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Roofing insulation concerns

spoons22 | Posted in General Questions on

Last fall I began a major roof renovation on my two story timber framed home located in southern New Hampshire, climate zone 5. We had major water damage in most areas of our roof from poor air sealing and poor flashing details. The assemblies are unvented cathedral ceilings. I have no insulation below the roof decking. These ceilings are exposed pine sitting on top of timber rafters. Before this project, and unfortunately during the early portions, I had zero knowledge of building science. After months of reading anything I can get my hands on and building along the way I have developed concerns about these assemblies. It is time for me to reach out for advice and feedback.

The assembly before on the first story roof, from the timber rafters up: 1″x 6″ pine shiplap, felt paper (in some locations), 4″ of EPS foam, 3/8″ plywood, felt paper, asphalt shingles. The second story was the same assembly, substituting 1″x 6″ pine for 2″x 8″ T&G pine. The south facing roof line had 4″ of polyiso foil backed foam, while the north side had a mixture of polyiso and EPS. 

The poor flashing details resulted in the EPS on the first story becoming fully saturated with water. Two by eight panels were weighing in well over 100 pounds. That resulting water intrusion resulted in rot and ant infestations and the replacement of over 500 square feet of 1″x 6″ shiplap. The poor flashing details on the second story skylights, and what I now suspect as also poor air sealing, resulted in water damage to the 2″x 8″ T&G. Over one hundred square feet of boards had to be removed and replaced. Luckily, any rot and water damage to the timber rafters was minimal and only required minor repairs. 

My assembly as it sits now from the timber rafter up: 1″ x 6″ shiplap or 2″ x 8″ T&G, 2″x 6″ framing 24″ O-C, 5″ – 5.5″ of ccSPF inbetween framing bays* directly on top of pine board decking, 1/2″ zip sheathing, seams and penetrations taped and/or liquid flashed. 
*This leaves me short of the code minimum R -49 roof assembly. All things perfect the ccSPF has an R36-32. Knowing I had a bad install during the second stage of construction has left me worried about its true R-value. Calculating the effective R-value of my framing bays has left me with a Ueff of around 30. (best case scenario)

Lack of knowledge, research, and budget has left me well short of the minimum roof assembly in these sections. I have an unvented flat roof assembly on 2″x 12″ rafters I plan on detailing: R-30 batts directly to underside, 5/8″ treated plywood, Grace ice and water shield, picture frame edges with treated lumber, 1-two inch layer foil polyiso (foil side down), 1-two inch layer XPS, 1/2″ Advantech sheathing, seams and penetrations flashed with Zip liquid flashing, PVC roofing. From all the helpful information on this site and elsewhere I believe this should be sufficient.

There are another two assemblies, both unvented cathedral ceilings, which are traditionally framed with drywall interior ceiling. These two have yet to be stripped and rebuilt. I still have a chance to get the detailing right with these assemblies. These won’t be getting ccSPF ($). I’ll have to use a combination of rigid foam and batts.

Because of my poor due diligence and proper planning of this construction project I have developed concerns that I’m making major mistakes if I don’t address the inadequate R-value. I also have concerns of thermal bridging with my assembly. 

I have contracted East Coast Metal Roofing out of Webster, MA to install PermaLock Aluminum Shingles on my roof once I’ve completed all roof assemblies. They think I will have enough R-value because of the thermal properties of aluminum. I’m still skeptical. I believe I’m buying a great roofing product and want to make sure if I’m going to put a “Lifetime” roof on my home, I’m getting the important factors correct, and not facing the same issues in 25 years.

I would appreciate any responses and feedback towards my situation.

Thank you.

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Spoons,

    I understand that you probably don't want to change the timber frame aesthetic from inside the building, but it seems that a simple solution to increasing the R-value of the spray foamed roofs would be to all insulation below the roof deck. Because you have plenty of spray foam outside of the sheathing, you could fill these interior rafter cavities with fibrous insulation like cellulose. A drywall ceiling will give you an opportunity for more air sealing and if you wanted to install a wood planks over the drywall for style, you could.

    When it comes to the cathedral ceilings that are yet to be insulated, keep in mind that the only insulation recommended for the underside of the roof sheathing in your climate zone is closed cell spray foam. If you are planning to use rigid foam, you should install it above the roof deck (at least R20 for zone 5) and then fill the rafter bays with fibrous insulation to meet your target R-value.

  2. spoons22 | | #2

    Thanks for the feedback Brian.

    I do have a preference for the full exposed timber rafters and decking, but I would be willing to sacrifice certain sections of the home to that assembly. That option being more beneficial to my budget if I could wait to do after the roof install. Are there any concerns with my assembly as it is after the roof is installed on it besides less than optimal thermal performance? Do my concerns on the poor install of the ccSPF warrant considering adding rigid foam and another nailable base?

