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Community and Q&A

New Roof for Old House

josephny | Posted in General Questions on


I am renovating a 100 year old house with original pine board roof decking, no insulation and a shingled roof that is way past its useful life. The roof has multiple dormers with the associated valleys.

I have been struggling with getting a solution for weeks now, speaking to roofers and insulation installer and reading everything I can online.

I have a very good spray foam installer that I’ve worked with before and have been happy with the outcome. But that was on new construction.

I am concerned that if we use closed cell spray on the underside of the 100 year old pine board roof decking a couple of things will happen: Foam will seep up through holes and cracks in the board and the roof decking will not be able to breath (and therefor rot).

I’ve gone through various ideas on what to put on top of the pine boards: vapor permeable or vapor impermeable membrane and then shingles; rigid insulation then osb decking then felt or other membrane then shingles; furring out decking to create a soffit to ridge vent then decking etc.

I am looking for the simplest and least expensive solution that will create a heatlhy, long lasting roof.

I am trying to get the roof done before the weather gets too cold. Spraying will have to happen at a later time.

Thank you for the help!


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  1. user-2310254 | | #1


    Where are you located? Advice is climate dependent.

    The best approach is to put all the insulation on the exterior, but this strategy is not always practical or cost effective.

    Ideally, you want the sheathing to be able to dry in at least one direction. So if you installed closed cell foam on the interior and asphalt shingles on the exterior, you could set the stage for long-term problems.

    How is your attic framed? Is everything open from the top plate to the peak? Are you using the attic for storage or extra living space? Is any equipment located in the attic? Can you post some pictures?

  2. user-723121 | | #2

    I would contact a local builder well versed in this type project. The opinion of roofers and insulators matter not, you need a professional opinion. Most of the roofs on these type homes are under framed and under supported. Dormers add to the complexity and make for a weak roof in hand framed situations. Hips are strong, valleys are weak generally speaking. Ridges are typically 1 x boards and are just a connection for the rafters with little support strength.

  3. Expert Member
    Akos | | #3

    SPF is usually the simplest way to insulated complicated roof on old houses. There are no issues with SPF and board deck. As long as the SPF is installed under a dry roof deck you won't have any moisture issues.

    Asphalt shingles have a small amount of permeability, as long as liquid water is not making its way in, your roof deck will stay dry. SPF is the common way of insulating these older houses around me (Toronto).

    If you can find a contractor to do the install, exterior rigid also works great. A layer of peel and stick can seal up your current roof deck than install sufficient rigid for your climate for condensation control followed by a new roof deck before shingles (see here for ratio ).

    Insulate the rafter space with fluffy and finish with drywall. Usually with dormers, it is simplest to insulate the walls exactly the same way. This would give you a very high performance roof (significantly higher assembly R value than SPF). Materials for this is cheaper but with complicated roofs, this labour does get expensive.

  4. ssnellings | | #4

    If closed cell is used on the interior side and the exterior roofing is properly installed there will not be moisture issues. I agree with Akos.

    I've had closed cell installed against board sheathing on a couple homes, but many of these had existing plywood already installed over the board sheathing by old re-roof jobs, or had very tight board sheathing installation with no gaps larger than 1/8 inch or so. I shot off an email to the insulator on those jobs to get his perspective, but may not hear back until after the holiday.

    If you can't eliminate your jitters over it, just have your roofer put down a layer of plywood before the new roof to eliminate any possibility of seepage.

    Exterior insulation is a great idea, but it is going to get painful if the roof really does have multiple dormers to work around.

    1. ssnellings | | #5

      The insulator I use got back to me already. As long as there's sheathing and shingles above there isn't a concern with squeeze through. This only applies to board sheathing installed properly with a maximum 1/4 gap required by most asphalt shingle manufacturers. If the insulating crew can see daylight anywhere (indicating a 'clear path' that the expanding foam could take) they "pre-treat" the gap with a small bit of canned foam to ensure the full foam application doesn't create an issue - but this done mostly due to the insulator's personal very low risk tolerance.

