Replies

1. 
   kxm1 | Feb 02, 2014 10:11am | #1

   hello john...

   ...i can advise you accurately on your questions about your upcoming projects.

   I need more info though....

   is the drywall applied to the bottom of the rafters?

   I can offer you this bit of advice right now...if you expose stud cavities and/or rafter bays with drywall applied to bottom of rafter-use sprayed closed cell polyurethane insulation.

   fiberglass is not the best choice-particularly if it is rools or batts.

   looking forward to seeing your response

1. 
   Superanvil | Feb 03, 2014 09:28pm | #2

   insulating over a rafter roof

   The drywall is attached to the bottom side of the rafters.  I have been thinking about stripping the existing roof down to the sheathing, putting 2x12 over the existing rafters, drilling ventilation holes in the existing sheathing, insulating with 10 inches of fiberglass over the sheathing, putting rafter venting over the 2x12 and resheathing the roof.  Installing soffit vents and a ridge vent in the new roof  This will allow me to ventilate the roof and also let out any excess moisture from the interior.  What are your thoughts?

   Thanks,

   John 

1. 
   jimblodgett | Feb 06, 2014 12:50am | #3

   I'm no expert, Superanvil, and you can certainly build up your roof and walls and add insulation.  Pretty straight forward.  You even have the option to vent, or not vent the newly insulated built over area, there is a case to be made either way.  

   But the one thing you'll hear time and again, from those who advocate for venting cathedral ceilings and those who don't is that you absolutely, positively, do not want warm air moving into your roof/ceiling cavity from your living space.  Disaster awaits if you do not head this advice.  You'll be introducing moisture, which will cause strucural damage.

   Please.  Don't take my word for it.  Go find really smart people, who study and debate these issues for a living and read what they say.  Study it until you believe you understand a certain approach and why.  One misstep in an otherwise great plan and you'll negate all the good you're trying to accomplish, waste a lot of time, effort and money and get frustrated by the exercise.

   A great place to start would be at Taunton's Green Building Advisor website - just today I got their weekly (maybe twice monthly?) newsletter with a long article and discussion about this very subject.  

   Another place to check out - and I shouldn't refer to them as "another", in my mind they are the single best source - is Building Science Corporation's website.  Excellent source for accurate, easy to wrap your head around information - even for a hard headed old carpenter like me.

   What you are asking is reasonable.  And doable.  But it's definitely screwupable in ways that are not intuitive.  Give yourself a fighting chance of sucess and go read up on it.  In the time it will take you to work through the inevitable bickering and name calling you'll find here you'll be well on your way to a series of details that WILL do what you want, and an understanding of the principals underlying them.

   This forum is great for a lot of things.  But a reasoned discussion about building science is not one of them, and that's what you really need.

1. 
   Superanvil | Feb 06, 2014 12:23pm | #4

   Thanks for your advice I will take a look at those places today.  There is not much to do outside besides bring in more firewood.  I can do a lot of thinking for free.  Once the roof comes off, I will be more committed to my plan.  Better to do it right once.

   John

2. 
   jimblodgett | Feb 09, 2014 11:10am | #5

   Found a Book

   Hey John - I was digging through my home library this morning, ran across this book and thought of you.  It might be worth your time to track down a copy.  It was published in 1981, but it deals with exactly what you're asking about, with lots of detailed drawings, straight forward text. At a glance I don't find anything that raises red flags about credibility or dated information. 

   This "superinsulation" thing isn't new.  It's just that Americans lost interest in the early 80's when oil became plentiful again and the government cut many of the tax incentives that had fueled much of the alternative energy/energy efficiency industry.  Lots of builders got black eyes when suppliers folded (curbing further research and making warrenties worthless) and swore off anything that wasn't time proven as a result.  It has taken a full generation for American builders to come back around to these ideas, while European building practices kept right on evolving, tweeking ideas to where they are now so far ahead of us it isn't funny.

   We didn't have the internet back then and relied on print publications like "New Shelter", "Fine Homebuilding", even "Mother Earth News"  for alternative ideas. And good luck as a young person trying to discuss this stuff with the oldtimers on the job at lunchtime...they all wanted to listen to Paul Harvey and wax nostalgic.  I wouldn't call it the dark ages or anything, but there was only so much light, that's for sure. 

   Anyways - here's some info that I hope will help - The Super-Insulated Retrofit Book , byBrian Marshall and Robert Argue, published by Renewable Energy in Canada, isbn # 0-920456-43-x (paperback) isbn #0-920456-45-6 (bound)

3. 
   DanH | Feb 09, 2014 02:40pm | #6

   Yeah, the key is to be aware of the temperature/dewpoint profile, from one side of the wall to the other.  You want the temperature to be above the dewpoint at every point along that profile.