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2x4 rafters, skip sheathing, no insulation, help with roof choices

The shingled roof on my 1940 farmhouse in Zone 3b is due to be replaced. My wife and I are considering the virtues of metal for our very hot, very dry climate where electricity often exceeds $0.40/kw hr.

I plan to remove what little insulation is presently on the attic floor. The hvac ducts are also in the attic with little insulation and are likely poorly sealed.

The roof has hips and gables and valleys, oh my. The rafters are healthy (nearly 1 3/4" thick) 2x4's on 24" spacing with every other one (every 48") comprising a site built truss. Sheathing is 1x3 with 2" spaces. There are separate, exposed rafter tails comprising a shallow 12" eave but no gable eaves.

I'd like to put an inch or more of polyiso on top of the existing skip sheathing with a radiant barrier/air gap somewhere above that supporting metal roofing. This sort of assembly makes sense to me as a means of reducing the insane solar absorption by the attic in my neck of the woods which I presently have very little given how well the cedar performs and leaks. I haven't seen a detail yet that includes skip sheathing as a base so if anybody has come across one, please share. If I can simply treat the skip sheathing as if it were plywood, I will need to find a reference to guide our building inspectors.

Does it make sense to complete the air barrier and insulation at the roof assembly in this case or drop back down to the attic floor to attempt to seal and insulate? If completing the air barrier and insulation at the roof assembly, how can I achieve code minimum insulation? I fear the 2x4's do not have enough depth to support the normal means and I fear the roofers around here won't have the slightest how to finish a roof deck with excessively thick insulation on top.

Suggestions or questions?

Asked by Edgar Pankey
Posted Apr 20, 2017 7:51 PM ET


11 Answers

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It's possible that your rafters are structurally deficient. It's also possible that your local building code requires properly fastened plywood over the existing ship sheathing. In order to evaluate these issues, you should consult an engineer.

Assuming that your rafters and the skip sheathing don't raise structural or code concerns, it's certainly possible to install rigid foam above your existing sheathing. Here is a link to an article that explains how this is done: How to Install Rigid Foam On Top of Roof Sheathing. If you end up installing rigid foam on the skip sheathing (rather than plywood or OSB), it's important to tape the seams of the rigid foam to limit air leakage.

You may also want to read this article (which explains why it's important to install insulation at the roof plane rather than on the attic floor when there are ducts in the attic): Creating a Conditioned Attic.

I don't have an easy answer to the problem of contractor ignorance, so I can't really advise you on what to do if your local contractors balk. The best approach is to keep making phone calls until you find an interested contractor.

-- Martin Holladay

Answered by Martin Holladay
Posted Apr 21, 2017 6:41 AM ET
Edited Apr 21, 2017 7:56 AM ET.


To hit IRC 2015 code minimums for roofs on a U-factor basis you need a "whole-assembly-R" of ~R33 or better. At a framing fraction of 10% (higher than average due to hip/valley framing) An unvented 2x4 roof with blown cellulose (to fill in the spaces on the skip sheathing will come in at about R13, including the R-value of skip sheathing, roofing, interior & exterior air films, etc. Adding a layer of 1/2" plywood above the skip sheathing and 1/2" gypsum on the interior side adds another ~R1.

Adding 3" of continuous polyiso held down by a 1/2" OSB/plywood nailer deck through-screwed to the skip sheathing would bring the whole assembly up to IRC 2015 performance, with huge dew point margin at the skip-sheathing/roof deck level- no need for interior vapor retarders of any type. That adds only 3.5" to he roof thickness, which is pretty easy to deal with aestheically/cosmetically, but with hip & valley roofs there is potentially a high scrap rate on the foam.

To take the sting out of the foam costs, there are probably several square miles of reclaimed 3" roofing polyiso from commercial building re-roofing & demolition in salvage yard and foam-reclamers' warehouses, possibly even one near you. In my area recliamed 3" fiber roofing iso in good to near-perfect condition runs about $15-25 per 4x8 sheet ($0.47 - $0.78 per square foot.), compared to $55-65/sheet for virgin-stock goods f.o.b. the local building materials distributors. With a highly cut-up roof like yours buy at least 20-25% more than the actual roof area, to deal with the fraction of damaged foam and total scrap rate.

