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Most cost-effective way to insulate and vent a cathedral ceiling?

Spencer Jackson | Posted in Energy Efficiency and Durability on

Hi all, great site here, I’ve found lots of helpful information already.

I recently bought a 35 year old house in a Zone 6 area and it needs a new roof. This seems like the best time to do the attic insulation, but with all the expenses of our first home we’re running low on cash. We’ve budgeted around $5K for the whole project this summer and plan to do as much as possible ourselves. I’ve read through many articles here and on buildingscience.com but I’m kind of stuck.

The home is a rambler with an A frame type section through the middle which has the cathedral ceiling. I’ve made a little sketch of the attic area you can see at https://docs.google.com/file/d/0BzSs9AkeMJ5mWmxpNGVPZmptbVU/edit?usp=sharing

The cathedral section only has 7″ for insulation, less if an appropriate vent gap is left. I’d like to overinsulate to about an R-60. I plan to do this in the gabled wings with just 18″ of loose cellulose. That part’s easy. The difficulty is what to do with the cathedral. The options I’m seeing (other than having much less insulation or going into the interior) is to furr out the rafters and add enough space for cellulose or XPS, contracting someone to spray in polyurethane in the existing gap, or stacking 6″ XPS over the existing sheathing to prevent condensation and do a hybrid roof. I’ve read people on several forums still recommend putting in sleepers for vents on top of the XPS then more wood sheathing to prevent ice-damming. At that point I don’t see how it’s much different than furring out the rafters to simply make more room for insulation in the attic. Also do I need to worry that the rest of the attic will be vented or do I just dense pack the cathedral and seal it off from the rest of the attic?

I was thinking the spray-in foam was the way to go, but then I got a local estimate and it would use the whole budget on just insulating the cathedral section. I’m concerned furring or sleepers will add enough load to the house that it will need reinforcement, which I don’t know that I could do properly. I’m trying to balance my budget and limited experience (I’ve done basements, sheds, and shingles but nothing on this scale), but most of all I want to get it right the first time. I thought perhaps an unvented assembly would be easiest to do myself, but I keep worrying about moisture. I’d like any advice anyone can offer. Thanks for your help!

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Spencer,
    I think you have described your options. So it's time to sharpen your pencil, estimate your costs, and decide which way to proceed.

    You might look into buying some used polyisocyanurate to install above your existing roof sheathing. If you are installing a new roof anyway, that seems like a logical way to proceed. Don't forget to insulate the two triangular sections of cathedral ceiling that face your conventional attics on either side of the cathedral ceiling.

  2. Spencer Jackson | | #2

    Ok. Thanks. I perhaps should have specified it needs new shingles not necessarily a new roof. but the sheathing will be exposed so if a roof replacement/renovation is in order to get properly insulated, this is the time.

    I've never considered used building materials would local contractors be the folks to ask about it?
    Those two triangles will just be insulated with cellulose like the level sections of the attic.
    So if adding xps on the exterior of the cathedral, how do I make the transition from the regular vented sections of the attic? Seal it off?
    Also how valuable is it to add the vent channels by putting 2x4s above the XPS? You mention it in the "How to Build an Insulated Cathedral Ceiling" article, but I don't understand the advantages.
    I think there are quite a few details that I'm unsure of. I can start new threads as they come along if its more appropriate. Again, thanks so much!

  3. GBA Editor
    Martin Holladay | | #3

    Spencer,
    Q. "I've never considered used building materials. Would local contractors be the folks to ask about it?"

    A. The best-known source of used polyisocyanurate insulation is a company called Insulation Depot. I would start there. There are other companies selling recycled polyiso; you can ask around locally or do a web search for the names of more companies.

    Q. "Those two triangles will just be insulated with cellulose like the level sections of the attic."

    A. If you insulate above the existing roof sheathing, you'll need to be sure that you have continuity in the insulation layer, including the triangles of the cathedral ceiling that are hidden when you stand on the roof. These triangles should appear as exposed roof sheathing, visible from your two vented attics. (I am attaching the sketch of your roof that you linked to, so that curious GBA readers can understand what I am talking about.)

    Q. "Also how valuable is it to add the vent channels by putting 2x4s above the XPS?"

    A. I would only consider that type of venting if you your climate gets lots of snow during the winter, and you are worried about ice dams.

    .

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