Thank you Laren,
A much misunderstood concept among practitioners, and few publications and books even describe it clear understanding of proper use of vapor barrier in house. I have spoke to many contractors and find most don't take this matter very seriously and just do as they were taught, repeating mistakes and practices that have been passed down from one generation to the next. Primary cause of this is the fact that very few contractors are hired to clean up their own mistakes, so not once do they stop to question the cause and source of wood rot in the roof and walls.
Even in this DIY group you'll encounter conflicting opinions to having an airtight membrane, house envelope as they call it nowadays. Many old-timers will swear that the walls and the ceiling has to allow air to travel through so the house can breathe! I would be happy to show them examples of wood rot in my own house which was built in the 50s were insulation was at this the luxury afterthought. For example condensation water tearing down on insight of window panes is a good example of how warm dropped down to dewpoint from cold air leaks caused by poorly sealed windows and sliding. Where you see that, chances are that the same thing is happening elsewhere in your walls and ceiling.
So what's the solution, let's assume I was going to tear down the outer layer of my house, what would be the proper approach to insulate and seal, so as to achieve an airtight envelope?
In my case, I have a low pitched 2,1/2 in 12" metal roof, on top of a layer of plywood that is nailed on top of the T&G planks across roofs rafters, exposing a beautiful natural wood finish to the underside. Surely am not going to rip up all this old growth Doug fir wood, and I'm not about to sprayed with closed cell foam and cover with drywall.
So whatever the solution has to work on top of the plywood.
After the sheet metal is peeled off and set aside, and any dry rot replaced with fresh wood, you are my options, to cover the top in the following sequence working from inside out:
(1)
start_plywood || adhesive || rigid foam sheets || impermeable membrane (Tyvek) || roofing membrane || and finally sheet-metal finishing off the outer shell.
panels
Now hypothetically let's simulate what happens as warm air builds up inside the house and permeates through the wood planks up through the ceiling and through the plywood, if the collective layer of rigid foam about the plywood is sealed airtight on top, I would think that this warm air will never drop down to dewpoint I won't have to worry about any level of condensation building up anywhere in the cross-section of wood and foam.
or alternatively,
(2)
start_plywood || impermeable air barrier(Tyvek) || adhesive || rigid foam sheets || roofing membrane || and finally sheet-metal finishing off the outer shell.
panels
So I ask you the expert which of the above two options would you deem as the ideal approach to insulating the ceiling with no chance of air movement, hence no condensation?
There is a lot of opportunities for confusion and mistakes isn't there?
for example, following statement which confuses me yet again:
>> As you should. However, you should protect the roof sheathing with a layer of impermeable foam sheathing (ISO or XPS), against it, before
placing your fiber insulation (is it Rockwool?) It also needs an air barrier and vapor retarder as the interior finish, to limit air movement in and out of, and through the fiber insulation.<<
placing your fiber insulation (is it Rockwool?) It also needs an air barrier and vapor retarder as the interior finish, to limit air movement in and out of, and through the fiber insulation.<<
and this state and seems to conflict your earlier statement and I quote:
>>That is why we no longer use vapor barriers on the interior side of the building envelope. We want it to be able to "breathe" (diffuse) unwanted moisture, to the interior. But, we do not want air leakage.<<
Perhaps this is just a play on words which one may misinterpret, but you can see how statements like this can easily throw off the novice, or the self-taught contractor without a college degree (no offense intended).
Anyway this is a fascinating topic and one which I would love to find a live seminar to attend an exchange and debate ideas and approaches with practitioners.
Best regards, Sincerely,
FRK.
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