      This advice does not apply to skip sheathing.

  5. twoodson | | #6

    Not the same situation (flat roof with pine board sheeting), but a century house.

    We did 4 inches of polyiso on the top and 10 inches of open cell SPF on the bottom. The poliso gives us condensation control and the open cell gives us an air barrier. It was easier to install, an efficient use of materials and mostly foolproof. The other theory is that an issue with the tpo watershed will show up on the inside before it rots the sheathing. I'd rather replace a bit of drywall than large sections of sheathing. At least that's the theory.

    I will say, closed cell SPF has some serious "moving" power. We sprayed walls with that and had a few issues, so I get the concern with it leaking though the boards.

  6. josephny | | #7


    I would prefer to put the insulation on the outside of the house, but from my research, this is far (far) more expensive. I originally wanted to preserve the visibility of the underside of the old pine board decking and the 100 year old wood structual elements (which are completely open now).

    I want to use the attic as a 3rd floor habitable space.


    Hard to find a local building well versed in this type of project. More precisely, everyone claims to be well versed and I'm certain that's not the case.


    SPF is attractive to me specifically because it is a complicated, old roof, I am in an R49 roof code zone, and (I'm told) it adds to the structural stability.

    I keep reading and reading completely opposing understandings on the need for air flow above a deck that has SPF under it. In my case, the existing deck are old pine boards. Pretty good condition but I'm told a new layer of sheathing would be wise. So the question remains whether I can safely (from a moisure accumulation perspective) put down a layer of OSB or plywood, felt and shingle and not have to worry about trapped moisture and the resulting rot.

    Same totally conflicting advice about using Ice and Water over the sheathing (that has SPF under it). Some say that vapor and/or moisture and/or water will eventually make its way in and be trapped.


    It sounds like your insulator does not see the need for another later of sheathing. That would be a nice money saver. Sounds a lot easier to fill a few gaps from inside (pre-foaming), than resheath a 4200 sq ft complex roof.

    To be clear, you also believe that I don't need a complex roof assembly that includes a vent on the exterior of the sheathing (whether it's the original board sheathing or a new layer on top of that).


    I'm starting to understand the great benefit of insulating from outside -- no thermal break from the rafters, condensation control farther to the outside. To achieve 49, would require a combination of Polyiso and open cell of something in the area of 10" (average of ~R5/inch?). I have to explore this more.


    I have posted pictures.

    I haven't done anything yet, so if I'm heading in the completely wrong direction, I'd so very greatly appreciate any other ideas.

    Thank you all so much!

    Please see this photo album:

    1. ssnellings | | #8

      Yes, no need for another layer of sheathing, just do a scan for places where the gap might be larger than average between the boards and pre-treat with a small bead of canned foam. And you don't need to vent the roof given you're using closed cell spray foam on the underside. The original sheathing install looks nice and tight from the pictures.

      Thanks for the photos, cool house, looks like a lot of work!

  7. Expert Member
    Akos | | #9

    There is nothing wrong with both peel and stick on the outside and closed cell SPF on the inside provided the deck is dry before the install. It is a common roof around me with SPF since code requires eave protection, my own home has this for part of the roof.

    Closed cell tends to be expensive and usually not the best use of your dollars for insulation when sprayed between lumber. The thermal bridging of the rafters short circuits the high R value of the foam. Generally a better option is enough closed cell SPF for condensation control and the rest as open cell. Check here for the ratio of ccSPF and ocSPF based on your climate zone:

    Since your rafters are 6", you'll need to fur out your rafters a bit anyways if you need to meet R49. A bit of extra depth there is pretty much free to be able to go with open cell SPF for part of the insulation. Cross strapping also decreases the effect of thermal bridging of the rafters, you get a higher assembly R value overall, almost the same as having continuous rigid insulation installed.

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