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Dormers penetrating: yes or no?

canadianexpy | Posted in Green Building Techniques on

In design stage of building a 1 storey pretty good house, with hopes of going Net-Zero. The design has large front roof that has 4 dormers, we wanted the dormers to break-up the overall size of the roof.
I know Martin you will not like my roof design as it has dormers and an 8/12 pitch changing to a 4/12 pitch at the porch, but I need my wife happier than you!
Currently I was planning on the dormers being decorative only and not even penetrating the roof into the attic. The option is there to have the dormers penetrate into the rooms below as they do line up, The house has a full wrap around porch, so I was a little concerned that the front rooms might not have much natural light (or solar gains), but sealing and insulating dormers I know can be a pain. The front of the house faces southeast.
Location is 1 hour NE of Toronto, ON. (Osaca)
Do think there would be any advantage to having the dormers penetrate into the rooms below?


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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Do think there would be any advantage to having the dormers penetrate into the rooms below?"

    A. As you have noted, the advantage (if any) is an aesthetic advantage, not a performance advantage. Like all aesthetic issues, you will have differing opinions on whether creating a cathedral ceiling with high dormers is an improvement or an abomination. There is no disputing taste.

    If included within the thermal envelope of the building, the four windows will provide additional solar heat gain on sunny days, and increased heat loss on cloudy days and at nights.

    Including the four dormers within your home's thermal envelope will be significantly more expensive than building dummy dormers that don't do anything. Creating a well-insulated envelope (meeting code-minimum R-values, at least) and detailing the air barrier to be continuous will be a challenge.

    -- Martin Holladay

  2. Expert Member
    Dana Dorsett | | #2

    If the dormers primarily decorative, over insulated attic space, the last thing you want is have them open to a room below, complicating the air sealing, and amplifying the potential for heat leaks capable of starting ice dam issues.

    If they're basically roof ornaments, keeping the plane of the roof deck & it's WRB continuous is the right thing, with the ventilation of the dormer features separate from that of the attic as a whole.

    The depth of the porch roof overhang can be analyzed to determine how much wintertime solar gain you would receive, but you need not worry about daylighting given the number of windows in the picture. Direct sun through the windows introduces glare, reducing the efficacy of that light.. (By contrast, north facing windows are essentially glare free and shadow free, providing better visual acuity even at lower ambient light levels.)

  3. STEPHEN SHEEHY | | #3

    Dave- If I understand correctly, the attic space will never be livable space, so any light from real dormers is pointless unless you open the ceiling to the rooms below. Frankly, since you asked, I'll opine that fake dormers are a design abomination.

    Even if they are fake, yyou'll still need to attach them to the building, flash, roof, etc. You won't be able to easily vent or insulate the roof. And by putting on the dormers, you pretty much eliminate the possibility of solar.

    Your drawing exaggerates the roof pitch, so it looks almost vertical. Here's a photo of my roof, also with 8/12 pitch on the main roof. I don't think it's hideous, but I'd at least show it to your wife.

  4. canadianexpy | | #4

    Thanks you very much for your answers Martin and Dana I always respect the answer you two give.

    So I will stay on course with leaving them decorative and not penetrating the roof deck at all. Makes things easier, same reason I didn't want cathedral ceilings, keep it simple. At least my wife agrees with me on cathedral ceilings, she doesn't like the idea of trying to keep cobwebs out of the high corners.

  5. canadianexpy | | #5

    Thanks for the picture of your house Stephen very nice, but as I said the dormers stay (as per my wife) As I don't dislike your house, I do also like the dormers. They will not penetrate the roof deck. as for venting the plan is soffit to ridge vents and blown in cellulose for insulation.
    The solar is going on a separate storage building.

    Thanks for your comments !

  6. Expert Member
    Dana Dorsett | | #6

    It's probably worth putting down self healing membrane such as Ice & Water Shield under the dormer-ornaments, and including a ventilation/drain gap at the bottom of the down-slope wall to ensure that any bulk water that gets into the dormer leaves quickly. A ridge vent on top of the dormer would help keep it dry too.

    A down side of putting dormers on the broad SE facing pitch is that it complicates using that roof real estate for photon-farming. An up-side to making them an applique onto the the structural roof deck rather than integrated into the roof framing is that they can be easily removed or moved if there's ever a decision to go with rooftop PV. An 8:12 pitch is fairly optimal for PV for a SE oriented roof at your latitude, and steep enough that it can shed snow fairly quickly. It's likely to glide off in the first few days that temperatures break 0C after a snow storm, using my local experience as a guide. (Central Massachusetts is a degree or two cooler than Toronto on average for January-February, and somewhat snowier.)

  7. charlie_sullivan | | #7

    You could get Sistine Solar to make you some solar panels that look like dormers.

    Or that look like anything else you want.

  8. canadianexpy | | #8

    I was planning to have the whole roof in self healing membrane then covered in standing stem metal roof. The ridge vent on the dormer is an idea I will look into, also the front drainage.
    As for the solar, i'm also in the designing stage of a storage building roughly 48 X 90 with a 6/12 pitch facing due South.(pole barn or Eng slab) Plan on putting Max panels allowed in Ontario on the roof roughly 40 panels total 10 KW, which according to the one company should produce roughly 12.33MWh/year. with no winter clearing.


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