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Thermal Comfort of Upgraded Windows

point78 | Posted in General Questions on

Finalizing the plans on a new home build for zone7, with 11K hdd.

(double wall dense pack cellulose, r40 walls, r80 roof)

Currently I have planned Alpen ZR6 windows.  .17u

I’ve never lived in a house with good windows, and its cold where I live in the rocky mountains at 9700′.

My question is how is it going to actually feel (inner glass temp, etc) on a cold winter day, say 15F-0F standing next to a 8′ tall patio door with. .17 u,  and if there is a noticeable comfort difference if I upgraded to a ZR9 listed at .12 u.

I’m not really worried about the heat load calculation or energy cost(it was already good with the zr6), mostly personal comfort.  I have several big windows I’d like to be able to sit next to comfortably.


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  1. Jon_R | | #1

    Here you go:

    Don't miss the "Room-side Low-E Coating" option.

    1. Deleted | | #7


    2. point78 | | #8


  2. Expert Member
    Michael Maines | | #2

    Brian, anecdotally, I was skeptical of Passive House and triple glazed windows--interested but not a koolaid-drinker--until I attended a tour of the Ecocor house pictured here: It was about 10°F outside and during a 30-minute presentation, I leaned against the large, center window on the first floor, an Intus that was probably around U-0.15. Even leaning against the glass, without a jacket, I was not cold or even a bit uncomfortable. That experience convinced me of the comfort that triple glazing provides. I don't know how perceptible the difference between U-0.17 and U-0.12 would be--probably not noticeable for a small window, but for a large patio door I think you would appreciate the lower U-value.

    1. point78 | | #9

      good to hear, thanks for the info

  3. onslow | | #3


    For grins and giggles I tried the (now much more accessible) payette tool and found it does pretty much match my perceived "chill" factor for my four year old Alpen windows.

    I live in SW Colorado at 8000 ft with typically 8500HDD. The windows are .15 U fixed, .19 casement/awning. The largest triple window gang is 5.5' x 10.5' and even in the dead of night in February they do not present a chilly profile. We have put up cellular shades for sun or imagined privacy, not to keep warm. Too closely set shades will allow the inner glass to cool too much and the edges can start getting condensation in rooms other than bathroom, which is hopeless when showering.

    The breakfast corner nook has 3 windows on the two sides, 2N and 1E (east covered by the porch roof) that are roughly 4 1/2' tall and 4' wide (the ganged pair 8'). I sit with my back to the north windows and do not notice them despite a severe follicle deficiency. The backup electrical points I put in for under window strip heaters have never been used.

    I did take some surface temps on the glass of a couple of windows one night when it was -5F. While the edges had gotten down to the 40's and condensation was forming, temps from about 3" inch to the center held within a degree or two of 62F. We have a number of plants in the room, so our humidity levels tend to be a bit higher. It is worth noting the edge to center values will reveal themselves on some wintery days as patches of condensed mist on the exterior side of the window.

    All windows suffer from edge losses due to the framing and the edge strips separating the panes. The heat losses at the edges will warm the exterior most pane edges enough to beat the dew point on some mornings where the overnight chilled glass center will lag for some time. This is actually showing how good the window is performing. There is not much loss through the field area. Or the framing really for that matter. Overall I am a big fan and will be putting Alpens in my daughters house once we can build.

    One thing you did not mention was which SHGC you were intending to go with. I went with the lowest on all windows regardless of orientation. This was contrary to the mantra of winter energy benefits. At my particular altitude and having had prior experience with the local summer sun loads it penciled out as not worth the trivial gains I would net vs the excess I would face for the summers. Winter light hours are short and per window gains even shorter. The sun load at 9700' is not something I can judge, perhaps the summers are cool enough to not have solar input overdrive the house. You are seemingly talking a lot of glass.

    The depth of tint or percent of transmission is not an issue. Between the snow bounce and just plain more intense light at higher elevations, we have not felt like it is bad to have low values. It is really only noticeable if a casement window is opened next to a fixed unit or if you walk out of the house around the 1/2 hour sunset transition, which is kind of like taking off sunglasses and realizing the sun isn't quite gone yet. Not a big deal in my book.

    While noting you said energy efficiency wasn't a priority, do consider that if the power goes off or for some reason you ran out of firewood, having the extra edge of .12 windows will slow the cool down. Relative to your walls and ceiling, the windows are still big loss points. Total square footage of glass will drive how quickly the various parts of the house will chill off. Moving heat from the room with the woodstove can be a bit challenging when the power is down.

    And last thing, the size of the windows for the sweet mountain views. The largest window set we have is the 5.5'x10.5' facing east. It was designed by myself, so I can only say that the designer could have made the window only 5' tall and not have affected the view. This would have made the header choices a bit more flexible and maybe less costly.

