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Community and Q&A

Vented/unvented attic?

trynadoitbetter99 | Posted in Energy Efficiency and Durability on

Am building our SW facing energy-efficient home (1850sf+/-) shortly: Reddiform ICF for all outer walls from footings to roof, stuccoed, 3″ Dow SM under basement slab, all joints taped, infloor hot water radiant, engineered 20″ wood floorjoists which are topchord bearing on LVL ledgers for clearspan basement, with extra horizontal strongbacks to make a truly monocoque floor system, stapleup radiant tubing, 7.5/12 roof trusses 16″ o/c with 4′ overhang, single attached unheated garage, hardcoated windows = 6.4% of wall area, 2 freestanding see-through gas fireplaces (supplementary heat & esthetics, 1 per floor). Am also considering Solatubes ending in ceiling lightboxes to provide indirect mainfloor daylight. All windows are inset are much as possible, with window openings chamfered about 40 degrees. All plywood sheating is glued & screwed, and we will be using an HRV with ECM motor.

Here, in Ontario, Canada, code requires vented attic insulated to R31, or if not vented, R50 under roof. This house will be very tight, and I do not want a vented attic; would rather have 5/8″ plywood sheathing totally covered by waterproofing, and, under the steel roof with ridgevent, I would like to install vertical 1 x 2 strapping, then horizontal 1 x 3 roof strapping, to have sufficient venting. I’m wondering how best to insulate under/over the roof: 3′ or better DOW SM/Roofmate foamboard,
sprayfoam, or mineral insulation? I’ve considered extending the outer housewalls upward by installing plywood kneewalls between each truss, and covering in sprayfoam, which will also seal any very small leaks we may miss.

Advise or criticism are both welcomed.

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  1. user-2890856 | | #1

    " stapleup radiant tubing "

    May I ask why you chose or are entertaining doing staple up as opposed to something conductive using lower water temps and achieving higher efficiencies from connected equipment ?
    Did someone design this for you or was this method discovered by other means ?

  2. GBA Editor
    Martin Holladay | | #2

    It will take a little time to translate.

    I guess "Dow SM" is a brand of extruded polystyrene (XPS) insulation.

    I'm not a fan of in-floor hydronic ("radiant") heat distribution, especially in well-insulated homes. For more information, see All About Radiant Floors.

    I'm not a fan of gas fireplaces; they aren't particularly energy-efficient.

    I'm not a fan of Solatubes (tubular skylights); they puncture the ceiling air barrier and thermal barrier.

    For information on insulating a sloped roof assembly, see How to Build an Insulated Cathedral Ceiling. Here in the U.S., you would want to aim for R-49, not R-31.

  3. user-2890856 | | #3

    All About Radiant Floors. While informative is based on an article from 2002 . The systems referenced are of unknown construction and design other than they were in floors . I would be more than willing to address this on a point by point basis with anyone whom wished to engage civilly and scientifically . I will state that I do not agree with the warm floor slogan and wish it would go away .

    Radiant heat does not require in floor installation either . The sci3ence is real and radiant walls work and radiant ceilings are Better . Radiant can also do sensible cooling and latent may be handled by DOAS , which should be installed in these types of homes anyway .
    Everyone remember that water is the predicate heat exchange device all other things are compared to . There is a reason for this .

    Here is an example . Mitsubishi has a new series , guess what the sensor monitors , Mean Radiant Temperatures at the windows and walls . No other technology is able to control the Mean Radiant Temperature in a structure as well as radiant heat .

    Too many things that can happen to heat pumps that can leave you cold and wishing , this winter proved that .

  4. GBA Editor
    Martin Holladay | | #4

    Feel free to post comments about the "All About Radiant Floors" article on that page.

    I certainly agree that hydronic heat distribution is a well-established method of heat distribution. In fact, I used to design hydronic systems (using fin-tube radiation) back in the 1970s, when I worked for a plumbing and heating wholesale house in St. Johnsbury, Vermont.

