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Community and Q&A

Antique house #1: Cedar Roof

Curtis Betts | Posted in General Questions on

I will likely be taking on a crazy project in the next few months – buying an old house in Rhode Island, and improving it, on speculation.

The general scheme is to
–Do minimal work on the older (about 325 years) portion of the house,
–Expand/rebuild the newer (about 115 years) section with modern kitchen, baths, windows, etc.
— Remove ugly garage, breezeway, and dormer.

With luck, the major work on the old portion will be replacing the decrepit asphalt roof with cedar shingles, or something similar, with as little change as possible to the existing framing and sheathing (except removing the dormer, and perhaps adding a skylight or two). The attic is plastered, so inspection of the roof deck is impossible without removing the shingles. Presumably there is no insulation, and adding insulation within the roof assembly would seem to be a non-starter. I have not yet had “the talk” with the local Building Inspector, but anticipate that energy codes will not be an issue.

My current thinking is to:
–strip the roof, remove dormer
–add a layer of sheathing – either rough-sawn boards (diagonal?), skip-sheathing, or plywood.
–consider 2″ or so of insulation board
–another layer of sheathing – either plywood or furring spaced 5″ OC
–if plywood, some kind of ventilation/drying spacing or fabric
–cedar shingles, 5″ exposure
–site-built wooden gutter, lined with (?)

Roofing felt, or ice & water shield would likely be a good idea, but where in the assembly? Over the insulation? Under it? Certainly not too close to the old structure, or to the cedar shingles.

Fasteners are also an issue – I would hope to minimize intrusion on the original rafters – which are presumably spaced unevenly anyway.

Insulation board likely polyiso, though the house is very close to a big manufacturer of “Aerogel” insulation. There were some interesting discussions on this product on GBA recently, perhaps I could sweet-talk my way into some product…

So many issues, and this is just the start! I would appreciate any insights or ideas…

Curtis

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Replies

  1. Curtis Betts | | #1

    As noted, this is a historic house, with likely zero insulation currently. My first instinct is to NOT add any insulation in the old section, as historical preservationists would recommend. Adding several inches to the roof structure may not work aesthetically. But, while energy usage is not the primary concern here, it is still a consideration, and I am eager to make improvements consistent with the general appearance, details, and structural integrity of the building. The title of the posting was meant to indicate that the roof is only the first of many details that need to be worked out.

    Any suggestions, or reference to how houses of similar age have been modified (on a tight budget - the property is in a working-class neighborhood) are most welcome.

    As to "convincing the inspector to look the other way" - it is my understanding that the energy code applies to new construction, and not necessarily to buildings that have stood with only modest changes since 1690. This will probably be about the 15th roof replacement, and I hope not to screw it up too badly ( I am rather nervous about using newer technologies; I have recently been repairing mistakes in a house I built for myself 25 years ago, using then-trendy materials and techniques ;-)

    So - again - constructive suggestions are welcome!

  2. John Ranson | | #2

    Renovating a house while ignoring energy codes doesn't sound particularly green to me. In fact, it seems downright malicious to do as a speculator. Whoever lives in that house is going to pay, literally, for every shortcut taken.

    You're suggesting an R-11 roof in a climate where the zone indicates R-38 minimum. If you're seriously looking for a green, energy-efficient design, you'll have to do something different. If you would like those suggestions, ask, and I'm sure you will receive good advice.

    --John

  3. John Ranson | | #3

    Okay, that's a different question. What's the best you can do while preserving the historical character of the house? I can get behind that.

    If you're not planning on touching the plaster on the inside, and you don't want to mess with the character on the outside, you lose most of the insulating options available.

    Since sloped ceilings are unlikely to have rosettes and other beautiful features of old plaster, I personally would consider removing the plaster on the inside of the roof and using closed cell spray foam. Here's why:

    A. You can do a much simpler roofing project overhead. Rip off the roof, repair the rotten boards. Put down felt along the center and grace on the edges and valleys. Add purlins or maybe a ventilation underlayment. Apply your shingles. No edge detail necessary to hide 2" of rigid foam.

    B. Well applied spray foam also acts an air barrier. Houses of that age will leak like a sieve, and fixing the roof is a start.

    C. Even if your roof is framed with 2x6 rough cut lumber, you still get R-30 to R36.

    --John

  4. Nate G | | #4

    Let me be a contrary voice and say that I think you may be on the right track.

    A house that has stood for more than 300 years is automatically a green house since it hasn't been torn down and rebuilt multiple times, and insulating it without being extremely scrupulous may result in damage that ends this favorable longevity streak.

