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Building energy efficient on a budget Zone 4

user-5935867 | Posted in Green Building Techniques on

My wife and I are planning to build a house and we would like some advice, suggestions, caveats, etc. We want it to be as energy efficient as possible within our budget. The house will be built in central Kentucky, zone 4. We tried to keep it as simple as possible. It will be a modern barn style. Four walls, four corners, straight gable metal roof, vertical board and batten siding, no dormers or bumpouts. The long side will face almost due south so we can take advantage of passive solar. The first floor is 1600 sq. ft. The second floor has only 2 bedrooms and a bath and is 750 sq. ft. The rest of the space is open to the first floor.

The foundation will be a 4″ slab with 2″ rigid foam underneath and around the perimeter.

The walls will be 2×6 studs on 16″ centers. 1″ rigid foam will be attached to the outside of the studs as sheathing. Then house wrap (rain screen combo?) and then the vertical siding. The walls will be reinforced using diagonal metal t-straps. The wall cavity will be filled with mineral wool. Drywall on the inside. The foam plus batts should give R-value of R-28 and address thermal bridging. I would like to increase the R- value by spraying 2″ foam then mineral wool but that would depend on the expense.

The attic will be conditioned to be used as storage and for mechanicals. Can I treat the roof in a similar manner to the walls? Can I attach 1″ rigid foam to the outside of the rafters, topped with a layer of plywood? Then 15# asphalt, 1x slats then the metal roof. I have seen the 1x’s run down the roof, across the roof, and diagonally. Does it matter? Not sure yet what insulation to fill the cavity with. If flash-and-batt the walls then may do the same here.

The windows will be casement and fixed. I have read that the U factor and SHGC ratings should be different for north and south facing windows. Is this as big a deal in Ky. as in some other climates? If so, how large a difference should they have?

Any advice would be greatly appreciated.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Kevin,
    Q. "Can I treat the roof in a similar manner to the walls? Can I attach 1" rigid foam to the outside of the rafters, topped with a layer of plywood?"

    A. No. In your climate zone, this type of insulated unvented roof assembly needs a minimum of R-15 in the rigid foam layer (and a minimum of R-49 of total roof insulation). One inch of rigid foam won't give you R-15, so your suggested roof assembly won't work.

    Here are links to some articles that will explain what you need to know about this topic:

    Calculating the Minimum Thickness of Rigid Foam Sheathing

    How to Build an Insulated Cathedral Ceiling

    How to Install Rigid Foam On Top of Roof Sheathing

    Combining Exterior Rigid Foam With Fluffy Insulation

  2. GBA Editor
    Martin Holladay | | #2

    Kevin,
    Concerning your wall plans: you didn't really explain what you will be using for an air barrier. Ideally, your stud cavities (which you plan to fill with mineral wool insulation) will have an air barrier on the exterior side of the studs as well as on the interior side of the studs. In theory, your exterior rigid foam can act as your exterior air barrier -- but only if the seams are carefully taped and penetrations are sealed.

    Similarly, you can use the Airtight Drywall Approach if you want to use the drywall as your interior air barrier.

    I'm pointing this out to emphasize the importance of air sealing if you hope to have a high-performance house.

  3. GBA Editor
    Martin Holladay | | #3

    Kevin,
    If you are interested in discovering how changes in window specifications will affect your home's energy budget, you'll need to input your home's specifications into an energy modeling program.

    For more information on glazing specs, see All About Glazing Options.

  4. user-5935867 | | #4

    Martin,
    Thanks for your replies. I had visited most of the links you mentioned but did revisit them. The R-15 minimum for the roof is what is required on top of the sheathing to keep the sheathing warm in winter. I am not proposing putting any on top of the sheathing. The 1" layer would be under the sheathing. Primarily to break the thermal bridge. Wouldn't the furring strips under the metal roofing make it a vented roof?

