Design options for conditioned attic retrofit

I have a customer retrofit and want to get some opinions on designing the retro. It is a conditioned attic with batts on roof deck from the side attics to the upper attic. Upper attic is blown cellulose on ceiling. Lots of bypasses. They installed poly over the batts in the side attic. We are climate zone 5 in northern arizona 7500 hdd, and pretty dry here. Don't like a vapor impermeable air barrier so we will probably take that out. Here is the sketch. Conditioned%20Attic%20Sketch.pdf (not to scale)

#1 I would like to enclose the ventilated attic and go with a spray foam to deck and go unvented, however I am not sure what to do with the portion of the roof which is enclosed by drywall from the conditioned space(the angled ceiling). It currently has batts. Not sure if the batts could be removed and cavity filled with foam or if we should just leave them be and encapsulate them top and bottom with foam plug, then foam the rest of the attic.

#2 Sealing all air paths and removing the poly sheeting covering the batts at the lower roof attic sections. We would then install a more appropriate vapor permeable air barrier and keep the insulation/air barrier at the deck for lower attic. Need to make sure new air barrier is sealed at all 4 sides.

Do we then encapsulate the batts at upper and lower attics, between roof deck and drywall?

#3 Sealing all air paths and moving the insulation/air barrier to the wall/floor of the lower attic (spray foam) and removing roof deck insulation in side attic

Do we then encapsulate the batts at upper and lower attics, between roof deck and drywall?

The floor of the lower attic would be very difficult to insulate due to sheathing installed for attic storage.

If we go with unvented attic, we will be installing a vapor barrier on the crawlspace floor.

--
Craig Bird, BPI Building Analyst, HERS Rater
ReGroup Performance Contracting
Energy Audits, Efficiency Retrofits and Green Building Consulting
www.RegroupConserve.com
Cell - 928-255-5564

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Permalink Reply by robert post on December 11, 2010 at 10:05am

I'm in Pa. Generally, I get rid of all fiberglass in these instances. Dense-pack or spray foam if budget allows. I also advise homeowner that when re-roofing to seriously consider introducing ventilation on top of roof deck since rafter bays are too shallow to vent below and have an meaningful thermal value. If bays are more generous, we baffle under roof deck and retrofit soffit & ridge vents. I hope I understand this and you find something of use below. Best of luck!

#1 Not sure if the batts could be removed and cavity filled with foam or if we should just leave them be and encapsulate them top and bottom with foam plug, then foam the rest of the attic.

We cut a 5-8" trough of drywall out of the middle of the slope and pulled out the fiberglass. Then you could use .5lb foam and let it expand out the new trough, cut excess, and patch. Or, you could patch, then dense-pack with cellulose.

Bye bye batts. Continuous spray foam would need no additional barrier.

#3 Sealing all air paths and moving the insulation/air barrier to the wall/floor of the lower attic (spray foam) and removing roof deck insulation in side attic

We try not to put the thermal boundary here because of reasons mentioned and the likelihood that it will be disrupted by homeowner or future remodeling activity i.e. cable guy, electrician, etc... I would stick to #2 with foam.

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Permalink Reply by Sean Lintow Sr on December 13, 2010 at 7:53pm

Wow, Flagstaff is only 7500 hdd?

If it's in the budget & for a cleaner looking / better performance job, I would remove the drywall on the entire angled part & spray foam the entire deck with CC (if not you could look into removing the fiberglass and going with a slow rise foam after spraying the lower section off)

As for the plate issue, they should be able to seal that with the foam also and eliminate any issues with vapor barriers, etc...

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Permalink Reply by Craig Bird on January 27, 2011 at 9:28am

Sorry for the delay in getting back and thank you for the suggestions. While I agree with the solutions provided - alas they are above the budget. We decided to keep the existing alignment of the air barrier/insulation. We removed the poly and lifted the batts to expose the air pathways. We spray foamed all air pathways into the conditioned space on top plates and other various penetrations in interior walls as needed. Side attics are now all conditioned space vs. confused space.

To beef up our alignment of the air barrier/insulation and prevent heat transfer to the roof deck in the side attics, we installed 1 inch of rigid polyiso under the batts and sealed all edges and seams. We spray foamed all top plates and penetrations in the upper attic and left the existing roof ventilation.

--
Craig Bird, BPI Building Analyst, HERS Rater
ReGroup Performance Contracting - Flagstaff, AZ
Energy Audits, Efficiency Retrofits and Green Building Consulting
www.RegroupConserve.com
Cell - 928-255-5564

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