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Insulating slider

user-6023872 | Posted in Energy Efficiency and Durability on

Major retro; Midcoast Maine. 11 ft cathedral ceiling with a Fujitsu heat pump near the peak and directly above south facing sliding patio door. A ceiling fan is located in close proximity. Portion of the roof is foamed; other is original; 6″ FG.. Experiencing stratification; 4-5 degrees with outside temps of 40. With ceiling fan on down blowing, 2 degrees; with han in updraft, 2-3 degrees. As temps get lower, stratification seems to increase. I sit directly next to slider. I propose using refectix or better to make a modern “window quilt” for the slider, at least for the fixed panel.. I am looking for suggestions as to how to proceed. Thank You.

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  1. user-2310254 | | #1

    If you are renovating, it would make sense to improve the insulation in your cathedral ceiling. See this article for more info:

  2. user-6023872 | | #2

    Construction is completed; Old portion of building was mostly untouched for cost considerations: FG 6" in roof, New roof is foamed. Thanks

  3. GBA Editor
    Martin Holladay | | #3

    First of all, it certainly sounds as if the indoor head of your ductless minisplit is mounted too high, especially since you live in Maine, where heating matters more than cooling. Relocating the minisplit head to a new location, as low as possible, is one part of a possible solution. You can either mount the minisplit head at 6 feet or 7 feet above the floor -- or you can even mount the head near the floor (see photo below).

    Here are a few hints about occupant discomfort and temperature stratification:

    1. If you sit in a chair near the sliding doors, and you feel cold, one solution is to increase the thermostat setting until you are comfortable. The slider cools you off because (a) it leaks air, and (b) it represents a large surface with a low R-value, meaning that the glass is cold and your warm skin is radiating heat to the glass. Turning up the thermostat will make you warmer. If you are feeling cold when the thermostat is set at 72 degrees (near the ceiling), set the thermostat to 76 degrees.

    2. If you want to reduce temperature stratification, you need to tackle two problems: air leakage and low R-values. Your slider is one source of air leakage. You need to reduce air leakage through the door by replacing the weatherstripping, or replacing the door with a better door (perhaps French doors), or, as you suggest, installing a big window quilt.

    You also need to look for air leaks near your ceiling -- perhaps at electrical boxes or recessed can lights -- and fix these problems as best you can.

    The section of your ceiling that is insulated with only 6 inches of fiberglass is clearly contributing to the stratification problem. You'll have to learn to live with the problems associated with this thin insulation, or else save up the money to fix it.


  4. Expert Member
    Dana Dorsett | | #4

    There will always be some offset between the temperature sensed at a high-mounted mini-split head and the average room temperature. When it's 9'+ off the floor that effect is exaggerated. There is some amount of adjustment to that senseed temperature inside Fujitsu heads that can be tweaked (in the electronics, but a Fujistu certified tech) to have a more reasonable correlation between the sensed temperature and the room tempeature 3-5' off the floor, but to fix it completely would require a wired wall-thermostat (a proprietary custom item from the manufacturer.)

    A retrofit low-E window film applied to the interior of the slider will increase comfort without obscuring the view, or eliminating daylighting (the way aluminized bubble pack would) but will also increase the amount of window condensation you experience. The window film can be applied to both the fixed & movable panes. eg:

    This is not a particular product endorsement- there are many others.

  5. user-6023872 | | #5

    Thank you Martin;Height to bottom of Fujitsu is 8' 6". Should the ceiling fan be run in the up or down mode? Potter

  6. GBA Editor
    Martin Holladay | | #6

    My recommendation is that ceiling fans shouldn't be operated during the winter. (Moving air usually makes people feel cooler, not warmer.) For more information on this issue, see Using Ceiling Fans To Keep Cool Without AC.

  7. dinnerbellmel | | #7

    I would get a window insulator kit (the clear plastic sheet with shrink film) before putting in a "window quilt". A few companies, like 3M, makes a kit for sliders for around $16 or so. It will help with drafts and you won't feel the cold from the glass which is probably a problem if you sit next to it. Plus you will still be able to see outside. The downside is you won't be able to open the slider so you'll need to use another door for getting in or out.

  8. Expert Member
    Dana Dorsett | | #8

    Run the fan when you are actually in the room, and want to feel cooler. Otherwise leave it off.

    Leaving ceiling fans on 24/7 in summer can use more power than keeping the temperature cool with a mini-split, and the power used increases the actual room temperature. But when the cooling loads are low, operating a ceiling fan with an occupancy/vacancy sensor switch can keep you comfortable at a higher room temp, and you can leave the mini-split off. A 100 watt ceiling fan emits about as much heat into the room as a sweaty adult human, and offers zero comfort for humans who aren't even there. A mini-split idling along at low speed uses about as much power as the fan, but is emitting it's heat outdoors, not into the room it's actively cooling. And unlike the fan, it actively removes moisture from the air, which is also more comfortable. When there is any significant cooling load, run the mini-split, not the fan, and don't run them at the same time.

    At 8' 6" off the deck there will be a substantial offset. You can have the tech tweak the calibration to correct for it, but it's never perfect, and it WILL vary it's offset with the season. If rock-stable room temperatures are a requirement, the wall thermostat is really your only option, but most people can get along fine without it, even with a high-mount head.

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