Replies

1. 
   Bill_NC | Apr 20, 2014 07:33pm | #1

   PS:

   Air barrier would be by taping both layers of foam. Both layers of plywood and the foam would extend down and lap both the 1st and 2nd story band joists.

   Drywall would include airtight details as overkill.

2. 
   wjrobinson | Apr 20, 2014 08:25pm | #2

   The steel is on all of the wall? I would try to get the steel adjacent to the air barrier and it will also also be a moisture barrier having no perm. Then set up the rest of the layers to dry out and in away from the steel. So steel next to foam and go with 3/4 foam layers.

3. 
   Bill_NC | Apr 20, 2014 10:32pm | #3

   The steel would be on the entire perimeter of the bedroom/bathroom.

   OK, a double layer of 3/4" XPS or EPS would be somewhat of a vapor retarder, if not a vapor barrier. So the wall should ideally be able to dry from the foam both to the exterior and the interior. So the steel (a vapor barrier) needs to be next to the foam to avoid a double vapor barrier. Makes sense.

   Apply plywood to the exterior of the studs. Then steel plate, taping the seams. Then 2 layers of foam, taping each layer. Then the second layer of 3/4" plywood, which would have to be fastened to the steel plate, or, through the steel plate to the 1st layer of plywood (or through the steel and 1st layer of plywood into the studs).

   I might consider substituting a 3rd layer of plywood for the steel, applying a double layer to the exterior of the studs, taping the exterior of the second layer, then the double layer of foam with both layers taped, then the exterior layer of plywood.

   Many thanks AJ.

4. GBA Editor
   Martin Holladay | Apr 21, 2014 07:01am | #4

   Bill,
   Q."Will the following wall section perform well as far as energy and moisture are concerned?"

   A. Yes from a moisture standpoint. From an energy standpoint, the walls are so-so. Thicker rigid foam or a higher quality insulation between the studs would improve the thermal performance of the wall.

   Q. "What type of foam would be best?"

   A. Most green builders have concluded that polyiso is the most environmentally friendly type of rigid foam. For more information on this issue, see How to Install Rigid Foam Sheathing.

   Q. "Since this wall section would have to have some drying capability to the exterior, what should I look for in a housewrap?"

   A. As you have evidently already figured out, the wall needs to be able to dry out in both directions from the layer with the lowest vapor permeance -- in the case of your wall, from the steel layer. Almost all brands of housewrap are vapor-permeable, so I wouldn't worry. For more information on this issue, see All About Water-Resistive Barriers.

   Q. "Would a vented roof system or sealed perform best?"

   A. For a story-and-a-half (Cape Cod style) house, it makes sense for the insulation to follow the sloped roof assembly. If you take this advice, the rafter bays can be either vented or unvented, as long as your roof has no dormers, skylights, chimneys, valleys, or hips. If you have a complicated roof geometry with some of these listed features, you should create an unvented assembly.

   For more information on these issues, see:

   Two ways to insulate attic kneewalls

   How to Build an Insulated Cathedral Ceiling.

5. 
   Bill_NC | Apr 21, 2014 09:40am | #5

   Martin:

   Many thanks.

   I'll explore the costs of higher R value in the 5-1/2" stud depth.

   Foil faced polyiso is what I used on the exterior of the walls on the original house.

   I'll read up on water resistive barriers.

   The roof is a simple 10/12 gable. There is a wood stove chimney near the ridge, and a level ceiling at 8'. So after re-reading the several articles on this site about cathedral ceilings, I was thinking I'd vent the roof with a 1-1/4" air space (1x furring and 1/4" plywood or OSB); insulate between the rafters with either fiberglass or closed cell foam; 3" foam under the rafters; air tight drywall with no recessed fixtures. Insulate the roof slope from the eves to just above the ceiling, insulate the ceiling flat to min. R 40, fabricate a well insulated, airtight scuttle, vents only at the eves and ridge, pay attention to air tightness where the chimney penetrates the 1st floor ceiling and the 2nd floor ceiling.

   I think I can get the chimney penetration of the level 2nd floor ceiling flat more airtight than trying to get the penetration of the sloped roof air tight. Insulating the level ceiling also allows a more cost effective higher R value.

   I have experience getting drywall airtight. A rental house that I built a couple years ago passed the Energy Star blower test even when a window was open 2". So I feel pretty confident I can get the GWB air barrier in the second story very tight.

   Please let me know if I appear to have missed anything.

   Thanks again.

   Bill

6. 
   Bill_NC | Apr 22, 2014 05:47pm | #6

   After reading the article on water resistive barriers, I came away with a question.

   Are you advising using a house wrap such as Home Slicker Plus Typar with vinyl siding?

   There would be an airspace, but it would be continuous vertically only through the nailing slots. The vinyl would not absorb any moisture.

   Would you think that the vinyl is loose enough not to require a wrinkled house wrap?

   Thanks,

   Bill

7. GBA Editor
   Martin Holladay | Apr 22, 2014 07:02pm | #7

   Bill,
   Home Slicker Plus Typar is not necessary with vinyl siding. Vinyl siding is inherently well drained. All you need is ordinary housewrap or asphalt felt behind vinyl.