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Retrofitting roof deck insulation with large % glazed area

Nathan Moore | Posted in Green Building Techniques on

I have a client (“John”) interested in improving the insulation in his home west of Denver (dry, cold, 7300 feet), who may be replacing his roof (with insurance money) from hail damage. I am suggesting to him that now is the time to insulate his roof deck, at the very least over his vaulted ceilings. He has a fairly complicated roof line, and a tremendous amount of view windows.

How much rigid insulation can we put on his roof? Is there a practical limit of length of fasteners? Any other limits? I am partial to Polyiso for its R/inch, despite its $ cost (partly because of Alex Wilson’s article on greenhouse impacts of insulation – https://www.greenbuildingadvisor.com/blogs/dept/energy-solutions/avoiding-global-warming-impact-insulation). Is cellulose a potential solution for insulating a roof deck? Any better ideas?

Since much (40-50%?) of his wall area is double-pane clear windows, does the heat loss through there mean it’s not worth putting R50 in the roof? Where is that point of diminishing returns?

For the parts of his house with an attic (and existing blown fiberglass insulation) does it make sense to ignore that and continue the rigid insulation along the roofline?

Thank you,

Nathan

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Replies

  1. Riversong | | #1

    Since much (40-50%?) of his wall area is double-pane clear windows, does the heat loss through there mean it's not worth putting R50 in the roof?

    I think you answered your own question.

    There has to be some balance to the various elements of the thermal envelope. A significant increase in roof insulation would make sense only if it was part of a plan to upgrade all the other weak parts of the envelope, starting from the excessive and poor glazing.

    The more R-value in part of the envelope, the more the poorly-insulated sections drags down the average.

    For instance, say you have a 24' x24' room with R-19 vaulted ceiling, R-11 walls with 50% R-2 windows (ignoring the floor, which may be over heated space). That yields an effective R-value for that envelope of 5.4. Now upgrade that R-19 ceiling to R-50 and you've improved the overall R-value to 5.9. Is that a good return on investment?

  2. GBA Editor
    Martin Holladay | | #2

    Nathan,
    I agree with you -- reroofing is an excellent opportunity to upgrade ceiling insulation. If you want to know the effect of the ceiling insulation retrofit work on energy bills, use an energy modeling program.

    You can do 6 inches of polyiso if you want -- Alex Cheimets did:
    https://www.greenbuildingadvisor.com/homes/old-house-gets-superinsulation-retrofit
    ...although he said that driving the long screws was difficult.

    I think it's fine to address one component of the house at a time, even if there remains a window problem. Maybe in 6 or 10 years, it will be time to replace some of the windows or even reduce the total window area. But if the roof is being replaced now, I'd say -- upgrade the insulation.

  3. Expert Member
    Armando Cobo | | #3

    If John has a vented attic, installing rigid insulation on top of the roof decking won't do much for energy conservation either. You would need to change the attic to non-vented and spray foam insulation under the roof decking to work best.
    I agree with Martin, if John can afford to insulate now, specially with insurance money, go ahead and do it. There are many other ways to temper those windows until they can be replaced. John can go to a home improvement store and buy window-tinting material, its very inexpensive and good weekend project plus there are 30% of material tax credits up to $1,500 if you buy by Dec 31, 2010.
    Also, there are solar screens, shades, etc., etc, that can be remedy his problems until he changes windows.

  4. mike eliason | | #4

    um, wouldn't tinting the windows reduce the SHGC and make the house even colder in winter?

  5. mike eliason | | #5

    sorry, solar heat gain, not SHGC

  6. Doug McEvers | | #6

    Dry and cold west of Denver? If this area has a potential for heavy snow make sure your use full coverage ice and water shield before roofing. Even standing seam metal roofs back up in the valleys and leak on sunny winter days.

  7. Riversong | | #7

    make sure your use full coverage ice and water shield before roofing

    The proper method for preventing ice dams is roof venting, but this complicated roof line may not allow that (though you could use a venting nailbase insulation like Cool-Vent II).

    Ice & Water Shield is a bandaid "solution" for a poorly-designed non-vented roof, and it's as likely to cause problems as to solve them. If there is any impermeable layer (such as closed-cell foam) beneath the roof deck and an impermeable membrane on top, the risk of decking rot increases exponentially, particularly if it's OSB.

  8. Nathan Moore | | #8

    All,

    Thank you for your thoughts. We'll do some more energy modeling and see what his insurance will cover.

  9. Doug McEvers | | #9

    Nathan,
    http://www.buildingscience.com/documents/profiles/etw-aspen-profile/

    You may want to read what BSC has to say about mountain roofs, they recommend a continuous underlayment.

  10. Riversong | | #10

    BSC, believe it or not, is not gospel.

    They may recommend full membrane roof coverage, but they also recommend the "perfect wall", which is the equivalent of a hermetically-sealed house. And that's a manifestation of either insanity or a complete ignorance of the requisites for life on earth.

    In the hostile environment of out space, it's necessary to contain human life within a sealed, technologically-controlled capsule. On earth, where human life co-evolved with the biosphere, it's necessary to remain in constant relationship with that environment in order to maintain homeostasis, or what we call health.

    To that end, all human life containers, which we call shelter, must be composed of semi-permeable membranes which only conditionally separate our bodies from the rest of the environment. In other words, human shelter must be breatheable and any shelter made from organic materials, such as wood, must also be able to breathe or it will succumb to decay and dissolution.

  11. GBA Editor
    Martin Holladay | | #11

    Robert,
    Every now and then, Robert, your tendency to engage in exaggeration reaches a point where I feel the need to respond.

    I certainly disagree with your statement that the recommendations posted on the Building Science Corporation (BSC) Web site amount to "a manifestation of either insanity or a complete ignorance of the requisites for life on earth."

    Homes built according to recommendations from the BSC are not "hermetically sealed." They have windows and doors which can be opened. The also have mechanical ventilation systems -- just like the houses you build.

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