Is a skillion roof with high windows less effective due to high ceiling heat loss?

Discussion in 'Designing, building, making and powering your life' started by dieter, May 19, 2015.

  1. dieter

    dieter New Member

    May 19, 2015
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    I do house plans and am just wondering whether these high ceiling skillion roofs with high north facing windows is really better, since it is more difficult to heat a room with a high ceiling. I understand the high windows could be double glazed with internal blinds, and you have the benefit of heating up an internal thermal mass wall, but are the benefits really worth it? Say compared to a flat ceiling say 2700 mm high with north facing windows on the north wall, heating perhaps just a slab on ground.
    Does anyone know any way to prove which is better in keeping the place warm in winter? Especially if the solar gain is not sufficient at times and one would need to add their own heating.

    High ceilings can be a bonus in Summer to keep the place cool, so perhaps someone knows how to include this in their calc's too?

    I'm assuming North windows being in the south hemisphere.
    Also, with a skillion roof, if the higher wall is on the north side and the roof slopes towards the south, it would be more difficult to install solar panels on the roof and get adequate sunlight in winter, correct?

    One more thing to consider in the calculations... if the majority of the radiant sun heat comes from the north, a south sloping roof would perhaps reflect heat easier than say a north sloping roof (if the south wall was the highest wall)
  2. S.O.P

    S.O.P Moderator

    Jul 8, 2011
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  3. 9anda1f

    9anda1f Administrator Staff Member

    Jul 10, 2006
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    E Washington, USA
    Semi-Arid Shrub Steppe (BsK)
    Hi Dieter,
    Although winter heating of a high-ceiling (i.e., large volume) home by conventional "heat the air" means might be problematical, if you are willing to look into alternative solutions the high-ceiling may be an advantage.

    As warm air in the summer will rise to your skillion peak, it can be effectively used to heat the earth beneath your house as a form of thermal battery using little more than a system of buried piping leading to near the ceiling's peak and a small PV fan or two. In the winter, that stored heat will be released as radiant heat which is effective regardless of volume.

    Some more info here:

    Note the air tubes at the right to draw warm ceiling air down into the earth in this greenhouse photo:
  4. Brian Knight

    Brian Knight Junior Member

    Jan 23, 2015
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    In general, more volume of space means more energy to heat and cool it. The details easily effect this logic but I think in most cases, a low ceiling is more efficient. While radiant floors may be more comfortable with high ceilings, it will probably take more energy to maintain a temperature setpoint than if the ceiling were modestly lower. In extreme weather, a smaller volume of air is easier to keep at a certain temperature and humidity thus probably more comfortable too.

    Do high ceilings keep a space cooler in the Summer? Even with passive cooling Iam not so sure and it depends on the details. The stack effect can be utilized to draft air out of high windows pulling in air from lower windows. This can work great but is limiting regarding climate, time of day and humidity. In our summer climate, even if the air outdoors is a bit cooler, it can be less comfortable and more problematic to ventilate with it when it has a higher humidity than what's inside. That said, I use this "nightime flushing" technique for keeping cooler in the summer and avoiding AC usage. I do this at the expense of some of my furnishings and finishes as the increased humidity can cause mold and mildew growth.

    If ventilating with high windows is the design goal, I like to question if the same thing can be achieved with windows on the same level. In my experiences, opening windows on the same level, being sure to keep bedroom/interior doors open, will achieve close to equal performance as ventilating with high windows or cupolas. That said, I have a client I suggested nixing the cupola for, but we kept it mainly for aesthetics. He seems to think the cupola makes a big difference in ventilation benefits, albeit they are at a higher elevation with lower night time lows.

    There are other ways that skillion roofs or high walls can increase the heating and cooling loads besides the increased added volume. More wall surface area means, more sunlight exposure and unwanted gains. The windows are the real concern though. Eastern and especially western facing windows can add loads of unwanted heat. Sun-facing windows intended for wintertime benefits should feature well designed overhangs to block it out in the warmer months. The stack effect hurts you in the wintertime when trying to contain conditioned air (or summertime if using AC). Those high, operable windows will surely be causing more unwanted air exfiltration, probably the biggest variable and concern when trying to keep a home or space warm.

    Your concern of the roof facing away from the sun for PV is real. It takes more money or effort to build the frames to compensate for sloping in the wrong direction and usually looks pretty silly.

    That's an interesting link Bill. I think most who have tried these methods for cooling their homes have ended in failure, hence the academic communities suggesting it wont work. I think most would agree it can work, but the main problems are usually the humidity and soil gas concerns introduced by underground air ducts and any mechanical circulation energy which can be substantial. Greenhouses are a bit different of course. Would love to see more information on the system but the site is lacking there and the act of charging for information doesnt do much to dissuade my skepticism. Do you or anyone else out there have more to offer about this citrus in the snow project?
  5. drendrewolf

    drendrewolf Junior Member

    Feb 18, 2015
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    Coming from the point of view of experience, I think there are a lot of factors that have to be taken into account when choosing this kind of roof and complimenting heating and cooling systems. I inherited control of my family farm her in Arizona (northern hemisphere) a few years ago and on it we have a massive workshop with a monopitch (skillion) roof. It actually works incredibly well for us, while other buildings in the area with similar roofs are major pains to their owners. The two biggest things that make the difference with our workshop are a) the exact orientation of the building, and b) placement of windows and vents.

    Orientation: The orientation of such a building has a huge effect on the skillion roof's ability to utilize the stack effect for cooling in the summer. Most people around here chose their building orientation according to the sun's zenith. We didn't. The generation that built our workshop chose it's orientation to maximize rainwater collection during our summer monsoons. So, prevailing summer winds were the deciding factor, not the sun. Our workshop is, effectively, wind-powered for heating and cooling as a result.

    Location of windows and vents: Since our building is oriented to face head-on into the strongest winds of the year (taller wall facing mostly south and slightly west) the main vents were placed at the apex of the roof on the east and west walls. One's first thought in using wind to cool a space is to create a "cross-breeze", but we've found that isn't very efficient with a mono-pitch roof. My parents had actually experimented with different positions of vents on smaller sheds that utilized the same kind of roof before committing to the design of the workshop, which is 3-stories tall with a 2,400sf footprint. They placed the primary vents on the sides of the building to maximize external draw. The winds that push the rain into the industrial gutters on the north side of the building also pull the rising air out of the building. The cool-air intake is on the south wall bottom story, shaded by deciduous trees in summer. Actually, it's a glasshouse that runs the entire length of the building. Large vents on the east and west sides of the glasshouse have simple air-scoops attached to them. This uses the power of the wind to force air into the glasshouse, through the indoor food forest, then through a series of vents that passively push the cooled air through the building.

    Another thing that was taken into consideration was how tall the building was compared to the average height of the deciduous trees planted on the south (sun-facing) side. There are no windows in the south wall about the "shadow line". The third story of the workshop has no south-facing windows. All of it's windows are on the west and east facing walls to keep them from adding to the heat load on that level. To be honest, that's not enough light, and we plan to cut in more windows on the north side, and maybe utilize flexible sun tubes at some point. There is much to do with this building to really maximize it's energy efficiency and utility, but it's heating/cooling and water collection capabilities are awesome.

    One thing I would like to say though, is that many of the elements put into our workshop may not be feasible for a smaller structure such as a house. There, aesthetics have to come into play, both on the inside and outside of the structure. Also, I think ours only works as well as it does because of it's height.The sheer size of the building puts it 35 feet into the wind, which is what lets it take advantage of the power behind it.

    Anyway, I have no idea if any of that helps. But, I thought I'd report my personal experience for posterity's sake.

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