A cathedral ceiling with only 2x4 rafters? It must be a fairly small room. Is it uninsulated now? Is it vented?

We've used Tiger Foam for some jobs, and it performs great but is a royal pain to work with overhead. It is super sticky and gets everywhere. It is also very touchy, in that it needs to be right around 80 degrees when you use it. Spray-in foam is very expensive, probably the most expensive insulation out there, whether you do it or have it done.

That's why for the last two similar jobs we ended up using solid blocks of foam, which we can cut to size and fit between the rafters. It has the highest R-value per inch, is very reasonably priced for the performance you get, and is friendly for the DIYer. I'd say that unless you are very adventurous and handy, spray foam is not a good DIY project.

Also, are you doing this with a permit? R38 is the code for California ceiling insulation, which I believe would be essentially impossible in your situation without making the rafter bays deeper. Solid foam will give you the most bang for your buck, and would get closest to the R38 requirement. Code also says you must have a ridge vent, which helps prevent overheating of the roof, causing the adhesive an asphalt shingles to fail, among other things.

We no longer work in the Bay Area, so I'm sorry I don't know any contractors to recommend there.

The roof is over an area of about 50'x25' - not that small unfortunately. Old construction - 99 years old. There is no
ridge vent, since there is adequate venting already - a fan
and several skylights that open.

The current insulation is badly installed fiberglass, with
lots of gaps. Nominal insulation is R-13. I wanted something more air/heat tight, with better insulation.

I had considered solid foam, but the bays are irregular, which would mean a lot of cutting and waste. Perhaps the best solution is solid foam, with cracks filled in with expanding foam from a can. I'll investigate further.

Charles.

Irregular bays are a pain, and preventing air infiltration is a BIG advantage if spray foam, so if you can afford it, it's a great way to go.

We actually use an electric carving knife and a drywall straight-edge to cut the solid blocks. With judicious use of spray foam and caulk, you can get very close to the same infiltration prevention as spray-in (as we have verified using energy audits) for less than half the price.

Does it matter what sort of caulk to use? Also, any recommendations on which solid foam? The thickest I've seen is 1" at home despot, which will mean a lot of layering. Any suggestions for alternate suppliers?

cheers, Charles.

We use expanded polystyrene foam, and I haven't seen a major difference between the major brands we've tried. EPS uses no CFCs, and it is 100% recyclable. You can get blocks in any size or shape you want, though standard is 4x8' by 1.5". We've purchased them as thick as 6 inches, and have had them cut to the exact rafter bay size we needed, but it's far cheaper to get the big sheets and cut them yourself. The best tool is a hot wire knife, but I don't like the smell. You can also get the blocks with a foil thermal barrier or moisture barrier already on one side, but we prefer to put in separate barriers (inspectors seem to like it better that way).

As for caulk we generally use fairly standard builder's latex/silicon, which we buy by the truckload, practically. Just make sure it has a fairly high temperature tolerance, as it can easily reach 120 degrees under the roof sheathing of unventilated roof.

Finally, if you are in the hot part of California, I'd strongly urge you to consider a ridge vent, especially if you have asphalt shingles. Without ventilation under the sheathing the shingles can overheat, and the adhesive holding them down fails. You'll know during the next big storm in your area, when shingles start flying off.

If you end up going w/ the sprayed foam, I'd probably hire someone to do it. We have a bonus room w/ paneled walls against the attic. Crazy air-leaky construction. I thought about detailing it all myself from the attic side- caulk, put in FG and then sheath rigid foam over the studs. Foam seemed like a much quicker, one-pass solution. And I'm valuing my time more lately than I used to.

The cost for me in GA of buying kits vs. paying an installer was a couple hundred for this job. In my other, relatively limited experience, it seems that getting insulation done is usually only a minimal extra cost above DIYing it. Someone who's buying it all day every day just gets a better material cost than you or I do.

If you want to do it yourself and keep the costs down, use the sheets of 1/2 inch foam from Home Depot or Lowes and cut them to fit and stack 6 or 7 of them in each bay between your 2 X 4's

We bought an old brick house where the bricks ARE the exterior walls - No frame walls inside them to insulate. We removed all the sheet rock from the exterior walls and it was mounted on 2 X 4's laid flat against the bricks, so we insulated between them with 3 layers of the 1/2 inch foam and put new sheetrock back up. TONS cheaper than having someone come out and spray the walls

Don

You can read an article on this subject at the link below

Here is a link that might be useful: Insulating Unvented Attics With Spray Foam