Attic Project – Venting and Insulation

spaceboy

First time caller here… been reading/researching projects for some time on the site – but this is one I just can’t seem to get a straight answer on.
Here we go…
Live in Chicago and am looking at finishing an unfinished attic space without any current insulation in it. House was build early 1900’s. From my research attic & cathedral ceilings in my zone require R30 to R60 insulation.
My attic framing is 16” on center and I was planning on going the DIY spray foam route (soythane). My issue and question is related to venting. From what I have read – an area, such as Chicago, with major temperature swings would require me to vent behind the spray foam. My initial plan was to run Durovent baffles from the soffit to a ridge vent (not yet installed). And from there – spray foam into the cavity. My intent was to drywall the ceilings all the way to the top of the roof and leave the collar ties exposed for a lofty look.
BUT…. I have also read about sealing the soffits and cavities with spray foam and going the “hot roof” route. This just does not make sense to me – but I’m not opposed to it if it is the right thing to do. I just think having airflow makes more sense – but what do I know.
The plot thickens….. If I go the durovent route – that takes about an 1-1.5” of space for airflow and only leaves me with about 2.5 – 3 inches of space for insulation. With the spray foam – that depth only gets me about a R9-R14 r value. I do plan on zoning this space with its own furnace and AC units but given my location - im thinking that I need to address the insulation aspect of this project differently.
Again – I have heard both the hot roof/total attic seal will work as well as the venting behind the spray foam – that said – I just don’t know how to approach this.
In summary:
Question 1 – hot roof or Durovent to a ridge vent? Stop at the collar ties (will that alleviate some/any issues?)
Question 2 – Is insulation at the R levels described even worth all this trouble?
Question 3 – open to suggestions/comments/ideas

TrapperL

I'd suggest taking the tour at building science.com. They pretty much can advise you what is the best for your particular needs and environment. If you get a chance to go to one of their seminars, they are incredibly interesting.
http://www.buildingscience.com/index_html

spaceboy

Will do - im sure i will have some additional questions after.

That said - I sill think this is a very opinionated subject and not so sure there is really a "correct" answer.

Also - not sure if it helps... but i have added a photo of the attic.

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spaceboy

So here is what I have decided to do (i hope someone in the same situation finds this helpful). I have researched, researched and researched more...

Taking costs and overall effectiveness into consideration - here are my thoughts and what i plan to do.

Fiberglass has less r-value per inch than rigid foam. Spray Foam would require me to install the Durovents to the roof deck (extra cost) and to get to an appropiate R value - I would have to somehow extend out my rafters as they are 2x4's.

It's not recommended to either have the insulation in contact with the roof sheathing, or to compress fiberglass.

What is the solution you ask... Not sure if this is right... but this is the best I can come up with:

Rip pieces of 2" or 2.5" rigid foam to fit between the existing rafters
- Keep the foam flush with the lower edge of the rafters leaving an airspace @ the roof sheathing (removes the need for the durovents)
- Caulk and Tyvek-tape the seams (can even tyvek wrap the entire interior if you like - i personally think this is overkill.) I have also thought about spray foam to seal but cant figure a way to make it flush and the added cost, in my opinion, cant be justified.

Next, apply full sheets of 2" foam to the whole ceiling, using long screws and washers.

Now, being aware of where the rafters lie under the foam, chalk out lines 16" on center from the peak to the plate, perpendicular to the rafters, parallel to the ridge and side walls.

Next, using long screws into the old rafters, install 1" furring strips along these guide lines. To this you will attach the drywall or whatever finish.




Overall r value will be about 28, but possibly even better because of the 1" dead air space with the furring strips, and the fact that the ceiling is thermally isolated.

I plan on starting this sometime in Nov and if anyone is interested - I can post progress and photos if you like.