Posted By Bluffer on 31 May 2011 12:10 PM
I am new to the forum. I have read several posts throughout the winter in preparation for my first insulation DIY. I would like to convert my attic from a vented to an unvented this summer. Why? High energy bills, ice dams, recessed lights, airhandler in attic, ducts in attic, etc. Pretty much the usual suspects. I'm in the burbs of Chicago and a local downtown cc SPF company offers local pickup. I figured I would proactively reduce my winter headaches this summer by attacking this project.
First off, I am not a professional builder. I am able to hold my own DIY-wise and can learn and apply new concepts quickly. I am intersted with becoming more sustainable in my practices and I am currently studying to become LEED AP BD+C. I purchased this home 2 years ago and inherited a load of issues. The following are the particulars for my attic case:
Tri level ranch in Climate 5A, Chicago
low pitch roof - shallow attic 34" clear at peak
2x6 rafters in attic
vented attic with soffit vents & NO ridge vent
aspahlt shingle roof
unfaced batt insulation between ceiling joists
20+ recessed light fixtures
attic hatch into living space
80% efficiency furnace horizontally mounted in attic
duct work supporting 3 rooms/1 bath throughout attic
mechanical fan on temp/humid switch
this attic is a 30x30 foot print and is part of a 5 year old addition to the existing building. 1/4 of this roof is built over the old roof.
I will need to tear out the ceiling a one of the rooms the AHU supports to spray the underside of the deck of the original roof. The pitch on this portion of the roof is almost flat.
I would like to spray the underside of the deck with cc SPF. If I plan on just spraying 2" on the deck how else should I insulate. Can I use oc SPF in the ceiling (not roof) rafters and box over the ic rated can lights? I have read this article. http://www.buildingscience.com/docu...of-systems but I can't say I understand all of the options. I wish I could afford re-roofing with iso board to avoid crawling around this swallow attic. Maybe that's the best way to do this?
Anu thoughts would be appreciated. I know I haven't painted the most accurate picture, please post questions of clarification and I will fill in the blanks
Thanks
D
Sealing the soffits and insulating with 2" of ccSPF on the roof deck the roof deck is protected from wintertime moisture accumulation, and you'll be adding ~ R12 to the stackup while blocking stack-effect infiltration losses. If you insulate over the rafters too with at least an inch of foam, the thermal bridging of the rafters goes away, reducing the ice-dam potential.
If the ducts aren't insulated, seal every seam & joint with duct mastic, and every seam on the air handler with FSK tape. It may be possible to seal & insulate the ducts with 2" of ccSPF at the same time you do the roof. But seal & insulate you must.
Using open cell on the attic floor could work, but isn't the best bang for buck if you already have established good air tightness with the closed cell foam at the roof. A cellulose overblow of the existing batts would be a fraction of the cost. (How deep are the joists, and how thick are the batts?) If you set up depth guages and blow to 12" up from the bottom of the fiber layer you'd be around R38 in fiber (after a decade of settling), and R12 at the roof, for ~R50 total, ~25% in exterior foam. With that foam/fiber ratio, in your area you'd be averaging ~35F in the attic in January. But with the anticipated losses into that space from the thin spots over the lights, conducted losses from the ducts & AH, etc it'll average above 38F, which is where it needs to be from a mold-risk POV. Don't go deeper than that on the fiber unless you intend to add more foam at the roof deck, or any exposed wood in the attic will run cool enough to risk condensation & moisture accumulation in winter. The tighter you make the floor, the lower the risk too, but with a sea of recessed cans & myriad other penetrations, that becomes an endless task, difficult to do well.
When it's time to reroof, an inch or two of XPS, or nailbase iso panels might still be worth it if you continue to run into ice-damming problems, and would also give more wintertime temperature margin for attic moisture issues.