# Q&A: A Forum for Green Building Experts and Beginners

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### Do I need to use R10 minimum on the roof deck in climate zone 4C in all cases?

I am detailing a low slope unvented roof. I will be using spray foam & then batt insulation to total R38 under the roof deck in Climate zone 4C. I could meet code just with that but would like to use rigid EPS board, R6, on top of the roof deck to prevent thermal bridging. This introduces a third insulation that is not covered in the code
- Do I need to use R10 to meet the minimum above deck value? Does this rule still apply since I'm using spray foam directly below the deck?
Thanks, Andy

Asked By user-7003242 | Jun 22 18

### roof vent size for vaulted ceiling

Hi
I'm building a garage with a vaulted ceiling and at the point where i need to vent the roof. I see there are formulas for calculating attic ventilation but i can't find a formula for calculating a vaulted ceiling.
Should I measure sq. ft. of each roof side and add both sides to get the total sq ft? For example, my roof is 25' long and 10' from wall plate to ridge. This is 250 sq ft per side bringing the total to 500 sq ft to ventilate.
So i'm thinking i can now use the formula which says i need 240 sq in at the soffit and 240 sq in at the ridge???

Asked By nepal | Jun 22 18

### Unvented roof — vaulted ceilings — moisture, rot, etc.

Im hoping someone can steer me in the right direction for an issue Im having with my current roof.

As best I can describe here is the situation

Full home remodel completed in 2015 including 2nd story addition and brand new roof.
Home is in Southern California about a mile or so from the beach
Vaulted Ceilings exist in every bedroom, however the problem seems to be localized to one side of the house where there is not attic space, no venting.
Canned lights exist in each room, they are just standard cans IE they are NOT fully enclosed/sealed

Asked By Meyersrl | Jun 22 18

### Conventional heat pumps with mini-split low temp efficiency?

There seems to be quite a few options for extra high efficiency heat pumps when you go with mini-splits, but I am struggling to find conventional heat pumps that stand out. It will be connected to a standard ECM motor air handler, like a normal a/c unit would. The ducting is basically in the plans, so there is no going back now.

Asked By Mai Tai | Jun 22 18

### Most efficient way to run Mini Split ACs

I'm currently cooling ~1900 sq feet home, 2 floor Colonial zone 5, with an AOU24RLXFZ hooked up to a 12K wallmount @ top of open stairway, and a 7K in the master BR which is 1/2 of the second floor. I'm very impressed with the cooling abilities so far as Boston has seen some days close to 90. I keep the stairway unit at 71 (1 degree offset in function settings), and BR is at 72 in economy mode, but on low fan so it is always running quietly (fan makes annoying ticking noise in auto fan mode). Entire home +/- 1.5 degrees of 71.

Asked By kjmass1 | Jun 22 18

### Thermal bridging solutions when closed-cell foam is used?

What is the best way to handle the thermal bridging of the wood framing of my house -- recognizing that some closed cell foam is specified for the wall cavities? We are constructing a house in climate zone 5 with 2x6 studs using the "hybrid wall assembly" as defined by Joe Lstiburek in his article "Hybrid Attics and Hybrid Walls." I have seen no comments about managing thermal bridging using this type of wall system.

Asked By Judy5 | Jun 21 18

### Roofing without sheathing

We are currently doing a retrofit that includes a vented attic with lots of cellulose. The baffles will be ripped PIC with FG facings. The current roof is metal over 2x3 (actual) battens with no underlayment. In the next year or two we will need to replace the roofing. For some of the other details we are working out now, I want to make sure I know how we will be handling the roofing when it comes time. So here is the catch, do we need to sheath if we already have the battens? So far I have come up with 4 options:

Asked By Matthew Michaud | Jun 22 18

### Log home heat loss

I am currently working on a project that the homeowner has asked me to help size a replacement furnace for a log home. Currently the home is heated by an outdoor woodstove, which used 15 cord last year. The outdoor stove heats water which moves to a water to air exchanger in an LP gas forced air furnace. The gas is used only as emergency heat. I'm planning on conducting a blower door test. (Last log home blower door test I conducted was 19 ACH50!) There is some roof repair that needs to be completed, should be able to figure out the R-value in the roof at that time.

Asked By Randy Williams | Jun 19 18

### HRV / HVAC engineering help in Ontario

My building inspector came to do my HVAC rough-in inspection today. After he had ignored my e-mails and phone calls for 2 months, I gave up and just bought the Zehnder unit I wanted. The last thing he had said to me was he was "sure it would be fine", and he'd call me back in half an hour (which he never did, called him again and got another promise of a return phone call that never happened). So now he claims he sent an e-mail telling me the Zehnder didn't meet the Ontario building code, and needs a report from an engineer to allow it to pass.

Asked By Trevor Lambert | Apr 26 18

### Best practices for fastening lath / 3-coat stucco over rigid foam

There seems to be some debate over how to anchor 3-coat stucco over rigid foam while maintaining <1/64 deflection to avoid cracking. It seems that for:

<=1" Foam, Standard Wide Crown 2" Staples into Studs are Adequate
2" Foam, Lathe & Plaster Washers and 4" #10 Screws every 6"
4" Foam, Furring strips or add additional OSB/CDX Sheathing over Foam and staple lath

My question is,

1. What would you all recommend for 1-1/2" Foam (R-Max Polyiso)?

2. Would there be any advantage to a hybridized system of staples and screws/washers - i.e. screws every 12", staples every 4"

Asked By Zane Bridgers | Jun 21 18