I put these on the classic forum too, but just in case anyone doesn’t hit both of them:

 3249.1

My ranch house has an addition that creates a dead valley. I need a new roof, and would like to insulate more at the same time.

It occurred to me that I should stop trying to band aid someone else’s poor design, and just re-frame it the way it should have been done in the first place.

See the first image – existing roofline is in black, proposed is in brown.

What I am proposing to do is use TJI beams to span the distance between the upper floor roof down to the first floor. There is an intermediate bearing point that is the outer wall for the first floor and second floor, and I would build a small wall on top of the existing wall (outside wall is 7′) to match the 4:12 pitch.

The long span between the bottom and top wall is 28’4″, and the span from the intermediate point to the peak is 8’6″. The means I would use 16″ deep 210 TJI beams. The 16″ beams are overkill for the top portion, But I figure it’s simpler just to use the same stock throughout.

This seems to me to be a simple and straight forward solution – anything I’m missing?

AND

3250.1

I have a related post about framing with TJI, but this is distinct enough for a seperate thread on insulation.

I’m looking at framing a roof section with 16″ deep TJI beams on 19.2 spacing.  One slope side is 37.5′ long, the other is only 8.5′.  It would be 33′ across.  I would be insulating primarily with cellulose between the webbing.  At the top of the webbing I would wedge in a 1″ piece of rigid foam, which would leave a 1 3/4″ vent channel.  Inside there would be about 7″ of cellulose dense packed.

I realize I could simply block the bottom of the web and get nearly 12″ of fill, but I honestly don’t need that much insulation!  The latest guidlines for my specific climate zone call for R30, and I’ll be using Energy Star white shingles on top of it.

So here are my questions:

What do I use to block up the bottom of the webbing?  It needs to be flame resistant since it is facing the inside of the house.  I was thinking using insulmesh would be a giant pain to staple along both sides of the inside of the channels.  Since I am blowing in cells, does this have to be able to breath or can I use something like a solid panel?  I was thinking about blocking it up with 1/2″ drywall across the inside of the channel, with thinner strips supporting the crosspiece blocking it up from the bottom piece of the TJI.  Like an upsidedown “U” of drywall.

What kind of foam should I use for the top blocking?  Whatever I use I would tape the seams so that if any water penetrated the deck, it would flow over the foam and out the soffit vent.  I was thinking about permiability.  Would using an EPS (white puffed balls in a panel) be better than foil faced polyiso, since it actually does allow vapor to pass though – potentially letting the cells dry to both sides?  I don’t have XPS available in my area.

I see online prices of $.38 per pound of cells, which would be $1.33 (3.5lbs) a cubic foot – Is my math right on this?  Is this about what it would cost for me to hire this to be blown for me?

Are there any better ways of doing this?  I saw a DOW product that allowed you to roll TYVEK across the rafters and tighten it into air hannels with U shaped pieces of plastic.  Would water roll down the face of TYVEK or TYPAR instead of penetrating to the cells below?