If you can't find local sources there is at least one reclaimer (Nationwide Foam: http://www.nationwidefoam.com/ ) that will ship pretty much anywhere in the US from regional depots, but there are many local reclaimers and scrappers trading in used roofing foam. With a ZIP code I might be able to bird-dog one of those for you.

Answered by Dana Dorsett
Posted Apr 21, 2017 10:36 AM ET


Interesting responses.

Martin, I've read both of the articles you mention as well as Lstiburek's Builder's Guide to Hot-Dry Climates which is why I'm trying so hard to find a way to improve upon local practices. Most local roofers would just lay up asphalt and blow cellulose but I'm convinced that by raising the temperature in the attic and increasing the thermal mass there I'll flywheel that energy well beyond the diurnal hours when the house currently becomes comfortable. I believe in the concept of conditioned attics especially where existing ductwork and 1940 attic floor complexity support it. My fear, and apparently yours as well, is regarding the structural acceptability of my existing roof framing for any change to my roof system.

Let's just assume for a moment that the existing framing is deficient for any new roof selection. What sort of options should I be considering? I presume I could always re-roof with cedar, clean up the attic floor, seal it as best as possible, and then blow in cellulose after also doing my best to seal up and insulate the existing duct work. Frankly, I'm impressed at how comfortable this attic is on a 110 degree day compared to others I've been in with asphalt shingles.

If I were to have to deal with a new roof structure, in order to justify the cost what sort of best roof systems should I consider?

Dana, I'm in California's central valley, I'll hit you up for more info on scrap polyiso if/when I decide what to do and now definitely after I talk to an engineer. In your first paragraph you describe filling the 2x4 rafters with cellulose with gypsum on the attic side. However, installing gypsum seems difficult given my rafters are open to the attic with (albeit minimal) trussing every four feet and a fairly low roof pitch. Would a netting or spray foam (given the dew point margin isn't a concern) make more sense?

Answered by Edgar Pankey
Posted Apr 21, 2017 4:59 PM ET


The structural issues are independent of the insulation issues, but it's certainly worth talking to an engineer before you invest in new roofing.

If you find out that you need to sister new 2x8 rafters to your existing 2x4 rafters, that may change your plans for how you want to insulate your roof assembly. Whatever approach you take, it makes sense to insulate your roof assembly (so that you bring your ductwork inside your home's thermal envelope).

-- Martin Holladay

Answered by Martin Holladay
Posted Apr 21, 2017 5:07 PM ET


Get radiant decking, eg: techshield osb.

Answered by Anon3
Posted Apr 21, 2017 5:55 PM ET


Skip sheathing is an obsolete building practice in California. With the age of your house, before considering metal roofing you should check to see if the roof is straight. It will cost extra to make the roof straight enough for metal roof. Give that you are in California the roof should be covered with plywood or OSB sheathing and properly nailed. It is an essential part of modern construction to have the roof act as a diaphragm to distribute the earthquake forces to the walls.

Answered by Tim R
Posted Apr 21, 2017 6:32 PM ET


I just spoke with a local structural who had little concern for the city's requiring any additional study of the existing framing. Not that it is perfect, but that it won't raise any eyebrows as long as I'm comfortable with it (which I am) and don't attempt putting something heavier on the roof deck. [edit]While I was writing this another salient point was added by Tim above regarding the roof system in seismic zones acting as a diaphragm which was also a part of my conversation with the structural but not added as it hadn't yet been a part of the conversation here.

Thanks again for everyone's suggestions. I'll probably dive deeper on the details for the roof edges now. It seems a little strange to have a square or plumb cut roof deck that's 4" thick over exposed rafter tails the same height. I'm not sure I've seen a level cut done with polyiso although I imagine it would have to be protected somehow.

How does one waterproof the top of the polyiso against condensation from the bottom of the metal while maintaining the radiant barrier it provides (when the metal is installed on furring strips) or is that impossible?

Answered by Edgar Pankey
Posted Apr 21, 2017 6:33 PM ET
Edited Apr 21, 2017 6:56 PM ET.


Q. "How does one waterproof the top of the polyiso against condensation from the bottom of the metal while maintaining the radiant barrier it provides (when the metal is installed on furring strips) or is that impossible?"

A. Building codes require roofing underlayment (asphalt felt or synthetic roofing underlayment) under roofing. If you wanted to skip the roofing underlayment, and instead depend on foil-faced polyiso taped with foil tape, you would have to negotiate with your local code enforcement official to determine whether that approach is acceptable.