    That said, adding insulation to the roof seems like it's possible to do safely. If you can sneak a few inches of foam or rigid mineral wool above the original roof sheathing, you may also be able to cut some holes in the sheathing and blow in dense-packed cellulose, which will be safe in conjunction with insulation above the sheathing.

    However, be wary of negative comfort effects. An uninsulated roof over a conditioned room is losing a lot of heat at night, which is actually semi-desirable during the summer in the Rhode Island climate, where air conditioning is rarely necessary for long. Insulating it to sub-code levels will probably keep the finished attic hotter in the summer than it is now even as it'll be warmer and more comfortable in the winter. AC bills may rise as heating bills fall. Might not be really all that cost- or comfort-effective unless you're willing and able to at least hit code minimum with the insulation levels.

  5. User avatar
    Dana Dorsett | | #5

    If you're going to do the foam-over anyway, Ice & Water over the antique plank sheathing follow by 4-5" EPS nailbase panels over the top through-screwed to the rafters 24" o.c. with pancake head timber screws (penetration must be at least 1.5" into the rafter, not just the sheathing), then #30 felt, then sheathing. With an appropriate facia board on the exposed ends of the nailbase it doesn't have to look completely out of character.

    R49 is current IRC code min for US climate zone 5 (that's you) or R38 continuous insulation. Hitting R38 with above the deck foam would definitely change the appearance in a big way. R20 above the roof deck would be sufficient dew point control for up to R30 (or even a bit more) under the roof deck, so even if there is some insulation above the ceiling it would be safe to air seal the attic venting (if any exists), with R16 -R20 above the roof deck, which takes a 4-5" EPS nailbase panel. You could use a 3.5" polyiso nailbase panel too, if that was better aesthetically, but the mid-winter performance is no better than a 4" EPS panel.

    Skylights are a recipe for ice dams and air leaks- not a great idea in the snowier parts of RI, but may be OK on Block Island or right along the coast.

    Putting R30-R36 of closed cell foam between the thermally bridging rafters is both a waste of good foam, since a very large fraction of it's performance is lost to thermal bridging, and it's a crime against the planet due to the HFC245fa blowing agent. EPS and polyiso are blown with pentane, which is far more benign. The 100 year global warming potential of HFC245fa is about 200x that of pentane, and about 1400x that of CO2. Putting a continous R16-R20 on the exterior outperforms a thermally bridged R30 (at typical framing fractions), and by putting all the structural wood inside the thermal envelope the wood stays warmer & drier- it's better protected.

  6. John Ranson | | #6

    I agree with Dana about spray-foam generally being nasty. That said, I would prefer to live in a house insulated with a decent amount of spray foam over a house with just a thin layer of foam above the roof. Now, a thick layer of foam above the roof would be fantastic, but it will require competent carpentry to retain the character.

    One detail to consider. As you mentioned, older houses are often framed irregularly, and with smaller framing and greater spacing than modern code would call for. If you add dead-weight to the roof with extra insulation or a second deck, and heavier shingles, you may want to consult with an engineer to make sure your framing (kneewalls, etc.) are up to the load, especially combined with your snow load.

  7. Dan Kolbert | | #7

    Spray foam is a really bad idea from a historic pres. standpoint - very irreversible. In addition to its other sucky qualities.

  8. Keith Gustafson | | #8

    I should like to mention that the first inch of insulation is the most important. While many here get hung up on massive amounts of insulation, this still holds true. You are looking at a house with zero insulation, other than the air film and value of the building materials. Adding just one inch of iso would lower the heatloss by a btu amount that will never happen again if you buried it in a mountain of insulation.

    Let not the perfect be the enemy of the good.

  9. User avatar GBA Editor
    Martin Holladay | | #9

    Curtis,
    I'd like to chime in with a few opinions.

    1. I agree that skylights are neither historically appropriate nor a good idea from a performance perspective. If you want to preserve the house for future generations, you don't want ice dams.

    2. If we can all agree that we want to preserve old houses, let's also agree that we want to preserve the planet as a livable environment for our grandchildren. That means that you have a responsibility to try to create a high-R roof assembly to limit heat flow and minimize energy use.

    3. A site-built wooden gutter? Really? Well, it's doable, if you have enough money to get it lined with 16-oz copper, with soldered seams. Make sure that the copper flashing extends far up the roof slope, and that the downspouts are also copper, soldered to the gutter lining.

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