New construction cathedral ceiling
The home I am constructing is in US Department of Energy climate zone 6 in Bloomville NY. I have a 30′ x 40′ 2-story house with a great room, 2 bedrooms, laundry room and 1 bath downstairs and master bedroom, bath and loft upstairs.
The downstairs living room is open to the cathedral ceiling. The upstairs back master bedroom, bath and loft are open to the cathedral ceiling also. The loft is over the kitchen and dining area and looks over living room.
The 2×6 (16″ OC) stud walls are filled with open cell Icynene for an R-21. The exterior walls have 1/2 inch plywood, 30 lb. felt, 3/4 inch faced polyiso foamboard, cedar breather rain screen and then western cedar shingles prestained on all sides.
The south facing windowed and doored wall and north facing wall have 2 foot overhangs. The first floor west and east walls have covered porches to allow cool breezes accross interior of house in summer. The windows and doors are double pane low e argon filled. The upstairs east and west dormers also have 2 foot overhangs. The dormer roofs are a 4/12 pitch and the cathedral roof proper is a 10/12 pitch.
The rafters are 2 by 10’s (16″OC). They have a 1 inch rafter to rafter vent from soffit vent to ridge vent. I went with a vent because I have heavy duty GAF Camelot series roof shingles.
Under the rafter vents I have 4 inches of Corbond III closed cell sprayfoam at around R-24. Then I have 4″ depth of rafter left. I plan on putting in HD R-15, 3 1/2″ Ecobatt or Johns Mansville unfaced insulation batting. Over the cathedral studs I want to put 1″ T&G Dow 4×8′ XPS unfaced foam board for a thermal break. Then the sheet rock.
The walls will get a 3/4″ unfaced foam board. Since the exterior already has a vapor barrier, the walls will have to dry to the inside. Therefore no facing (vapor barrier) on the interior foam boards. The electrical boxes are figured in for depth with foamboard on interior of exterior walls.
I would have loved to put all the insulation on the outside but I am building as I can afford and at the time I couldn’t afford it. My father and I are building it and we are in our fifth year now. Last spring we had the foam insulation put in.
So now I am trying to achieve an R-46 or R-48 in the rafters and almost an R-30 in the walls. I guess I am asking if this sounds good so far. I haven’t put the batts of insulation in the ceiling yet or any interior foam board up.
I want to make the envelope tight and install a HRV. Any reccomendations on finishing off the insulation and what brand of HRV to use would be greatly appreciated!
Bonus Question: Where would the dew point be in my exterior walls with the 3/4 inch exterior foam board (R5) the icenyne (R21) and the 3/4 unfaced XPS foam board(R3.8)?
Thanks, Kathy
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Replies
I don’t design homes in your neck of the woods, but in CZ6 I would want 2” min. rigid insulation for thermal bridging on the outside of the roof and wall sheathing.
You need to do a psychrometric chart analysis to determine a dew point, which is relative to insulation, indoor RH and delta T; maybe someone in your area will know the numbers.
Here is a formula to figure out your dew point in the assembly:
Ts=Ti-DT(Rc/Rt) where:
Ts= temp sheathing
Ti= temp interior
DT= (Delta T) difference of temps inside and out
Rc=thermal resistance of cavity
Rt= thermal resistance of assembly
Thanks for the formula. I would say we are on the border between zone 5 and 6. The codes here are R21 in the walls and R48 in the ceiling I believe. I put up what I could afford at the time. I have been building as I can afford it over the last 5 years. That is why I would like to beef up the inside a little. Amazing the difference the foam made. I am really pleased so far. Just wanted to know if anybody had any better ideas about finishing off the inside.
Kathy,
Based on Albany, NY’s coldest 3 months temp of 25°F: (Closest to)
1. If indoors temp 70°F & 30%RH then your dew point is 37°F. So you need R10 insulation outside of the sheathing to elevate the sheathing temp to 39.51°F.
2. If indoors temp 70°F & 40%RH then your dew point is 44°F. So you need R18 insulation outside the sheathing to elevate the sheathing temp to 45.76°F.
Right now, with ¾” insulation R3.5 your sheathing temperature is 31.42°F, well bellow both dew points from above. You definitely will have condensation inside your sheathing.
I think this is right... too early here for arithmetic... ;-))
Kathy,
IMHO ... I'd be a little bit nervous about not putting a vapor retarder on the inside, and still a little more nervous about condensation in the walls. (remember that vapor barrier and retarder are not the same thing, you should probably still have a vapor retarder on the inside). The wall assembly you mentioned would probably put the condensing surface square in the middle of the open cell foam. A good rule of thumb is to have somewhere around a 50/50 ratio between the insulation outside of the vapor barrier and the insulation inside the vapor barrier.
My fix (if possible) will have to be on the inside since the siding is already up. Wouldn't the interior foam board and final interior paint finish act as a vapor retarder... plus the fact that I will have a heat recovery ventillator in the house?
One more option someone told me was to put up a faced polyiso rigid foam board up forming a second vapor barrier and isolate the interior of the exterior wall. I would have to carefully seal the interior of which I have been told is a tall order.
My sister's house down the road has a 3/4 inch faced polyiso on the exterior with fiberglass on the inside. That's been 7 years and no problems yet on the sheathing when they broke through for an addition. Is that a differenent situation than the icynene in my walls?
Kathy,
Thin exterior foam sheathing is risky in cold climates like yours. But thin exterior foam sheathing does not guarantee problems; it sounds like the sheathing at your sister's house is OK.
Thicker exterior foam sheathing would have been better, but it's too late now. I would recommend:
- Installing a "smart" interior vapor retarder (MemBrain), and
- Maintaining a fairly low interior relative humidity during the winter.