Posted By Bob I on 22 Jan 2017 03:14 PM
best way is to insulate on the exterior - minimum of 8" of 2" polyiso insulation, with 2x4 strapping above and roof sheathing above that. The insulation is available used to keep the cost down. The thick cedar will be perfect to fasten the 2x4's to, through the foam. Since the cedar's continuous, it'll be a snap. Air tightness is critical, so take the joints and caulk the outside edges of the cedar.

+1

that should say "tape" the joints, using a long lasting acrylic tape such as 3M 8067.

Hi Parahomes: The owner will not be doing anything other than insulating the roof, the crawlspace cement wall and changing the windows. My concern is that going from zero to R-31 on the roof is a waste of money, given that the walls have no additional insulation and that the seams (on the single T&G walls) will not be attended to. Based on your modelling, can you tell me how much he will save by going from R-10 to R-31? I'd appreciate it if you could. Thanks, Rob.

Posted By Bob I on 10 Feb 2017 07:16 AM
that should say "tape" the joints, using a long lasting acrylic tape such as 3M 8067.

Long lasting? No such thing for room temp cures more less weathered cured film adhesives (best if FOD free hot bonded under pressure most don't do), never has been never will be, especially acrylic bonded to wood. When they thermally cycle, and see other life cycle fatigue from building dynamics the adhesives fail in a short time. The only way around this is in a factory autoclave with elevated pressure and temp cure ~ 250 - 350F under vacuum pressure(bagged or plate) in a clean room that does not produce voids or delams, like OSB/Plywood, then metal bonds will see 30 yrs on average, acrylics 5-15 yrs. Your welcome to look at the life cycle 3M data for lap tension/shear of this "long lasting" adhesive I doubt you find and post it?

I think this whole industry be better off if builders focused on the many challenges of robust low cost building and left the design to engineers.

Rob, R5 CS walls, 5 ACH @50 ~ $100 yr diff R 10 vs R31 roof, R7 ish walls. If you decrease ACH I posted the values and if you give me your window areas per facade old/new u-value and SHGC, door sizes and direction the front is facing I can tell you more.  Without more in the wall and less in ACH I doubt you see a whole lot of differences.

Interesting how consumed energy gets exponentially lower @ < 1 ACH. But, there is a price to be paid.

Let's just say that Mr Para and I disagree about the tape. We use these tapes and have used many others; these are our findings and they have been confirmed by several other sources.

Posted By Bob I on 10 Feb 2017 01:04 PM
Let's just say that Mr Para and I disagree about the tape. We use these tapes and have used many others; these are our findings and they have been confirmed by several other sources.

I'm baffled how you have more life cycle data than 3M. You should share it with them and post your data. The only way to do that is to do blower door testing @ 5, 10, 15, 20....yrs? Compared to the new construction test? 3M can do this in a hot box BUT it is very expensive, and it is still complex and limited unless it includes a test plan for international climates. Add at least 3 decades of more testing to that, and we are still just getting started. The test bed would have to be very large even more expensive. I don't know how much of this testing you have done, I have done alot. In 3 weeks I just got hired today as a consultant to go to another lab and do more of this type of certification and testing. The testing has to be done to a spec like ASTM or ISO. What spec did you test to?

Para how about ASPCA test