Insulation for a low sloped roof normally goes on top of this kind of roof deck and a roofing material like a synthetic fully adhered single ply membrane is installed over it.

If you don't want to do that, one inch of extruded polystyrene foam board will add an R value of 5. That's not a lot of insulation but it will be a huge improvement over what you have now.

will add an R value of 5

That's between the 4x6s.

Interestingly, the 4x6s alone have an R value of 6.88.

How about adding insulation to the walls?

If you add insulation to the roof, consider going up to the Energy Star recommended R25-R38 for southern California.

But before adding any insulation, tighten up the house. Loss of heated air through cracks, chimneys, vents and the like can account for 16% of heat loss.

Here is a link that might be useful: ZIP Code Insulation Calculator

Thanks for your responses. We've redone most of the house except for the roof, which is fairly new. In the process, we've put tyvek on the exterior and R-13 fiberglass in all of cavities of the exterior walls. All of the windows and doors are new and have dual pane, low E glass, so the rest of the house is about as tight as it can get. The roof is the one area that we'd like to address for heat loss.

I'd put the insulation on top of the existing roof-- 4-6" of rigid foam. Making it look good on the inside will probably cost more than replacing the roof and 1" of foam doesn't really give you very much insulation.

Speaking as a New Englander though, if your lows are only in the 40's a sweater would probably work as well for a lot less cost and hassle. How often do you even have to turn the heat on?

All of the windows and doors are new and have dual pane, low E glass

Some installers are still omitting proper sealing around retrofit windows, i.e., low pressure single component spray foam, such as Great Stuff, Handi-Seal etc. If you don't know if they used it, you can use smoke pens, pencils or puffers to detect air leaks.

I will address the openings around the windows and the doors and I think we'll take RW's advice and kick the can down the road on the ceiling insulation until we are ready to replace the roof.

Thanks again. I appreciate your help.

I second putting the foam sheathing on the roof
when re-roofing. stopping thermal conductivity
of framing members is a plus.

best of luck.

Hi I’m just jumping on here to get some clarity for myself.... we do not need a new roof right now. We want to expose our ceilings rafters in our 1920s home. It’s a two story with radiator heat and we’re in Colorado so we need insulation. From what I’ve read the insulation needs to go on the outside of the roof boards which sounds like we would need to take the roof off to do this. Is that the case? Exposing the ceiling would mean redoing the roof at the same time so we can add insulation and then put the roof on top of that?