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Community and Q&A

Roof Insulation Retrofit

apkeck1985 | Posted in General Questions on

We have a 2 story tall-and-skinny with rooftop access in Seattle that needs to be redone for drainage purposes anyways, it’s currently a cold roof (entire thing is 2% tapered trusses starting at 16” H to 12” H filled with batt insulation, vents on both sides are just 2” holes drilled through siding with bug screens) that does okay during hot days but the house is cold during the winter (high electric bills), would it be worth converting to a warm roof? I will be resloping with Type II eps before having a company install PVC and I want to build parapets (which already bring their own problems) for privacy, with that I considered running continuous exterior insulation on the roof but I don’t know if it’s worth it to plug the existing vents. In addition the second floor is full of windows (great for natural light) but none are energy efficient so the house effectively gets too cold with a cold roof but would get too hot in the summer with a warm roof (plus the kitchen and master bath are on the second floor). We are planning on staying in this house for a very long time.

Edit: On parapets, the consensus seems to be to wrap them in a continuous layer of insulation so they are part of the envelope. That makes sense when the structure as a whole is wrapped but does it make sense when the structure is not? I think individually it might make sense to wrap the parapet as added insulation on what is a thin exposed wall by itself, but it also seems like overkill when you can just vent the parapet instead. The issue of building a parapet above a cold roof is an added issue, as insulating seems like a wasted effort at that point. In an ideal world I would seal the cold roof and wrap the whole building, but that might have to wait.

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  1. Expert Member
    Zephyr7 | | #1

    Roof vents don't cause houses to be cold in the winter. Air leaks and inadequate or poorly installed insulation is what causes houses to be cold in the winter. Vented roof assemblies in many ways are the best way to go in terms of robustness, since the venting tends to prevent many of the moisture problems that can otherwise damage a roof.

    Chances are you have leaky upper floor ceilings (the "attic floor" area), and either insufficient insulation, or insulation that isn't installed very well. The best way to go is some type of loose-fill (blown) insulation, cellulose being the type that is most recommended on GBA.

    I would check those batts first, see if they are installed properly. If they aren't, or if they're too thin, I'd consider removing them all, doing an air sealing project on the attic floor, then reinsulating to at least code minimum levels with blown cellulose. This will usually be an easier/cheaper solution to a "cold in winter" home than trying to convert the attic to conditioned space. I'm assuming that you don't have any mechanicals (ductwork, etc.) in the attic, if you do, that changes things.

    You DO NOT want to plug any of the attic vents. If you have air leaks, as I suspect you do, plugging those vents as a first step is likely to cause moisture and mold problems in your attic.


  2. Joshua Salinger | | #2

    It sounds like you are going to be putting insulation on top of the roof deck as you mentioned 'resloping with type II eps'. You just did the hard work of creating a hot roof assembly... I would suggest making the small side of the sloped EPS be min R-20 (code minimum to prevent condensation in CZ4C). I would then take out the fiberglass batts, close off the roof vents and fill the rafters with dense pack cellulose. I would take particular care to make sure the ceiling is air tight and remove any can lights if there are any and replace them with LED 'pancake lights' and air tight electrical boxes such as the 'Air Foil' box available from Small Planet Supply.

  3. user-7853075 | | #3

    I am a commercial roofing supervisor, so I'll stick to what I know for my comments.

    - if possible, I'd suggest framing the parapet either over continuous insulation or filling the wall. This will give a better thermal break than having the wall being only half insulated if you're not doing the entire exterior with insulation. The top of the parapet will need wood blocking, and doesn't typically allow for insulation being installed below or above. This blocking at the top of wall will be used to attach your edge metal ( coping, drip edge, es1, whatever).

    - manufacturers provide us with tapered insulation diagrams based on an average r value, rather than a " code and up" r value. Where I am in central new York it is r-30 for commercial roof decks. This works out to be 5.2 inches of polyiso with fiberglass facer for what we always use. Making sure if minimum r values in the taper assembly is a good idea, your contractor may be going off the average r value of the tapered system.

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