Absolutely. Is insulating against extreme cold different from insulating against extreme heat?

It's all about heat transfer.

Right, the only difference between here and up north is the direction of the transfer.

All good suggestions, plus I would beg rent or borrow an IR camera.....

Would you recommend an air barrier if used on original 2x8 T&G beamed ceiling/roof? Currently don't have any insulation on our main roof section of the house. Originally built as an un-insulated beach house and then insulation was blown in later.

We've been considering adding additional rafters to our original beamed ceiling, insulating and then sheet rocking. I was originally thinking of the spray foam but, like the idea of the rigid, less mess and I could do all that myself.

SAT

Most people misunderstand how to cool an attic. Its air movement that's the most important not just thick or effective insulation.

How will rafter be added? Cutting a hold on the side and shove it in from the inside? Sounds like this will be a finished living space or storage? Why do you want to add thickness to rafters? Just to house insulation?

I've got one.

Ridge vent when you do the roof.

Absolutely!

Check out my Man Cave Thread, I've been doing a lot of insulating between rafters

Using 2 layers of 3" board, I bought a insulating board saw but found an old wood saw much easier. All my sheets are a tight fit between the rafters and I also aluminium foil tapped all the joints too

It does make a bit of mess but it's a clean mess so can be swept or vacuumed up easily

It a very easy DIY job

I don't have attic in the main section of the house. The whole south facing part of the house (Living/Kitchen/Dining) is built with lam ridge beam, 4x6 rafters on approx. 6' cntrs and 2x8 T&G planks.

http://forums.pelicanparts.com/uploa...1625069594.jpg

Adding additional rafters for two reasons, one being nailers for rocking the ceiling and two, create smaller cavities for the spray in foam to hold (if I go with spray). What you see is my roof. No insulation under the composite shingles up top. Was originally considering spray foam to mitigate condensation since there is no ridge vent and I don't have sofitts. Insulating up top would require additional rafters anyway to support the additional weight of insulation and then sheathing on top of that for the roofing material.

My question is whether the rigid foam could be used as an alternative to the spray in. Would the rigid foam induce condensation and then mold because it's not completely flush with the T&G?

Same, just a question of whether or not the blowing machines are available to rent. I wouldn't haul it into the attic, but blowing it in is easy.

This is very interesting. If I understand you correctly, your roof/ceiling is
shingle
decking (usually plywood)
TnG boards
rafters

I'd absolutely want some insulation up there! By comparison, my shoddily installed, old insulation is fantastic.

The 2x8 T&G is the decking. No additional sheathing up there. Built in '61 and the roofing material was originally tar and volcanic rock with french drains. We removed that, trimmed the fascias down and installed a traditional shingle roof with gutters.

We're right on the coast and through the majority of the year, our temps are great, coastal breezes used to cool the house (no AC). It's those rare occasional days in the high 80s/low 90s (minimal wind) that make me want to mitigate the heat from the roof. The section of the house with the bedrooms and baths are traditional sheathed rafter with attic space. I have eave vents and roof vents there though the insulation needs to be replaced.. It's blown in cellulose that was probably done in the '70s. Being the North facing, the heat there has never been an issue really though replacing that insulation would help. Shallow pitch and I'd have to open a ceiling to get up there.

Wow, very interesting!

Poor man's Eichler:D

The house is L-shaped and the interior of the 'L' is a flat roofed carport with the same planking. We did torch down there. The whole carport is big beam open construction. I love the look though and have been struggling with covering it up. The other advantage of rocking the ceiling aside from insulating is being able to add can lighting. Currently running track.

SAT, have you considered adding insulation above the deck, then roof over it, non vented assembly. I did my house with this method, works quite well with low slope steel roof.

Yep, back when we converted the roof to traditional shingle. We also toyed with going to a steel roof as well (love the look). In either case, I'd have to add additional rafters to distribute the additional weight.

The R rating back then wasn't very good either.. What R did you end up with?

Hogging Masraum's thread. Really do not know why they would need additional rafters, standing seam metal with foam and Z mounts does not weigh much. I have that configuration over the garage with 2.5" polyiso about R15, and in yesterdays 100 deg heat direct sunshine on roof it was 77 in there, no ventilation and little insulation on the main door, that is probably where most of the heat transferred in over the last couple of days, but the entire surface above deck is foam, no joists for thermal transfer. Most of the nearby houses were way hotter then my garage.

No worries, more information floating around is generally better than less.