I used to be a roofer, and I'm a firm believer in the use of roofing underlayment under metal roofing (because of the likelihood of condensation and drips).

A radiant barrier adjacent to an air space adds between R-1 and R-2 to the R-value of your assembly. It's something, but it's not a lot. If you want more R-value, it makes more sense to just install thicker rigid foam rather than worrying about R-1 or R-2 from the foil facing.

-- Martin Holladay

Answered by Martin Holladay
Posted Apr 22, 2017 7:09 AM ET


With R15+ above the roof deck and R13 cellulose (blown in netting or damp sprayed) in the cavities you don't need interior side gypsum from a dew point control perspective. You have HUGE dew point margin for a central valley CA location. With only R5 above the roof deck it would be wise to install an air tight class-III vapor retarder.

With blown ors prayed cellulose insulation the fit is perfect and the air retardency is quite high- it will perform at it's rated R without an interior side air barrier, unlike low density batts. Blown/sprayed fiberglass can be used too, but it needs to be at least 1.8lb per square foot density to have comparable air retardency. If 1 lb fiberglass is used there will be some convective loss of performance if there is no air barrier.

Spray foam is installed under the roof deck needs to have a thermal barrier such as half-inch gypsum board to meet fire code in most places. Some jurisdictions would allow an intumescent paint for fire protection, provided it has been tested with the foam product.

The top of the polyiso should have a plywood or OSB nailer deck through-screwed to the structural sheathing to hold the polyiso in place. Between the nailer deck and the metal roofing you'll need at least #30 felt or some other weather resistant barrier underlayment (often specified by the roofing manufacturer.) For the nailer deck, using the fastener specifications & spacing given by nailbase foam panel manunfacturers is used, it should be "good enough", though tighter would be OK too. eg:

https://www.hunterpanels.com/roof-polyiso-products/fastening-pattern-gui... (details start on p5)

Answered by Dana Dorsett
Posted Apr 22, 2017 10:12 AM ET


So, if I were to go a different direction with the help of a structural engineer to over roof a new 2x6 (or 2x8, whatever the structural requires for seismic considerations) assembly above the existing skip sheathing, what would be the most desirable infill insulation? Or is this just crazy talk and I'd be better off rebuilding the entire roof? If it's not crazy talk could the 22" Roxul comfort batts make sense here? Or is it just way more economic to dense fill with cellulose or fiberglass or something else?

Given I couldn't achieve R38 unless the cavity got especially deep I would imagine still needing either an insulated nailbase for sheathing or some combination of polyiso and osb which would also help provide a thermal break for the new rafters. Everything from the new rafters "up" is fairly well explained elsewhere I believe; however, would felt paper be preferable between the skip sheathing (1x3's with a 2" skip) and the new rafters to provide vapor control and/or mechanical retention if necessary?

I've seen quite a few articles and such on over-roofing with various insulating products, but not much on structural over-roofing. Pete Bennet wrote about it in FHB ten years ago: http://www.greenbuildingadvisor.com/sites/default/files/021190074.pdf

This is quickly getting out of hand! It's an old house that is the polar opposite of airtight, a major roof overhaul is going to beget questions regarding the walls, and then the original double hung windows, and then the crawlspace. argh

Answered by Edgar Pankey
Posted Apr 24, 2017 2:10 PM ET
Edited Apr 24, 2017 2:11 PM ET.


All of the different ways to insulate a sloped roof assembly are covered in this article: How to Build an Insulated Cathedral Ceiling.

There are two basic approaches: the unvented approach or the vented approach.

If you want to install fluffy (air-permeable) insulation between your rafters, you either need to take the vented approach (with a ventilated air space between the top of your insulation and the underside of your roof sheathing), or you need to install an adequately thick layer of rigid foam above your roof sheathing.

Your old full-sized 2x4s provide 4 inches of depth. The new 2x8s would provide 7 1/4 inches of depth. If you leave 1 1/4 inch for your ventilation space, you still have 10 inches for fluffy insulation. That would give you an R-value of R-37 -- not too bad.

-- Martin Holladay

Answered by Martin Holladay
Posted Apr 24, 2017 2:42 PM ET
Edited Apr 24, 2017 2:43 PM ET.

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