FYI:

Dow Building Solutions product or application, please contact us. U.S. and Canada: 1-866-583-BLUE (2583)

Owens Corning Just Call 1-800-GET PINK™ (1-800-438-7465)

CertainTeed: Consumers call: 800-782-8777

My wife and I discussed boxing in the beams and still haven't given up on that idea at this point.

Man, when I think of all the time I put into caulking and painting that ceiling 20 years ago.....:eek::eek:

Thanks 65. I'm pretty familiar with both the Dow and Owens sites. I've used the Thermax commercially for Antenna insulating. We used to use the spray foam back in the day....What a mess...

I agree that you should really try to maintain the look of the boards up there.

Maybe you could add foam panels on the interior, then put up another set of boards below them so it would look the same (?) You could use pickled wood instead of white painted wood for a more natural, but still light, airy look.

That certainly would be good - both for retaining the look, and for added rigidity.

I would think that painted ceiling would help you out if you choose to skip the venting. That is it's a vapor barrier to the wood.

So on skipping the venting; if it were my house, I would accept that the construction is what it is --Venting was not part of the original design-- and move along. ...Add another vapor barrier under the Cedar (or whatever).

I mean, sure, venting is a nice feature if you have a lot of solar heat load that you want dissipated, or if you have a cold roof capping a moist dwelling. (I didn't see an indoor hot tub, or a kettle for tea on an unvented stove top.) But to what extreme are you willing to go to retrofit to gain those features of venting? . ..are they really needed? . .Should you design the venting to help prevent the formation of ice-damns on the roof over hangs? --like I said, try understand the climate and the true needs.

btw, what is the big driver for insulation? is it to stay cool in the summer, or warm in the winter?

Do you really need to do anything?

/ramble.

Disclaimer, I'm not a builder or contractor of any type.

I would either do a proper vented roof, or spray foam so their is no air gaps or flow. (cheapest I would think)

I did have to repair my roof because of rotten sheathing & rafter's. More than half of my house is vaulted. The space between the bottom of the plywood sheathing and the sheetrock is 7.25 inches (2x8 rafters.) After tearing my roof apart I concluded that it was originally completely filled with fiberglass bats. The roof has eave vents and ridge vents. No air was flowing, rafters and plywood rotted. Previous owner replaced roof, attempted to sister the rafters and replaced fiberglass with hard foam. Although he left an air gap, he did block off 2 runs where a skylight was removed and also short circuited the ridge vents by installing a ridge vent on a lower peak. I buy house, 2 years later I had water coming in and a sag in the shingles in between the rafters that had the skylights. I open up the sagged area only to find that the rafters had rot from the past. I had to rip up the whole side. The guy used 2x6's to sister 2x8's. I cut out the rot on the rafters and replaced all the 2x6 sisters with 2x8's, replaced all plywood, and rebuild 2 chimneys. I removed the ridge vents because their wasn't enough ridge to get the proper amount of venting and added a power vent fan.

You don't want this.

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How about sister larger beams in there and nail on blockings (the spacing is too wide) to support the T&G with insulation and lighting inside. IC cans will do the trick.

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Grog - don't you guys use rain channel up there instead of tar paper?

Insulate and install new tongue and groove/shiplap on the bottom of the beams.
Still keeps the look.

Really to try and keep cool in the summer. The house was originally built as a beach home with zero insulation. Later they came and blew a little cellulose into the exterior walls and in the ceilings where I have a regular attic. It's never been adequate though. We don't have AC and really don't need it here. It's a rare event to get into the high 80s and above. Both rooms with the beamed ceiling are southern exposure and the temp spikes like crazy in the afternoons. I've already been systematically installing fiberglass in the walls from the inside all along the exterior. The ceiling is the last piece.

Pretty much where I'm at... The Thermax has 1 mil alum foil on both sides and spray foam isn't going to be as affective nor have a moisture barrier.

When we re-roofed, the 2" T&G was in great shape, even in the carport (you should see those beams).

I give, did a search and can't find any rain channel for siding. Before I replaced the cedar siding with hardy plank, I did a lot of searching on contractor forums. Seems most contractors stick with tar paper, they have done lots of repairs to houses with house wrap.

I'd suggest you research this further, but AFAIK creating an hollow area is asking for trouble.
Water vapor will work past the sheetrock/T&G/foam panels/whatever, and accumulate against the underside of the cold roof surface. Then it collects and rots.

My suggestion is to either vent under the roof sheathing from soffit to ridge(helps keep the shingles cooler during the summer), or fill it in completely with spray foam insulation. The open-cell type foam does "breath" a little bit(slowly) I've been told.
You can buy DIY foam from Tigerfoam and other vendors but it's tricky to use.

A few years ago, I found out the P.O. didn't install proper soffit venting with active circulation when she built the second story addition only 20 years before. The bottom of the sheeting wasn't in good shape because of this, and squirrels tore up most of the batting, so it was all replaced.
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it is green and has a brand name with "rain" in it - you use it instead of tar paper or Tyvek house wrap - and about a year ago it was required to be used around here

it is like corrugated cardboard in plastic - if any water gets in, it drains down the side of the house