Vented or unvented attic in a large house
Hello All
We’re developing floor plans for a large house in Climate Zone 5A.
First and second floor would be 2300 sq feet each, with second floor having less living space with duplex living room. There will be a basement, which I will insulate but not finish, around 1500 sq feet. A 4 car garage will be attached and hopefully decoupled from home. The garage area would be 1000 sq feet. Above the garage, we’re planning a conditioned unfinished space for a future ADU.
This house size would not be passive but I’m hoping for a pretty good house. I am hoping to get R40-50 walls, R90 at roof, and and ACH 50 score of 1 or less.
The builder I am working with has never done a conditioned attic. And also not done a completely air tight envelope with ZipR sheathing before. The builder does two HVACs with one is basement, and one in a “partially conditioned attic”, whatever that means. We haven’t done calculations with the Electrical person yet but we feel we’re gonna be better with two HVACs.
As you can imagine the roof would be large.
My concern is that the builder won’t be able to make a completely conditioned attic since he has never done it, and I dont have budget to hire a passive house project manager. Plus it will be way more expensive. And it will add a lot of load on the HVAC.
Is it a good idea to make a very good vented attic with close detail to air sealing of the ceiling, and have the second HVAC in the the conditioned space above the garage?
Pros and cons? Any guidance is really appreciated.
Thank you.
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Replies
Good for you for asking these questions in the planning stage, these details should be planned out before you break ground.
Generally a vented attic is easier to get right. The downside is that the attic won't be conditioned space.
In a house that size, especially new construction, there is no reason to run any of the HVAC in unconditioned space. If you can't figure out where to put the ductwork, raise the second floor ceiling and have a dropped ceiling.
Very easy to vent a roof assembly AND tuck conditioned space beneath the rafters. Does not need to be expensive. I use several methods. Will cost more than a flat ceiling.
Also, very easy to hit the air infiltration rate you mention- even in a 2300 sq ft house. Not difficult to hit PH standards. At the upper end of the size spectrum of a contract I’d take- I believe large is wasteful. While 2300 sq ft isn’t modest, it’s far from large. Also, tiny houses present more difficulties in hitting low ACH targets- a single small leak is much more impactful.
You do not need to build a certified PH, but you may consider working with a different builder who better understands high performance building. If you do stick with this builder, he should take a Boot Camp at Emu in Colorado BEFORE you complete the design phase as an informed builder is integral to the process.
Also, R50 walls and an R90 roof in your zone are well beyond the point of diminishing returns. Upspend on windows and air sealing instead. Further, you won’t achieve an r90 vaulted ceiling without dense packing parallel cord trusses or i-joists, allowing room for venting, that’d be a huge roof assembly. In a nutshell, R90 would require a flat ceiling. I can hit r60 in a vented vaulted, no foam assembly.
Two HVAC systems? If you hit .6 ACH @ 50 pascals, with the insulation levels you suggested, your heating and cooling demand will be minimal. I digress, see comment regarding different builder.
Edit:
Agree with Walt. Missed the R, thought you suggested standard Zip. Zip-R is a terrible product for a pile of reasons. Double stud walls are by far the best way to achieve a high performance wall; r30 Bonfiglioli walls as a pricepoint option.
Thank you for your reply.
I will try to answer some things. Please bear with me as I have never built a home before, and never even done any kind of remodel. I have been reading GBA and watching a ton of videos on YouTube, and trying to learn the building science.
Due to land being owned by the builder, and desire for this school district, I do not have builder options.
I was going to try to accomplish R40-50 walls as follows.
Builder stated 2"x6" stud wall assembly
Place Zip R6 or 9 outside the studs for air and water barrier, and then R15-20 Rockwool Comfort-board or GPS (?Halo) to get R29 on the external side of cavity. This will then have a rain screen, and then a brick wall. Then fill the cavity with 5.5" Mineral wool or cellulose to achieve additional R20-23. The wall will be thick. I will run cost numbers for materials and then make final product choice.
If I go with a vented attic, my understanding was I can do R60-90 cellulose, without much increase in costs.
Regarding vaulted ceiling, we briefly discussed we wont have any. The first floor ceilings would be 9"6', and second floor ceilings would be 8"6'. The duplex living room will have a drop-down roof with LED strips, and I am assuming more space on the roof deck, for a vented roof, and R60 cellulose, with internal rigid board and a smart vapor retarder membrane...I have to understand this better from the builder. The same thing for the large space over the garage.
In addition, R20 under the slab, and R40 basement walls.
I was planning triple glaze R 6-9 european built windows, but I am now concerned that the rates may go very high with the new tariffs announced today. Same for Zehnder ERV and Steibel Eltron water heater...
Please correct me if my understanding is flawed. I really appreciate your time.
From a dollars and cents point of view the R40-50 wall plan is never going to save enough energy to recover its very high cost. The same is likely true for the R90 ceilings.
Your garage bonus room comes with some big energy penalties. It almost requires you to move the insulation to the roofline doing that almost forces you to use spray foam insulation. The cost of spray foam is too high to consider R90 in fact it is so expensive a messily R20 is about the max to ROI. You still need to insulate the floor and all the walls. I think you will find even with the lowest cost fluffy insulation R 60 ceiling is about the most that can ROI.
I don’t see the appeal for Zip+R I feel it make a weaker building putting all the squishy foam between the framing and the sheeting regardless of how many extra nails get added. If you want foam sheets, put it on the outside.
Ductwork and equipment in a vented attic equals poor planning by someone who has zero regard for the operating costs of the system.
“Partially conditioned attic” equals a code violation with a high chance of mold and rot.
Conditioned attic equals something marginally better than equipment in a vented attic but it is poor planning and lazy design.
At 2300 SQF with good insulation one HVAC system is very possible. If the bed rooms are on the second floor 2 systems will give you better control.
If you builder can’t make a conditioned attic there is zero chance that that builder can deliver on your 1 ACH50 goal.
Consider investing 30 hours or so in learning how to computer model your home in BEopt https://www2.nrel.gov/buildings/beopt
16 training videos
https://www.youtube.com/playlist?list=PLHC0xDtkdjgec8QhVt7exJY3tpSLEFk-d
Walta