HVAC ducting on the second floor
Hello All
Two story floor plan in the making with an additional basement. Zone 5A. First floor ~ 2500 sq feet living space with two storey great room. Second floor would be around 2900 sq feet space (extra space above garage). The space above garage will be conditioned and part of the second floor. There will be a vented attic at the top, without any intention of making it a usable space in future. Plan is for R80 blown in cellulose in the attic. Exterior walls would be R40 (Zip + Rockwool comfortboard). Will have an ERV.
Two forced air HVACs – one in basement for Basement + first floor, and second on the second floor, primarily for the second floor.
Question is how to run the ducts in the conditioned space in the second floor. I do not want any perforations in the ceiling air barrier.
Current plan is to have a conditioned mechanical room in the space over garage (at one end of the second floor). We cannot find a good place for the mechanical room in a central location on the second floor. The second floor has a straight hallway in the middle, so the man trunk will straight run over the hallway (bulkhead/drop soffit) and give laterals to a master suite (with two closets and bathroom) on one side, and two other bedrooms (with closets and bathrooms) + office, on the second side.
Initial plan was 9″6′ first floor ceilings, and 8″6′ second floor ceilings. No ducts in Attic.
HVAC contractor has suggested to increase the ceiling on the second floor to 9″6′ to have a 12″ bulkhead, and use 2’x 12′ ceiling joists (upgrade from 2′ x 6′) for top lateral take-offs. This is adding to costs. Plus then the ducts will flow in the ceiling, and we will have to air seal ceiling supply vents. I just want to have no penetration in the air sealing of the ceiling (planning to use SIGA Majrex).
I’m unable to clearly find how bad are “Side” lateral take offs compared to “top” lateral take offs. My thoughts are that we can increase the second-floor height by 6′ (from 8″6′ to 9″) and use “side” take offs, and those take-offs can run parallel to ceiling, but below ceiling, and run through bulkheads in closets/other spaces, and the supply vents would be on the lateral walls of the bedrooms, close to the ceiling. Making a 12″ bulkhead will then only drop ceiling height from 9″ to 8″ in the hallway.
Does this sound okay?
Any other ideas?
I googled, and some ideas are to run an inverted soffit in the attic, with that space being conditioned, but that sounds complex to me to pull it off. I just want no penetration in the second floor ceiling/attic.
Thanks in advance!!
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Replies
There was an article here in the last week or so about putting a floor on top of the ceiling joists and making that the air barrier. This approach would work for you as well. You can still install your R-80 cellulose on top of that and an added feature is that the plywood floor is much more robust and able to take the weight of the cellulose. Majrex won't do that unless it is well strapped.
This way, all of the second floor joist space become available for mechanical and electric chases. If you do decide to go to a 12" thick floor for ease of access, you should consider I-joists. In the larger sizes they are usually cheaper than sawn lumber and they are certainly easier to work with. Drilling and notching rules are looser, giving more options when you have to go crossways with infrastructure.
Thank you. I think I found what you mean here.
https://www.greenbuildingadvisor.com/article/ceiling-air-sealing#0
I will discuss this with my builder.
I am thinking a cheap OSB over the ceiling joists, and then SIGA Majrex for vapor on top of the OSB, and continuously attached to the Zip R on the exterior walls will work, and could be cheaper than using the Zip 7/16 sheathing on top of ceiling joists. SIGA would allow moisture to move from 2nd floor to the attic, but not in the opposite direction.
In this set up, the moisture from 2nd floor will cross the drywall (below ceiling joists) into the cavity. Would it damage the joists or OSB above joists?
Update on final plan.
Will raise the 2nd floor ceiling height to 9 feet, and then have a 1 foot bulkhead in the central hallway. The 2nd floor ceiling will have 12 inch ceiling joists for lateral take-offs (top take-offs) from the main trunk in the bulkhead. We will make Zip sheathing as a floor over second floor ceiling, and in continuity with Zip on the walls. So, the air barrier would be above the ceiling joists, and all the space within the joists will be available for mechanical/electrical and plumbing. Drywall will act as the first air barrier. I will place Rockwool comfort batts at the periphery of rooms within the joists, wherever I won't have the space for R80 cellulose (where rafters will meet the top plate above the ceiling). They do not use raised heel trusses here.
Why not just do floor grilles to serve the second level? Route your supply air ducting in the floor system of the second level - I recommend open web floor trusses, but I-joists could work too if you plan well. You could do a downflow air handler on the upper level, or move the mechanical closet for the second level down onto the first and use an upflow unit. I'd recommend ducting the return up to the ceiling of your second level, if possible, to help even out the temperature. Make yourself a shaft in one of the bedroom closets and get a return grille up high in the main corridor.
Thank you for the response. We thought about that, but issue is that I do not have space on 1st floor to convert into a mechanical room. Also, if I keep the HVAC on the 2nd floor, the mechanical room will be in the conditioned space above garage (garage is not conditioned). And air handler cannot enter the floor above garage.
Could you put both HVAC units in the basement and have a chase up to the first floor ceiling and put the duct below the second floor with floor diffusers on the second floor?
We had a meeting with the HVAC contractor. Placing both units in the basement added more turns and take offs. Decided to keep second unit on second floor with a straight main trunk in a bulkhead over the central hallway, and then take top take-offs to each room. This would be a long straight trunk with 45 degree take-offs.
I'm with Epato.
Run the ducting through the floors of the 2nd floor. That space is already there and is free. It is pretty much how it is done here.
If you can't run the trunk through the floor, you can run it in a bulkhead bellow. Random real-estate picture, step is the ceiling is the trunk:
https://cdn.realtor.ca/listing/TS638817117475130000/reb82/highres/4/w12117084_30.jpg
You can also run a bulkhead like the above in the hallway on the 2nd floor bellow the ceiling. From there you can run to wall registers to each room. Floor registers work better in heating climate though.
There is no such thing as no space for equipment. A ducted heat pump has a footprint of about 2'x2'. There is no way you can't find space for that inside a house that big. I know people don't want to give up sqare feet, but having long runs like what you propose is asking for an HVAC system that doesn't work well.
If you really don't have the space, you can look at a slim ducted heat pump. This can be installed in the ceiling of a closet or in a dropped ceiling in the hallway. They need about 1' of ceiling space but they don't take up any floor space.
Probably something like this is in the ballpark:
https://ashp.neep.org/#!/product/184478/7/25000/95/7500/0///0
Update on final plan.
Will raise the 2nd floor ceiling height to 9 feet, and then have a 1 foot bulkhead in the central hallway. The 2nd floor ceiling will have 12 inch ceiling joists for lateral take-offs (top take-offs) from the main trunk in the bulkhead. We will make Zip sheathing as a floor over second floor ceiling, and in continuity with Zip on the walls. So, the air barrier would be above the ceiling joists, and all the space within the joists will be available for mechanical/electrical and plumbing. Drywall will act as the first air barrier. I will place Rockwool comfort batts at the periphery of rooms within the joists, wherever I won't have the space for R80 cellulose (where rafters will meet the top plate above the ceiling). They do not use raised heel trusses here.