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Cold Roof (isocyanurate on exterior of roof deck)


Jeffrey Olson
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Hello All,

Am considering having licensed, bonded, insured contractor install isocyanurate panels (sheathed in oriented particle board)on top of roof decking, synthetic roofing felt over that, and 30 architectural shingle over that. Will have strutural eng take a look to determine if roof is strong enough for this weight (and frequent heavy snowfalls). What worries should I have? What energy cost savings might I expect? Dramatic? Might this qualify for energy tax credit? Thanks, Jeff

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Hello Kurt,

I don't have digital pics. It is a 1.5 story expansion. Previous attic is now marginally livable with knee walls running parallel with ridgepole. There is currently some fiberglass insulation on the interior side of the roof decking. There is no ceiling insulation past the knee walls ( in the crawl space). In general, the house is poorly insulated - 1955 era balsam wool.

Did I read that you are a boatbuilder as well? Cheers, Jeff

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It's hard to know what the best or right method for insulating the property is without knowing a lot of other stuff. What you described could be one method, but there's a lot of options that I'd want to run through before I went with the one you mentioned.

If you're trying to maintain the interior in it's current state, there's not a lot of options. I'm kind of a "do nothing, or do everything" sort of guy. If you want to bring the place up to a respectable level of energy efficiency, I'd gut the 2nd fl. and foam it with icynene.

I'd save money in lots of other ways before I did a half measure on the roof with some polyiso board. Maybe new high efficiency equipment...(?)......or new windows...(?).......

Yea, I built some boats. Restored a few antique boats too.

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Hi,

It sounds like he wants to add a sips deck on top of the existing deck. Sips decks are very strong when splined together properly and they can sustain a lot of weight. They do suffer from moisture issues though if the interior isn't properly ventilated and they aren't sealed well where they're joined together.

OT - OF!!!

M.

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If you're tearing off the roofing why build up on the exterior? A local roofing company has done a few retrofits similar to what your considering, but all the insulation was added between the rafters. After removing the original roof deck, sheets of poly iso were installed against the ceiling material and the rafter bays filled with spray foam, then new decking and roofing were installed. I would also consider a radiant barrier as as part of the project, there are several plywood panels with reflective surfaces as well as radiant barrier underlayments.

If you insist on buiding up the exterior, you should be using multiple layers of foam oriented opposite each other with the seams staggered to prevent air gaps that could thermally bridge the inner and outer decks together. This bridge effect is common with SIPS and eventually leads to condensation and rot at the exterior skin, ventilation will prevent this but that kind of defeats the cold roof concept.

One last thing, as long as you're dumping buckets of money on the roof, consider 50 year archetectural shingles. After a project of this scale you won't want to have to think about the roof for at least that long.

Tom

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Every SIP's home I have seen has had the attic area uninsulated and it is actually part of the conditioned airspace. This is a SIP's home and not a retrofit SIP's roof panel home. I'm with Mike, moisture is a major concern if they are not done right. I would be very leery of a retrofit on an older home.

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Hello All,

I really appreciate all of your responses. I am a bit freaked out about spending a bunch of money on the roof - on the one hand my existing is way past it's prime and I can't wait another year to get that done. So, it seems an opportune time to get 'er done right in an energy efficient way. New windows, better wall insulation, better sealing to prevent warm air from escaping into attic crawl space are all on the docket. If I did interior (rafter bay) insulation - should there be a vapor retarder (6 mil plastic) between the underside of the roof deck and the insulation? Is icynene a spray-on product? Cheers, Jeff

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Hi,

Well, you don't want to install insulation against the underside of the roof unless you plan to turn the area into conditioned habitable space. Lots of old homes are insulated wrong with insulation on the floor of the kneewall spaces and insulation tacked up on the underside of the roof in unheated attics without insulation on the back walls of the kneewalls or in the ceiling cavity below the attic floor.

If you only want to use that area as storage space but don't want to heat it, just air seal better and insulate the entire floor of the attic all the way through the kneewalls to the outer walls. If you want to use the attic space for storage, insulate the floor of the kneewall attics, the backside of the kneewalls and then the underside of the roof. You can insulate the underside of the roof with sprayed-on foam, use dense-packed cells or use batting while making sure to leave a ventilation channel above the insulation and ensure there is free airflow from the eaves, through the kneewall attics, above the insulation and out the ridge. If you want to increase the depth in the roof plane, fur the rafters down and use thicker insulation. You don't need to nail SIPs pannels to your roof.

In Minnesota, you want the vapor barrier on the backside of the interior space before the insulation, not between the insulation and the cold roof, or you'll end up with a wrong side vapor barrier and moisture trapped in your ceiling/wall cavity - a mold farm.

Go to BuildingScience.com, look up the preferred building practices information for your area, study it, study it again and then study it again until you can recite it in your sleep. Then contact some insulating contractors that have a good rep in your area and discuss this issue coherently with them.

Asking for information on a forum frequented by home inspectors from all over the planet, many of whom have no experience insulating homes or even an understanding of building science, living in dozens of different climates, used to many different techniques, isn't going to get you really useful answers.

ONE TEAM - ONE FIGHT!!!

Mike

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