    What I have now is a certain upgrade from before. I'm not looking for the super insulated, energy efficient roof. The main source of heat for the home is a coal stove. Being half constructed on the inside means the air leaks are plenty, which is why I choose to spray foam the decking from above. I want to make sure I'm not putting expensive roofing material on a poor performing assembly.

    1. GBA Editor
      Brian Pontolilo | | #4

      What is wrong with the spray foam install?

      1. spoons22 | | #5

        The installer was inexperienced. They sent one guy to come spray 600sq/ft on a second story by himself. They had to come back and cut out sections that was sponge-like. I could poke my finger right through it. It was installed in improper conditions. It was one of the hottest days of the year and the decking had been in direct sunlight during the install. Unlike the first install I had where they cut it flush with the rafters, the installer's poor spray and uneven technique left major fissures throughout the foam and up to 1" below rafter tops. I'm sure I'm not getting the full R/inch out of the product as I should be. I'm worried of future separation from the decking and rafter studs.

        1. GBA Editor
          Brian Pontolilo | | #6

          That's unfortunate. I assume that it is covered up now so you couldn't pay someone to inspect it. I don't feel qualified to advise you here, but the truth is, I feel better that the underside of the roof is exposed. You may have an inefficient assembly, but you're going to know about any problems right way.

          1. spoons22 | | #8

            The sheathing has been installed over the spray foam due to sun exposure and weather concerns. I wish at the time I had the access to the resources to have it inspected and repaired beyond the little make-up work they came back and did. It is unfortunate since the first install they performed exceptionally and professionally. This time I feel fleeced for my money.

  3. Expert Member
    Dana Dorsett | | #3

    >"R-30 batts directly to underside, 5/8″ treated plywood, Grace ice and water shield, picture frame edges with treated lumber, 1-two inch layer foil polyiso (foil side down), 1-two inch layer XPS, 1/2″ Advantech sheathing, seams and penetrations flashed with Zip liquid flashing, PVC roofing."

    That's barely sufficient exterior-R for dew point control on R30 fiber under the roof deck, and it exceeds code minimum performance due to the R20-ish thermal break over the rafters. Transitioning from foil faced polyiso to XPS is unnecessary. (And in this stackup if going with foil faced polyiso it doesn't matter what side the foil is on- both facers are vapor barriers.)

    For less money and same or better performance try 4" of fiber faced 2lb density roofing polyiso (labeled R22-R23) which is usually "walkable" under a membrane roof, and it'll be OK to skip the half-inch Advantech. (Or keep the Advantech to make it a "stompable" roof. :-) ) In southern New England there are multiple sources of reclaimed roofing polyiso, typically priced at 1/4-1/3 the cost (67-75% discount!) of virgin stock goods.

    Derating to R5/inch for winter temperatures and age it would still have R20 on the exterior. With 2" foil faced and 2" XPS, derating the XPS to it's warranted R9 and derating the foil-faced to R5.5/inch for temperature it's really the same. But with R20 on the exterior of the structural roof it would only need about R20 between the rafters to hit a code max U0.026 for a ceiling U-factor. Limiting the cavity fill to R23 rock wool or R21 high density fiberglass snugged up tight to the roof deck it will have ample dew point control at the roof deck.

    If aesthetically it can tolerate 7" of above-deck foam it could get there without any cavity fill. It can be cheaper to just do 7" of foam and no cavity fill if using reclaimed goods. Reclaimed 3" & 4" polyiso is usually on the order of $20-25 per 4' x 8' sheet, 2" runs $12-15 (sometimes cheaper) even in nearly perfect condition from places like the Insulation Depot/Nationwide Foam in Framingham MA, or Green Insulation Group in Worcester MA (the two largest vendors) but there are others. Pricing and stock varies quite a bit, and factory-blemished foil faced goods are usually available at about half price in a variety of thicknesses.

    1. spoons22 | | #7

      Thank you for the advice Dana.

      Aesthetically, I don't imagine it can tolerate that much added height. Your first suggestion I believe would be the better option. The roofing contractor is requesting 1/2" substrate for my flat roof. They said they would then put their underlayment, 1" of insulation, then their 50mil PVC membrane. Since I won't be ready until October, I'm trying to encapsulate the polyiso from getting wet within the AdvanTech and picture framing. The flat roof will be used as staging and storage to demo the next section of roof.

      The next unvented cathedral assembly to be deconstructed has from the interior: drywall, 2” x 6” framing 16”o-c, fiberglass batts, 3/8” plywood, exposed fastener metal roofing. I’m interested to see how it looks inside. Because of second story window locations, adding the required amount of rigid insulation above the roof sheathing won’t be an option. I’ve never seen any signs of failure from the inside with this assembly. When I remove the sheathing I’ll get a better picture. Should I be concerned if I replicate the same assembly, replacing the batts with new insulation and adding zip sheathing panels taped and flashed, to which the contractor will install their underlayment and roofing material?

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