HVAC – Vaulted ceilings, what to do ?
We have just came to an agreement with our builder to move forward on our new house. We have selected our HVAC folks of our own choosing based on their reputation. While they are very good at what they do, I want to have some additional input.
Attached is a pretty poor attempt to convey what I am trying to explain but hey, drawing is not my thing.
The house is a 3 story, ground level is parking and storage. The second floor is our main living area.
2 bedrooms, kitchen, great room, laundry etc. ALL floors have 10′ ceilings. Here’s the rub, The REAR of the second floor will have a vaulted/volume ceiling which will look great. The great room/kitchen is
39′ L x 18″ D. The question is how to properly address HVAC for this area. The house will have two separate systems, one for the main living area and a second system for the 3rd floor.
The HVAC folks expressed a concern regarding a possibility of mold in the ceiling of the great room if it were not addressed up front. It makes sense, wide open spaces look great however heating and cooling those is another story.
For insulation etc., roof will be closed cell foam and all exterior walls will be open cell. Attic will be conditioned space. We will have 2, possibly 3 big ceiling fans in the room to help to move the air.
My question concerns what to do in designing the system to address any possible issues up front now vs later, we get one shot at this. Do we put the return supply lines up in highest part of the roof etc OR ?
From what little reading I have done, cooling and heating rooms such as this is a big challenge. Invariably I am not the first with the issue and surely there is a way to do this right. There are no do overs, if it is not done right now well….. bad day.
Suggestions ?
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Replies
Thomas,
Where do you live? Advise is often climate-specific.
You should hire an independent HVAC engineer or RESNET rater to perform an aggressive Manual J calculation. HVAC contractors rarely complete an accurate assessment. Your outside consultant can also specify the components for your HVAC system.
You also need an ventilation strategy. See this article for why: https://www.greenbuildingadvisor.com/blogs/dept/musings/designing-good-ventilation-system
Cathedral ceiling can have moisture issues if not properly designed. See this article for details: https://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-insulated-cathedral-ceiling
If you ultimately decide to use closed cell foam in your insulation plan, consider using one of the new and greener products. Most closed cell products use blowing agents that are very damaging to the environment.
Steve, forgot to note. We are in Pensacola FL and we will be hiring an independent engineer to do a Manual J. Actually he's out of GA and was recommended here on the board, heck you may even have recommended him. Our climate is HUMID in the summer. For the most part, we use air although we have a couple of months or so of heat.
We will be using David Butler at Optimal Building for our Manual J. Just located info I wrote down.
In keeping with the same thought process here. Would sizing the unit for the 3rd floor as if it were all heated and cooled be the way to go and use that system in tandem with the 2nd floor system to help keep that upper vaulted ceiling cooled down ? Running supply lines to vents up in the ceiling area ?
Just to keep things straight - David Butler with Optimal Building Systems is from Huachuca City, AZ. Allison Bailes with Energy Vanguard is from Decatur, GA. Both very capable of doing Manuals J,D,S & T (You need all four) and ventilation strategy.
Thanks, my bad very much appreciated Armando.
Hi Thomas,
If you are working with David Butler, you should run these questions by him. He can explain his design decisions and respond to any concerns you have over heating and cooling the great room.
If you are building the type of "pretty good house" often mentioned on this site, you are making it much easier for someone like David to ensure your house has uniform temperatures and comfortable conditions 24/7/365.
Spaces that are tall compared to people are sometimes hard to heat, if the heat all goes to the top leaving the people in a cold area at the bottom. This is called "stratification" Cooling them generally is not so much of a problem. So you might not have that much of a challenge.
But if you have a building that is very well insulated and air sealed, you won't have a problem with stratification even in a climate where it's hard to heat and cool. If the heat can't get out, it will bounce around inside the space until the temperatures are equalized.
So making sure you have excellent insulation and air sealing is the first step. You might think that by getting spray foam, you are all set with that. But that's not necessarily the case. In an insulated stud wall, the heat leaking through the studs becomes a bigger fraction of the heat flow as you make the insulation better. So once you have pretty good insulation between the studs, you can't do much better by making it thicker. You get more benefit by putting continuous insulation, such as polyiso foam boards, over the whole wall.
And in the roof, the problem is that even though closed-cell foam is effective insulation, it's also expensive, and so people tend to either just barely meet code requirements, or sometimes actually fail to meet code requirements, where as with other types on insulation, you can go well beyond code at low cost.
For the details on what Steve was talking about on which closed-cell spray foam to choose, see
https://www.greenbuildingadvisor.com/blogs/dept/green-building-news/next-generation-spray-foams-trickle-market
Thank you gentlemen, and yes I will absolutely run this by David. I would say we are building a
"pretty good house" by the standards I often see mentioned here. We were originally thinking to go with an open cell foam on the roof and exterior walls. But, after reading a recent article here on a long term study with open cell foam, I basically nixed that idea. The obvious issue with closed cell is expense and it is very expensive. I don't have any hard numbers on the difference between the two as of yet. We are going to shoot for doing a closed cell foam at this point. To me, it does not make any sense to go to the extra expense and not meet code with insulation. Hopefully we won't experience extreme sticker shock when the numbers come back in, I know they'll be higher and hopefully priced to where we can make that work for us. If it is just out of the equation due to cost, then "Houston we have a problem". Not sure which way I'd turn. The big focus for us on the house is to make it as energy efficient as we can. It won't be a net zero house but it will be tight. Lastly, the cost vs payback comes into play to a point. We are in our 50's so if a calculate break even point comes back and we are 95 years old, well time to rethink things a bit !
One interesting thing to note is the the new HFC foam formulations can be used to do a thicker layer in one shot, so for thick insulation, the overall cost can, at least in theory, be lower than for the conventional closed-cell foam. So make sure you have quotes from at least one company that is familiar with those, such as Demilec's Heatlok XT HFO or Lapolla Foam-lok 2000 4G.
Cupola vent.....
In your climate zone as little as 1" of polyiso or 1.5" of EPS above the roof deck would be sufficient to let you use cellulose or fiberglass (if fiberglass, 1.8lbs minimum density) on the under side of the roof deck without increasing moisture risks. Of course more is better. A 4" polyiso nailbase panel up top and R15 fiberglass underneath tight to the roof deck would beat code-min thermal performance and would beat a code min solution of closed cell polyurethane thermally bridged by rafters by even more (due to the performance robbing R1.2/inch wood bypassing the R6/inch foam.)
Air seal the roof deck with tapes/caulks, or (if local codes allow-probably do) glue the roof decking to the rafters/trusses with construction adhesive, which will enhance it's hurricane resistance.
Thanks Dana, a good option if the closed cell prices are out of reach.
I like Dana's option, even if the closed cell pricing is accessible, because you'll get an overall better assembly R-value, considering thermal bridging.
Thanks Charlie, great forum and advice!
Best,
Thomas
Perhaps your HVAC folks can explain the cause of ceiling mold they have seen. Lacking that, I'll guess that they have seen ceilings with air leaks and when cooling, stack effect pulls hot humid air inwards - making the ceiling damp. And hot, moist air rises, so it stays there.
If this is the mechanism, then I'd focus on very good ceiling/roof air sealing (as CC foam will do). If that's not adequate, then I'd look at slight building pressurization. Better to stop moisture infiltration than try to dry it after it happens.
A Manual T should help with even distribution.
Double air barriers are a good idea.
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