not an expert on cold climates...but this link may be of help.
if not search buildingscience.com's site for specific info.
page 5 seems to have info about climate zone 7
http://www.buildingscience.com/documents/reports/rr-0404-roof-design

best of luck.

here is the link

Here is a link that might be useful: link

Do you have trusses or rafters? I think you may be over thinking it. If you have truss spaces, do not bother with spray foaming the roof deck area. It is a waste of money, and adding the XPS is a waste of time and money.
Assuming you have an attic space, spray foam the ceiling plane to create an attic seal. This does not mean laying down several inches, this only means foaming tops of walls, plumbing and electrical penetrations, and I even have them foam attic baffles in place as well. This prevents wind washing the thick insulation to come. After the attic is completely sealed at the ceiling plane, simply come in and blow an R60-70 min of cellulose of fiberglass (typically use fiberglass). This creates an air tight living space, a much higher r than what you are proposing at a greatly reduced cost, and a fully tried and true, tpyical vented attic so you have no concerns of moisture.

I seen that info before, 7 or 8 are closest but neither has the airspace.

I suppose i forgot to mention that I would like to retain the space as climatized storage, its a small house.

Thanks for the responses.

The strategy for insulation depends on whether the attic is insulated and conditioined or un-insulated and un-conditioned. If your HVAC equipment and/or ducts are in the attic they should be considered in choosing the appropriate strategy.

Building Science is a good source for information: http://www.buildingscience.com/

Good luck with your project.

There is nothing in the attic now.

As i said i want to use it as conditioned storage.
It will probably house a radon fan, air exchanger and range hood fan by the end of the job, i cant think of anything else mechanical that would go up there.

Is there an issue with my proposed assebly?

The joists are 2x8s, that would give me 1-3 inch ridged and 7-5 closed cell spray. I am thinking thicker ridged would be more cost effective.

As stated the house is small so this space is wanted as conditioned space.

Edit: oops , forgot to mention 1 inch air space, so those numbers above are all lowered 1 inch.

This post was edited by Noctua on Mon, Sep 9, 13 at 21:03

to answer your question, no, there is nothing wrong with your proposed assembly. Most people would forgo the air space and take the closed cell right to the roof deck, but its your choice. It certainly doesnt hurt anything other than loosing out on an additional r5 to 7 worth of insulation.

The airspace between the insulation and the underside of the roof makes no sense. Build the roof right and it doesn't leak. And to the extent that moisture still might enter, it will disperse through the semi-permeable insulation without damaging it.

Foil and heat shielding is pointless in a cold climate; not much help in a hot climate either.

Instead of trying to come up with novel approaches, consider the variety of proven and tested methods devised by qualified building scientists for use in your climate. Consider the one below for a cold climate. To save attic space, also consider foam board insulation on the top of the roof deck and spf beneath.

Thanks for the cut n dry answers, i would much rather hear i am making mistakes before i make them.

I was under the impresion the air gap was the best protection against ice damming, last year we had horrible ice dams and i really want to avoid that again. So i was really looking for assemblies with air gaps.

Inadequate insulation can lead to ice dams and your rough R value of around 40 is below the prescribed MINIMUM levels for your climate which is R49 in the 2012 IECC.

You can insulate below the rafters as the BSC diagram shows but better practice would put the foam sheathing outside the roof deck which is probably only cost effective if re roofing.

Ice dams are usually the result of warm, interior air exfiltration to the roof deck which your proposed assembly could make worse if its not air sealed appropriately. Seal it all up tight and avoid recessed cans.

lzerarc ...nice to see someone else understanding the wind washing of insulation. nice pic that you included for foaming top plates.

off topic a bit...but having no experience with
ice dams & cold climates I have a question.
if you put foam on attic floor...2-3" then blew
insulation over it...as long as no mechanicals
of ductwork was in the attic,,,and everything
was properly vented...would this eliminate ice dams?
seems to me if air barrier is complete @ ceiling plane
& this same area was insulated that there would
be no warm moist ...or just warm air to enter
the attic to cause issuse.

OP...if you want to use the attic for semi conditioned storage then moving the thermal barrier (insulation)
and air barrier to the roofline is how you achieve this.

did you take a look at the link I posted several days
ago?

I did read the info in the link.

Previously i read the air space was needed to reduce ice daming.
I thought the space allowed drying and, well, venting from the soffits.
I thought in the warmer monthes the space absobed the heat and allowed it to vent through the ridge venting i would be installing before it entered the home from above.

I read also that no airspace was also an option but that the spacing method was the tried and true method. ie. the easier method to reduce icedamming

I was thinking the foil back could help with heat reflection but that the main reason for it would be low permanence (level 1 barrier).

I am sheathing the entire home in 2.8 inch foam right to the roof line so i was going to try my best for an air sealed structure. This only leaves my rafters and rat tails for thermal bridging. I real would like to not remove my roof to add foam.

As you can tell, I am not a pro so all this info helps me alot.
I am trying my best, i want my home to ge great for my family.