  5. Dana1 | | #5

    Venting over the roof cladding can sometimes work in zone 4 if the underlayement is vapor permeable but it doesn't meet code. It's a very difference situation when the venting is under the roof deck. With venting over the roof deck moisture has to cross the vapor retarder (the roof deck itself), before venting to the outdoors. The roof deck itself spends many hours below the dew point of the interior air, but not nearly as much as in zone 5 or higher. With venting under the roof deck the interior moisture drives hit the vent space first, which is being continuously purged by drier outdoor air.

    If you want a cheap but relevant thermal break on the rafters, use 1.5-2" of polyiso cut 2-3" wide on the underside of the rafters, and long-screw the ceiling gypsum in place, but vent the exterior. You'll have to fill in the 1.5"-2" of space with fiber insulation, which can be done with split batts trimmed to width.

    Putting foam between rafters & sheathing or studs & sheathing requires an engineering analysis, since the sheathing is structural. In your area using 1.5" Huber ZIP-R sheathing and 2x6 w/R23 rock wool for the walls and is probably going to be simpler to build than what' you've proposed.

  6. GBA Editor
    Martin Holladay | | #6

    Kevin,
    Dana is correct: You can't put rigid foam between the top of your rafters and your roof sheathing. Roof sheathing needs to make a structural connection to the rafters, and the rigid foam is squishy and interferes with this connection.

    He is also correct about the definition of a vented roof. Your plan to include an air space between the underside of the roofing and the roofing underlayment is fine, as far as it goes, because the air space helps keep the sheathing dry (as long as you remember to use a somewhat vapor-permeable roofing underlayment like asphalt felt). That said, it isn't a real substitute for a ventilation channel on the underside of the roof sheathing.

  7. user-5935867 | | #7

    Thanks to both of you. That clarified things for me. The ZIP-R sounds like the best way for the walls. It will also alleviate some concerns I had about getting the foam, sheathing, and house wrap each taped properly. Seemed like a lot of opportunity to have leaks. I know how much trouble I have just taping a cardboard box to mail. I will check pricing but would assume that material and installation would still be at least no more that materials and installation of foam, OSB and wrap separately.
    Dana, thanks also for the tip about cutting the polyiso into strips. Any particular reason you mentioned polyiso in particular?
    I am still working through other parts of it so will surely be asking more soon.

  8. Expert Member
    Dana Dorsett | | #8

    "Dana, thanks also for the tip about cutting the polyiso into strips. Any particular reason you mentioned polyiso in particular?"

    Yes.

    Polyiso has a higher R/inch, a favorable R/$, and a lower environmental impact than XPS (due to blowing agent differences), and in the event of a fire it chars in place instead of melting and flowing the way polystyrene insulation (EPS & XPS) do. Spray polyurethane foam behaves similarly to polyiso in a fire, independent of installed density.

    When the average temperature through the polyiso layer is between 55-70F as it would be most of the time when used as interior side rafter edge strips, it will outperform it's labeled R value.

    Foil facers make it somewhat easier to mark and cut into strips than unfaced EPS or XPS too. A handy tool for cutting 1.5-2" foil faced polyiso for rafter edge strips is to sharpen & hone the side of a 4" wide steel putty knife.

