planned second floor living space, as that will probably have greater constraints on usable space.
Your best bet is to really commit to installing the mechanicals within the top floor itself, with a mechanical closet, ductwork in soffits, etc. This does take up space and it can be an architectural challenge, but it will provide the best energy results. Then you can create a seamless air barrier at the ceiling and pile insulation in the attic. The more you insulate, the less mechanical equipment you need, and the less space it takes up.
You could consider a high velocity A/C system that uses 3" ducts. These can save substantial space, but they do have somewhat higher fan energy use. You also still need a large return duct and a loop of 12" rigid duct for sound control and proper distribution. But overall, these take up significantly less space than a traditional system.
You can also consider conditioning the second floor with minisplit ductless heads in the bedrooms. Then you have no mechanical closets or duct space to worry about. This is a viable option in all climates.
As far as your venting question goes, Martin partially answered it - Clay tile roofs as considered to be sort of self-venting because the tiles themselves provide so much drying for the sheathing. As Martin said, use a vapor-permeable underlayment. Your wide overhangs should allow you to put plenty of soffit venting to meet code. If you only have soffit venting, you are supposed to have 1:150 ratio of vent to floor area. But with good air seals and deep insulation, clay tile roof, and ridge vent on whatever ridge you have, 1:300 venting ratio would probably work just fine.
PJ,
The code allows vented unconditioned attics to be vented with soffit vents alone, with no ridge vents. If you take this approach, you'll need soffit vents with 1 square foot of net free area for every 150 square feet of attic floor area.
For further details, see this article: "All About Attic Venting."
Thank you all. I believe I understand the considerations brought up.
Now searching for Tardis-like space to place equipment on living floor level. If that proves terminally infeasible, I will have to figure best ways to minimize energy loss penalty of placing above ceiling. Either way, I am definitely to the point of having to make compromises.
A reminder to GBA readers: The best time to finalize insulation details like those that PJ is struggling with is early in the design process. You want to avoid compromises that might occur if the decisions are delayed.
For every project, it's essential to know -- at an early stage of the design process -- what type of insulation you'll be using, the location of the mechanical room, and the location of all ducts (if any).
To read more on this issue, see "Plan Ahead for Insulation."
That is great advice, and whereas it would have been beneficial to have known then what I know now, I am happy to know now what I did not know then.
In my particular case, 2 years into the design process, I effectively finalized the insulation details at least 3 separate times. Competing constraints, unwillingness to compromise, and an evolving understanding of the details has been my storyline.
Designing a custom house that performs well, appears beautiful, and can be economically constructed has been an exceptional challenge. My current opinion is that getting all 3 at the same time might very well be impossible.