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Replacing siding on an old Toronto house

Jon Haque | Posted in General Questions on

Hello,

I need to replace the siding on my house. I’ve read through this article, but still have some questions:
https://www.greenbuildingadvisor.com/blogs/dept/musings/roofing-and-siding-jobs-are-energy-retrofit-opportunities

Some info:
The house, a small semi-detached, is about 100 years old, brick on the first floor, plaster walls throughout the second floor, which is the one that has existing siding. The existing siding is a kind of asphalt shingle on what looks like fibrous cardboardy material. It looks like this was put directly on top of old wood shingles. These asphalt shingles are in bad shape, which is prompting the project.

I would like to take the opportunity to insulate the walls from the outside with rigid foam, but I’m not sure how to go about it, especially given the age of the house and the plaster on the inside walls (I get the feeling the plaster will have implications for moisture control since it’s impermeable).

I can’t replace the windows as well as this will definitely go over budget but I also don’t want that sunken window effect and I get the feeling that installing J or C trim correctly might be challenging for a lot of contractors.

My thought is to rip out all the old siding, put XPS and aluminum siding on top. I don’t know what to do regarding any barriers and whether anything needs to be able to dry.

What would you suggest?

Thanks very much,
Jon

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Jon,
    Q. "I get the feeling the plaster will have implications for moisture control since it's impermeable."

    A. The plaster may be an air barrier if it's in good shape, but it isn't a vapor barrier. Plaster is vapor-permeable. Of course, if it has many layers of paint, these paint layers reduce the vapor permeance of the plaster somewhat. But it's hard to imagine that your air conditioning will ever be cold enough during the summer, or that inward solar vapor drive will ever be significant enough, or the interior paint will ever be vapor-impermeable enough, for you to worry about this phenomenon.

    Q. "I can't replace the windows as well as this will definitely go over budget, but I also don't want that sunken window effect, and I get the feeling that installing J or C trim correctly might be challenging for a lot of contractors."

    A. You need to think through the implications of this sentence. Here are my reactions: (1) If you add exterior rigid foam, you will get the "sunken window effect" unless you remove and reinstall the windows or purchase new windows. (2) Flashing the window openings requires much more than just installing J-trim; it requires a thorough understanding of flashing techniques, especially if the existing windows will remain. You may want to read this article: Installing Windows In a Foam-Sheathed Wall.

    Q. "My thought is to rip out all the old siding, put XPS and aluminum siding on top. I don't know what to do regarding any barriers and whether anything needs to be able to dry."

    A. Start by reading this article: How to Install Rigid Foam Sheathing. A common mistake is to install a layer of exterior rigid foam with an R-value that is too low to keep the wall sheathing above the dew point in winter. You're in Climate Zone 6; if your house has 2x4 studs, then your exterior rigid foam needs to have a minimum R-value of R-7.5.

    For more information on this issue, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

  2. Jon Haque | | #2

    Hi Martin,

    Thank you for the responses.
    I'm also considering simply adding aluminium siding on top of the existing siding. Is this a bad idea? If not, should I be putting a layer of tyvek underneath the new siding? A contractor has recommended this, but I get the feeling that might not be best if there's existing siding underneath.

    Thanks again

  3. GBA Editor
    Martin Holladay | | #3

    Jon,
    In most cases, it's possible to add a layer of new siding over older siding, as long as (a) the underlying siding is co-planar enough (smooth enough) to allow the new siding to be installed without problems, and (b) there are no problems with attachment. (There needs to be solid sheathing behind everything, and the siding installers need to use long enough fasteners to reach the solid sheathing).

    The siding installers would certainly need to install a new water-resistive barrier (WRB) -- for example, plastic housewrap -- between the existing siding and the new siding.

    Flashing the windows properly is a big challenge -- one that is too complicated to address here. Suffice it to say that you need to think about whether the walls are flashed well enough to prevent rain from entering the walls.

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