    Hydronic systems can use in-floor tubing, wall-mounted radiators, ceiling panels, or fin-tube radiation. Almost every building in Europe is heated with a hydronic system, and these systems work well.

    If you are designing a small, well-insulated green home in 2015, however, you will probably find that a heating system incorporating a boiler and hydronic distribution is overkill.

  5. trynadoitbetter99 | | #5


    It may be that I have inadvertently used a misnomer, but the mainfloor will be heated by radiant hot-water tubing, held up or fastened against the underside of the subfloor, and it will be shrouded by insulation underneath. If this does not fit your statement of "something conductive using lower water temps and achieving higher efficiencies from connected equipment", I am of open mind, and would certainly be interested in more detail. I decided on radiant because of 2 things: all the positive homeowner comments in regard to comfort and affordability, and because of health concerns, such as asthma. We are lucky in that we have both LNG, at the road, and propane available for our either/or boiler. I'm in my mid-70s, and an old-car guy, and have built my workshop with provision to underground hot water to the insulated radiant floor in that shop. I'm just an guy who has, for decades, foreseen enormous heating bills coming down the pike, and who has believed, for decades, in energy efficient buildings, built with ICF and totally leak-proof (and yes, you can do it if you take that extra small bit of time & care), and who is not wealthy by a long shot.


    You are correct: Dow SM" is a brand of extruded polystyrene insulation that is used, worldwide, mainly, I understand, to insulate large factory/warehouses with flat roofs. Roofers remove all the SM, no matter what the condition, and replace it with brand new material; the used SM is then sold for re-use. New, it is expensive, and I don't know what I would use if the second-hand material were not available.

    In regard to "not a fan of in-floor hydronic ("radiant") heat distribution", I don't know what else I would use: don't want radiators anywhere, nor anything electric ($$$$$$ here), nor ceiling or wall radiant, nor any kind of forced air heating system. Wouldn't touch ground source ($$$$$$$ to install, longtime, if ever, payback) or heat/air exchanger. What other choice is there, for us, with a super-insulated, super no-leak building? I cannot argue with the mandated heat loss design (which shorted me on a few rows of tubing, and it's a bad layout), which says I have a small need of supplementary heat; I really won't need them, they were always intended to be there, as stated, for supplementary heat & esthetics, and will see very low use. They are zero-clearance as far as the chimneys go, and I will do a painstaking job of insulating and leak-sealing, if I decide to use the Solatubes, which will give us lovely NE indirect daylighting year-round, without adding extra heat in the house year-round or running up the hydro bill.

    The R31 is for the mainfloor ceiling, the code here mandates R50 minimum for under-roof insulation.


    I'm hoping we will not require any other cooling than opening windows in nice weather; with the 4' roof overhang facing SW, we will get hardly any direct sunlight on the front of the house until mid-evening, but time will tell. Having spent a lot of time looking at our houseplans from every conceivable (to me) angle, I don't know (hence the latent) where any latent heat would come from, other than the sun: I don't know what "DOAS" are. I have all the radiant tubing in the lower areas, so any leaks will flow across the basement slab into the sump pump drain, locate one step down in the utility room. I have a very compact wall-mount Quietside DPW-099A, purchased some few years ago (we are behind schedule), works on gas or propane, & will heat or domestic water. Can't relate to the "heat pumps that can leave you cold and wishing", but we have a propane/gas fired backup power supply for the house, 'cause I'm to old to freeze. So's the wife, come to think of it.


    Since our system will also be supplying heat to the workshop when it's not supplying heat to our house, I really hope we are not in either an overkill or underkill situation. The nice thing is, if we do not have enough available supply to heat the workshop, it's just a matter of mounting a separate boiler out there.

    At this time, I might have to make an apology: I did a posting that didn't make it to the forum; tried again, no go. Later, I did another different posting, which showed up. Imagine my confusion when the first posting showed up. If I've contravened the rules, I'm sorry, but it won't happen again.

    Thank you, everyone.

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