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Considering improving the R-value and ventilation on re-roof (roof over)

Mike Kolder | Posted in Green Building Techniques on

Hello new here. Northern 5A area, Iowa.

I bought a 16×80 1990 mobile home, 6″ walls and vaulted ceiling in half of it. I need to re-roof it and might just simply replace the shingles.. But since I plan on keeping it for a while. Should I consider spending more money and time insulating and improving ventilation? Right now it only has 8 roof vents close to the peak but no intake since it doesn’t have a soffit.

I always over analyze things and have to keep in mind what the trailer is worth.. Right? But the other side is how much propane will I save if I spend allot of time filling the entire truss space with blown in fiberglass from above and remove and patch the 8 static roof vents. Cover the entire clean 1/2″ existing deck with a good underlayment and screw down 2×4 rafters on top of the existing trusses to create space for 2″ insulation boards and ventilation. Allow the new rafters to hang over 6″ to create a ventilated soffit. Install 2×2 battens over the trusses and install a good metal roof with ridge vent.

Is this to much effort for a old trailer? Or could I actually recoup the material costs (forget my time) over 10 years?

Thanks

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    ReRod,
    It's very hard to tell whether the work is worth it. The variables include (a) The current value of your trailer, (b) The anticipated value of your trailer in 10 years, (c) The cost of the insulation improvements (which depends on whether you are doing the work yourself of you have to pay a contractor to do it), (d) The cost of propane, (e) The energy savings attributable to the work.

    Trailers can rise in value over 10 years, but they can also decline in value (more than houses).

    I'll make up some numbers. Let's say you spend $1,800 per year on propane, and insulation improvements can cut your energy bill down to $1,450 per year. Over ten years, you could save $3,500. Can you install insulation above your roof sheathing for $3,500? Probably not, unless you do the work yourself and can scrounge some used materials.

    If you decide to go ahead with the work, you should know that the best way to improve the thermal performance of your roof is to add a continuous layer of rigid foam above the roof sheathing. Here is a link to a relevant article: How to Install Rigid Foam On Top of Roof Sheathing.

  2. Mike Kolder | | #2

    Thanks Martin.

    That article is well written but isn't aimed at a mobile home's inefficient roof R value and ventilation. I see some guidelines suggest when using sips type roof deck, (plywood over polyiso) that ventilation isn't necessary. But I have yet to see any re-roofs using polyiso with plywood over.. I would still be faced with ventilating the underside of my metal roof. The sips style would be good for shingles but reduces their life.

    My above method does cover the deck with polyiso like you recommend, yet goes beyond to add a ventilated soffit. It does not address thermal bridging though.?
    My only question at this point is how to treat the now vented truss space. Leave it alone except remove the static cans but leave the opening open under the new roof? Or remove enough plywood to blow in more fiberglass insulation to just under the deck and seal off the space since the new ventilated roof deck is now 4" above the old?

  3. GBA Editor
    Martin Holladay | | #3

    ReRod,
    If you decide to install rigid foam above your roof sheathing, and you follow the recommendations in my article ("How to Install Rigid Foam On Top of Roof Sheathing"), you aren't required to include a ventilation channel. You can create an unvented roof assembly instead.

    Some people who install rigid foam above their roof sheathing include a ventilation channel under the roofing. But this ventilation channel is optional (especially if you plan to install metal roofing).

    If you are concerned about thermal bridging, make sure that the rigid foam layer is continuous. You certainly don't want to cut up the rigid foam into narrow rectangles and install the rigid foam between framing members.

    Q. "My only question at this point is how to treat the now vented truss space. Leave it alone except remove the static cans but leave the opening open under the new roof? Or remove enough plywood to blow in more fiberglass insulation to just under the deck and seal off the space since the new ventilated roof deck is now 4 inches above the old?"

    A. It's essential that you prevent air flow through your old ventilation channel. To do a good job, you need to track down all air entry points and seal them. This is fussy work. You also need to track down all of the high vents in your roof -- whether mushroom vents or ridge vents -- and seal them. A two-component spray foam kit may be needed for this work.

    If you have a 4-inch high ventilation channel, it would also be a good idea (after the air sealing work is complete) to install dense-packed cellulose insulation in the 4-inch ventilation channel.

    Getting these details right is a lot of work. Good luck.

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