Apologies if there is a thread on this.  I tried searching twice but the search function was offline.

Our long-term plan is to add a deck/pergola/conservatory for passive/active solar on top of a one story room on our mostly two story house in a suburb of Chicago (Zone 5a).  BUT, the roof on the single story leaks and will be unshingled/re-shingled NOW.  We'll use this opportunity to remove decking of the flat part of the roof (it angles from the flat part down to the three walls), access the inaccessible attic, and air seal/insulate.

I don't know anything about building, so please read what I mean if I misuse terms.  Some in on the project want to do a great job of air-sealing around the tops of the walls by spraying closed-cell foam in the angle of the ceiling and air baffles that will be mounted under the decking.  I'll guess this would go on the top plate and over the ends of the joists (cellulose will be used toward the interior), I think it's likely the best way of air sealing/insulating this hard to reach area, and for a normal attic might be the way to go.  BUT, seeing the rafters will come off in a few years and a new live-load floor added as a second story, I'm guessing new joists might have to sit flush on the top plate, their ends will have to be flush against the rafters or rim joist, some might have to be sistered flush to existing joists, a subfloor would have to be level on top of the combo of the old and new joists, etc.

Do surfaces that new joists/subfloors would rest on/against need to be so flat/level that a really good job of removing foam must be done where new lumber will be added?  If so, is closed-cell foam easily enough removed to accomplish this without much extra cost?  What about open-celled?  If neither, is it reasonable to say that though carefully stuffing batts or cut board in the angle won't seal/insulate as well as foam, it's a good solution if some surfaces must be made level in the future?  Thank you for your time.

Gordon

Climate matters- where are you located?

Open cell foam is much cheaper, easier to remove & work around, and at 3" or more usually a better air-seal than closed cell. Closed cell foam is more vapor retardent (which can be both good or bad, depending on the stackup), but it quite rigid- at 3" it's considered structural. It will in fact glue the wood surfaces together. But it's no more difficult to chain-saw out than the wood.

It defiantly helps to know how your climate is. Does it snow? Is it humid? Is it dry? If you are looking for a cheaper project, Dana1 is right, open cell foam is cheaper but remember cheaper isn't always good and many time you end up spending more over a long period of time. If you are in a place that gets a lot of rain, I always feel as if closed cell foam is better, simply because of the fact that is is more vapor retardant. Hope this helped. Good luck.

Thanks to Dana1 and Inspiredled. Location/climate info has been edited in. There will be insulation put in, and there will be an addition on top of this area in about five years. Sounds like Dana1 recommends using whatever is the most appropriate insulation for the climate, even if it's closed cell because the closed cell can just be chainsawed out where new joists, subfloors, etc. will need to be placed in a level manner. Does anyone agree, or dissent? Thanks.

Do much sawing Dana? I was on a 5 horse 8 hours Saturday last. If you have to remove or find your way around any kind of foam, you will wish you had never heard of it. Best to get all mechanicals in before foam.

I would glass it for short term comfort. There is no ROI in short term insulation.

I would guess that damp sprayed or dense packed cellulose would do a pretty good job and be reusable in a few years.

Hi, Gordon. I’m Dave Wolf and I’m pretty new to Green Building Talk. I’m the senior research and development program leader at Owens Corning. Building science is a passion of mine, so I wanted to jump in and provide some insights.

It is not necessary to apply spray foam insulation over the entire floor of the attic. Most of this surface will be the drywall or lath and plaster that comprise the ceiling of the room below. These materials are air impermeable, so there’s no need to coat them with any sealant, except where these materials are penetrated (e.g. a light fixture) or terminated (e.g. an interior or exterior wall in the room below).

The air sealing only needs to be done at these locations, which will make your current task more economical and will be easily reversible with your future renovation.

After this air sealing, the attic floor can be insulated with blown fiberglass insulation, which is also easily reversible in the future.

If you decide to take this route – just to let you know – we actually provide a sealant called EnergyComplete, which can be installed by a contractor.

Hope it works out – just let me know if you have any other questions.