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Insulated Roof Questions

patrickosu | Posted in General Questions on

Hello All
I have a 1910 house, 1-3/4 story in Denver, CO (zone 5b) with an unfinished attic. The insulation on the attic floor needs to be increased. My roof is getting old and will need to be replaced soon as well. I’ve been reading about adding rigid insulation to the roof, which sounds appealing since the attic doesn’t cover the entire upstairs, but have a few questions:

1) How would the cost of adding insulation on the rook compare to having it blown into the sloped part of the wall and adding insulation on the attic floor? I just read Joe Lstiburek’s article “Over-roofing: Don’t Do Stupid Things” which showed that it is more than just throwing rigid insulation on the roof.

2) The house is brick on the 1st story and stucco on the second. The stucco needs repair/replacing. Adding rigid foam to the exterior would create an overhang where the stucco meets the brick. Would spray foam on the interior work better?

3) I would like to add central air to the house, but lack return air ducts on the 2nd floor. I may add these in when I remodel the kitchen. Would having an insulated attic make a difference for return ducts?


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  1. Expert Member
    Dana Dorsett | | #1

    In order:

    1> Blowing insulation between the rafters would be super cheap relative to putting foam above the roof deck, but in zone 5a that would rot the roof deck, and it would still have the thermal-bridging of the rafters. You could make like Joe L. and spray 2lb foam in there without risking the roof deck, but it would usually be several times more expensive than putting fiber insulation there. In zone 5B you'd need at least R24 above the roof deck to be able to safely use fiber between 2x6 rafters, unvented. (Joe L. states in the article that had he put the foam over the roof deck first he could have used much cheaper cellulose between the rafters, and there never would have been the frost-striping on the roof from the thermal bridging of the rafters.

    2> Spray foam on the interior of the sheathing doesn't thermally break the studs, and doesn't improve the whole-wall-R performance by very much at all. It also impedes the ability of the sheathing to dry toward the interior. Adding 1" of XPS between the stucco & sheathing improves the whole-wall R by about 50% (cutting heat loss by 1/3) with 2x framing, and isn't a huge overhang. Adding some trim & drip edge but leaving the cavity between the stucco & foam would be a GOOD thing, allowing the stucco to dry rapidly, and limiting moisture drive from the stucco though the foam to the sheathing. (Many stucco guys have problems dealing with foam thicker than an inch too. ) I'm sure you can work this out.

    3> If you have a sealed & insulated attic with the insulation at the roof deck running ducts in that attic (either supply or return) is just fine, since it's fully within the thermal & pressure boundary of the house. Any ducts that go outside of the insulation & pressure boundary (as in an insulated attic floor) increases the heating & cooling load on the mechanical systems and creates potential infiltration leakage and air-handler driven pressure difference to drive that leakage, whether it's a return duct or a supply duct. With return ducts there's no/low risk of condensation on uninsulated ducts the way there would be with AC supply ducts. Building in return paths via door grilles/transom grills or using partition-wall cavities as jump-ducts that are completely within the thermal & pressure boundary of the house is preferable.

    But also consider, since the cooling loads of upper floor don't track those of lower floors very well, a ductless mini-split solution may be preferable than ducted AC from both a comfort & efficiency point of view. Zoning ducted systems inevitably leads to room-to-room pressure differences infiltration driven by the air handler unless you have very free return paths within the house.

  2. wjrobinson | | #2

    Budget is first. Say, $10,000 for split air and some air sealing. Say $60,000 for a magazine picture beautiful kitchen.

    Or be happy, done.

    Or sell and build small perfect green home.

    Dana posted well for you. Cost of his project plus kitchen would equal building a new home.

    Patrick, great looking home you have posted. Call contractors, you will learn much from meetings to implement a sound plan that suits you and your home.

  3. patrickosu | | #3

    Thanks for the great input!
    Dana -
    I hadn't heard that it might rot the roof deck. Wouldn't putting a gap between the rigid insulation and roof deck would prevent that?

    When you mention putting in blown in insulation, are you referring to both in the attic as well as the sloped part of the roof?

    AJ -
    I considered mini-split but thought I would need several units - 2 up, 2-3 down? Or would 2 up cool the whole house?


  4. user-945061 | | #4

    If the sloped and enclosed portion of the rafter is no greater than 6', you should be able to slide a ventilation baffle (use accuvent's cathedral extension baffle) into the rafter cavity from above, below, or both. This would allow you to insulate a well ventilated rafter cavity with high-density cellulose. If there are kneewalls continue the accuvents in the rafters behind them (lap them to drain potential roof leaks to the soffit), enclose them in 2" Thermax polyiso, and dense-pack this cavity with cellulose also. Make certain to connect the rim joist behind the kneewalls to the Thermax - we usually pull the sub-floor and apply 2" cc spf. While not quite at the performance level as compact, unvented roof retrofit, this assembly is usually a significant improvement.

    Stucco - use EIFS. In my area I can get EPS scored for drainage up to 5" thick. Apply it over a liquid water-resistive barrier for bomb-proof results.

    Like Dana, I like mini-splits. The problem with mini-splits isn't capacity, it's distribution. But serious envelope improvements will make distribution much less of a problem. My experience with many mechanical contractors is that they often bemoan the lack of distribution with minisplits, but then install duct systems with far worse distribution problems which also permanently compromise the thermal integrity of the building.

  5. Expert Member
    Dana Dorsett | | #5

    Putting a gap between the insulation and the roof deck would be necessary but not sufficient on it's own to prevent roof-rot- it has to be vented at both ends to the outdoors, and the gap needs to be at a minimum 1.5" (to meet code), and on lower-pitched roofs than 4:12 you'd be well-advised to leave more. It looks like you have plenty of pitch to go with a minimal 1.5", but if there is blocking at the kneewalls that prevents good flow from soffit-to-ridge there could still be issues.

    Whether or not you can get adequate cooling for the whole house with just two ductless heads depends on the layout, your R-values & U-values, and the solar gains of the south & west facing glass. They work great in high-R houses, not so much in barely insulated antiques with wavy-glass single panes. Without knowing the orientation of the roof pitch & windows it's hard to hazard a guess, but there are plenty of homes in zone 5 that can be cooled adequately with one head per floor, but also many where that wouldn't be at all satisfactory.

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