    It is fun to look at the mountain home porn photos with giant soaring windows, but in actual use they are often a bit silly. If the goal is to frame a mountain view such that an entrance point to home or room produces the "wow" factor, fine. Just know that huge glass is expensive, subject to wind issues and will never be an R30 wall. But then maybe I am just jealous.

    1. point78 | | #5

      Great info, thanks.

      Im up in Leadville, house (1400sq') located in front of and directly pointed at Mt Elbert, then also looking down the range twards Buena Vista, and further the MIsquito mountain and that range directly behind me.

      Good problem to have, but I have too many mountains to look at as I'm surrounded, to many meteor showers to watch at night while laying in bed, and to many Colorado mind blowing sunrises and sunsets.

      So Im putting in picture windows- two 6'x6', one 8'x6', and one 6' x 54", and two big single swing style patio doors. 8'x43" tilt-turns

      The south west windows are all the "Solar Control", low SHGC. I lived in a house where it was a wall of south facing glass with cheap double glazing. If the sun was out, I had to have windows open regardless of how cold it was outside dead of winter. Then in the morning- I could tell you the outside temp by how low the inside temp had fallen, the radiant heat couldn't keep up below about 20F.

      There is a lot of sun glare, so I'm looking forward to the windows hopefully blocking some.

      I'm planning a single Fujitsu floor mounted mini split in the "great room" for entire house heating. The basement will have a second mini split, if I ever finish it. Currently it won't have any heat.

      7.7Kw solar up on the garage roof, all electric house, grid tied.

  4. Expert Member
    AKOS TOTH | | #4

    Even the best windows are terrible compared to walls.

    About the only way to get good comfort, especially on clear winter nights is with supplemental heat.

    For larger windows, you want either some floor heat or registers to make up for the losses through the window. You just need enough heat there so you won't get the cold air falling and stagnating in the area around the windows.

    I have some triple pane 0.19 windows, not as cold climate as you but it does occasionally get down to -20C. With the floor heat off, the area around the larger windows is cold. With the floor heat on, perfectly comfortable.

    No amount of money will buy windows that will be comfortable in cold climates. Spend the extra money on a bit of supplemental heat.

    P.S. If you are using forced air, make sure it is a modulating unit, you don't want the heat source to cycle because as soon as the heat stops the windows will feel cold. If it isn't a modulating unit, install a bit of resistance floor heat instead.

    1. point78 | | #6

      Roger that with the modulating.

      Also, I was planning on putting a wired thermostat for the mini split on the far side of the room- as I've read some people have slightly more even temps when using these in cold areas.

  5. onslow | | #10


    I think you will be fine with the mini floor mount. A friend with a very similar build method and same altitude has no problem keeping warm. If anything the sun load is still capable of providing a good boost. Maybe he will chime in.

    My walls are whole wall R32 and ceilings/roof R54. I use all resistance heat cove heaters, which is admittedly a bit insane. Bad mini-split options available 5 years ago and a spouse who did NOT want to look at mini-split wall heads determined the choice. I did not want propane, so I guess we're even. And yes the electric bills are awful, but man is it quiet up here.

    You will not need to have any additional heat in front of the windows based on my experience. I have checked wall and floor temperatures when it is about 10F outside and I got 2-3 degree differences under the big window at night. When its sunny the window sill is quite warm. Maybe the sliders will be a bit dicey given the threshold, but a .17U slider is very good compared to normal ones. Maybe an area rug by the slider will keep your toes from noticing anything.

    Sounds like the view will be something. Enjoy the new digs.

    1. point78 | | #11

      Thanks for the info. I hadn't even thought of the floor temp in front of the window.

      I was more wondering if I'll need a shirt on or not, sipping my morning coffee while sitting in the 12" deep window bay, or standing looking out the door while its blowing snow sideways out there.

  6. point78 | | #12

    Went with a Mix of both the Alpen zr6 and on some of the bigger windows the Zr9.

    Master bedroom and great room got the 9’s.

    Foundation should be poured by next week, and hopefully the basement ICF walls shortly after.

    Was able to get the new Nudura XR35 ICF. 4” of foam on each side.

  7. Raysee97 | | #13

    Brian, I'm just starting the journey you have most likely completed. I'm looking hard at Alpen zr6 right now and a week away from needing to put in my order. Any advice post purchase? Regrets, positives?

    Much appreciate any and all feedback.



    1. point78 | | #14

      They are just putting up the sheet rock in the house now, so I haven’t had any actual time living with the windows yet.

      If I did it again, I’d put the “balanced” glass option where I have the “high gain” windows (facing south). There’s so much glare off the snow where I’m at & there is a slight difference between the brightness between the two different windows.

      It’s still ok to look out the high gain windows glare wise, but it’s just a little better out of the balanced & solar control windows.

      (I do have pretty sensitive eyes.)

      The Alpen windows seem very nice.

      I liked the balloon they leave attached so the gas can expand and contract. They pinched off the lines after they’d been installed a couple of days.

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