  9. user-5935867 | | #9

    OK, so based on the advice of both of you, I have changed everything. Please let me know what you think.
    The walls will be 2x6 studs with R-23 mineral wool in the cavity. The outside will be sheathed with 1 1/2" ZIP-R (R-6.6) and wrapped by house wrap. I read another discussion that the consensus was that the extra layer was beneficial especially for the window flashing. Is Huber woods OK with punching nail holes in their protective coating? Does this affect the performance of the panel? I also would like to place a strip of house wrap between the slab and the sill plate (along with the sill seal) and fold it up onto the ZIP sheathing. I saw this elsewhere on the site and it seemed like an inexpensive way to add a little more protection.
    For the roof I am looking at metal roofing attached to furring strips attached to ZIP roofing panel fastened to the rafters. The rafter cavities would have 5" CC spray foam (R-35) and 5.5" mineral wool (R-23). Strips of 1 1/2" polyiso fastened to the underside of the rafters would provide a thermal break. Then drywall.
    I had planned to stay away from CC spray foam because of the expense but I think it will save in other ways:
    *I should be able to use smaller rafters, By adding the polyiso to the bottom of the rafters I have room for 5" of CC spray foam 5.5" mineral wool for a total of R-58 (I realize that is not the whole-roof R-value)
    *Without venting the roof I could eliminate the overhangs from the framing and tape the ZIP-R on the walls to the ZIP on the roof, forming a sealed envelope. I could then add the overhangs. The photo I saw elsewhere on the site showed 2x4's up the roof when adding the overhangs. I need to do this anyway to attach the metal roofing.
    *About half of the roof will be a cathedral ceiling. While there seems to be no perfect way to insulate a cathedral ceiling, this appears to be one of the preferred methods.
    Martin, as far as the air-tight drywall, I am still researching. It seems from other discussions that when done correctly, it is great, but sheetrockers are reluctant to follow ever step. I will use flanged electrical boxes on the exterior walls and seal where the wires enter the box. Since the walls, ceiling, and corners will all be taped and sealed, would it be enough to foam/seal between the bottom of the drywall and the floor before the tile is laid (tile floors throughout)?
    Please let me know if I am on the right track this time.

  10. Dana1 | | #10

    Closed cell spray foam between rafters wastes the performance of the foam due to the thermal bridging of the rafters. At 5" it would have to be installed in three lifts of no more than 2" each with a cooling off period between, which is both a quality and fire-hazard-while-curing issue. Furthermore, the high global warming potential of the HFC245fa blowing agent (for almost all vendors except LaPolla and Aloha Energy) makes it even worse.

    For dew point control at the roof deck in zone 4A takes a minimum of 30% of the total R to be on the exterior. If you went with 2x8 raftered R30 batt solution under a structural OSB roof deck (not necessarily Huber) with added a 3.5" polyiso nailbase panel on top of that the deck, with a standard underlayment etc for the metal roof you'd beat the U0.026 code-max for ceilings with some margin and have just enough dew point control for the sheathing. Follow the nailbase panel manufacturers' specifications for faster types & spacing. It would be a heluva lot cheaper and higher performance than 5" of closed cell polyurethane. It would also be greener, since it would be only about half the polymer, and it would be blown with pentane which is extremely benign compared to HFC245fa. (About 7x CO2 rather than about 1000X CO2 global warming potential.)

    eg:

    https://www.hunterpanels.com/product-documents/hpanels/speciality-products/108-h-shield-nb/file

    https://www.hunterpanels.com/product-documents/hpanels/speciality-products/104-h-shield-fastening/file

    https://www.hunterpanels.com/product-documents/hpanels/speciality-products/106-h-shield-nb-fastener-1/file

    https://www.hunterpanels.com/product-documents/hpanels/speciality-products/107-h-shield-nb-fastener-2/file

    http://www.atlasroofing.com/download.php?uid=822

    http://www.atlasroofing.com/download.php?uid=842

    Nailbase panels are similar to ZIP-R, but are not designed be used as the structural sheathing. They work well under metal roofing and most other roofing types.

  11. GBA Editor
    Martin Holladay | | #11

    Kevin,
    Q. "Is Huber OK with punching nail holes in the protective coating [of Zip-R wall sheathing]?"

    A. Yes. After all, every house has siding, and siding needs to be fastened to the sheathing (or to the studs through the sheathing).

    Q. "Does this affect the performance of the panel?"

    A. No.

    On another issue -- whether or not you want to install closed-cell spray foam between your rafters -- Dana Dorsett is correct in his advice. Installing rigid foam above the roof sheathing is the preferred way to go. You don't get much of a boost in R-value from spray foam between the rafters, because of the thermal bridging through the rafters, and the spray foam is expensive.

    Nailbase panels can be used, as Dana suggests, or you can simply install one or more layers of ordinary rigid foam above the roof sheathing, followed by a layer of plywood, OSB, or purlins.

    Once again, I urge you to read these three articles:

    How to Build an Insulated Cathedral Ceiling

    How to Install Rigid Foam On Top of Roof Sheathing

    Combining Exterior Rigid Foam With Fluffy Insulation

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