Insulation question and new construction…. thoughts?
PensacolaPI
| Posted in Energy Efficiency and Durability on
New construction, we are doing a non-vented attic to keep our HVAC ductwork in a conditioned space with foam on the roof deck. Come to find out, we can put foam in all the walls vs using bibs for the same price which surprised me. This will make for a tight house and this brings the question in my mind “what type of mechanical ventilation”? House needs to breath and have a good exchange of air.
House is a 2 story and about 2700 sq ft. 2 HVAC units will be installed. Biggest concern is a good ventilation system..
Thoughts? Recommendations for ventilation? We are in Pensacola, a very hot and humid environment.
Thanks much in advance.
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Replies
Thomas,
Start by reading this article: Designing a Good Ventilation System.
In your climate zone, you will probably end up with a central-fan-integrated supply ventilation system.
Two caveats:
1. Make sure that your contractor includes a motorized damper on the outdoor air duct, and make sure that your system includes an Air Cycler control.
2. Make sure that your contractor knows how to commission the system. Air flow rates must be carefully measured and adjusted to avoid overventilation -- if you overventilate, you will invite too much humidity indoors.
Thomas,
You also should read this article: https://www.greenbuildingadvisor.com/blogs/dept/musings/open-cell-spray-foam-and-damp-roof-sheathing.
Because of your climate, you may want to reconsider installing open cell foam to the roof sheathing.
Steve,
Thomas said that he is planning to install "foam on the roof deck" for a new house. I assumed he meant "rigid foam above the roof sheathing" -- but perhaps he meant "spray foam under the roof sheathing."
He never mentioned whether he is considering open-cell spray foam or closed-cell spray foam for the walls.
Guess I should specify, OPEN cell foam under the roof sheathing. The walls will be open cell foam, closed would be great but we do have a budget to contend with.
Thomas,
Evidently Steve guessed correctly. He's right; you don't want to install open-cell spray foam on the underside of the roof sheathing. Closed-cell spray foam is a much safer choice.
Here are links to two articles on the topic:
Open-Cell Spray Foam and Damp Roof Sheathing
High Humidity in Unvented Conditioned Attics
With two HVAC units it's hard not to oversize them to the point where it affects both comfort and efficiency. Be sure to have a qualified third party (professional engineer, RESNET rater, etc) run an aggressive Manual-J cooling and heating load calculation on it before specifying the equipment. Were you thinking heat pumps? Gas furnace w/AC coil? Package units? Mini-splits?
Odds are pretty good that with 2 units covering 2700' of conditioned space that the design load of one of the units (or even both) will come in at 1-ton or smaller level, and many equipment lines start at 2 tons. A 1.5-3 ton multi-split (ducted or ducted, sized appropriately for the whole house load) would give you options of more zones, but could be reduced to two if desired even for the larger units.
Thanks Dana, greatly appreciated. I've got a lot of confidence in my local HVAC folks. I researched HVAC companies for at least 6 months.
I have great confidence in a number of HVAC companies to install the equipment well, but the list gets a lot shorter when it comes to sizing it accurately. YMMV (Hiring a third party professional engineer to run the load numbers is still my advice.)
Martin, thanks for those links. I'll get with our builder on pricing out the closed cell for the roof. Invariably it'll raise the price a good bit. The balance is going to have to be open cell or bibbs and I'd dare say the open cell would be better than the bibbs for the same price. Really timely articles and spot on for our area!
From my limited experience, there is a large cost difference (3x) between BIBs wall insulation and spray foam. If the prices are the same, one or both might be wrong. Cut rate foam is no value - neither is overpriced cellulose.
You're dead right Mark, you can best believe I am doing my home work there.
Thomas,
As a follow-up to Dana's advice, you may find that a properly sized (and smaller) system will more than cover the cost of using an HVAC engineer or Resnet advisor for the Manual J. It is also important to remember that an accurate design will deliver more comfort and energy savings over the life of the system.
There are no local Resnet folks here or I would go that route. I'll look into finding a HVAC engineer though, the HVAC folks I've got are really pretty good but nothing like a double check. On insulation, what R values would the braintrust recommend and where? Roof deck I am going to shoot for R49 with a minimum of R38. Not sure about the outer walls.
Your HVAC engineer does not have to be local. I used David Butler at Optimal Building Solutions, and all his work was done via phone and email. He does a very aggressive Manual J, which is what you want.
Outstanding, BIG thank you Steve!
Steve, do you have the contact information for David Butler at OBS? I had a quick look on Google and did not find him. Thanks !
@Thomas
http://optimalbuilding.com/about_us.php
Thanks VERY much John, good looking out!