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Cold climate windows

AKDigs | Posted in General Questions on

We live in Anchorage, Alaska and purchased a home built in the mid 1980s.  The windows in our home are likely the originals:  wood framed, single pane Pellas with removable storm windows.  Our house is very drafty and cold in the winter and our gas bill proves it!

We need to replace our siding and our windows but have a limited budget.  We are leaning toward a mix of 3000 and 4000 series Coeur d’Alene vinyl windows, some pictures/fixed (3000 series), and some double hung (4000 series).  For the picture windows, it looks like we will be restricted to double panes.  For the double hung windows, we can install triple panes.  Two very large picture windows face west.  Two very large picture windows face east.  The double hungs will face south, east and west.  In total, most of the windows face east or west though.

In winter, our indoor humidity levels run 25%-30%, depending upon how cold it gets outside.  At the moment, we have a gas powered forced air furnace, which contributes to the dry air inside our home.   We keep our thermostat at 68 degrees but we still see gas bills in the neighborhood of $300-$400 in the winter.  At winter solstice, our area gets approximately 6 hours of daylight.

In summer, our indoor humidity levels increase to 40%+.  We don’t own, nor need an air conditioner…EVER!  Our home does get very toasty in the summer but we can easily fix that by opening a few windows.  Our gas bill drops to double digits in summer (basically June, July, and August) as we really only use gas for cooking and for heat on the occasional cold summer night.  At summer solstice, our area gets approximately 18 hours of daylight.

Based upon what I’ve read on this website and elsewhere, we’re leaning toward using LoE 180, possibly combined with LoE i89 on our windows.  We want the lowest U value, the highest SHGC, and the highest VT possible.

Our concerns are these:

(1) For the large picture/double pane windows (approx 106 x 64) we’re considering two 5mm panes to increase the strength of the windows.  Our home is in an area that gets occasional high winds (gusts over 100 mph), and we do get frequent earthquakes here.  My concern is that two 5mm panes will significantly decrease our SHGC and VT.  We’re wondering whether 4mm panes would be sufficient instead.

(2) On which surfaces should we place the LoE 180 and possibly the LoE i89?  At present, we’re considering this:
Picture/Double Pane Windows:  5mm with 180 on #2 surface / Argon / 5mm with i89 on #4 surface.
Double Hung/Triple Pane Windows:  3mm with 180 on #2 surface / Argon / 3mm Clear / Argon / 3mm with 180 on #5 surface.
Is there a better way to do this?  Again, we want the lowest U value, highest SHGC, and highest VT possible.

(3)  We’re a bit concerned about condensation on the large picture/double pane windows since their CR is significantly lower than the double hung/triple pane windows.  Coeur d’Alene’s warranty specifically excludes issues relating to condensation.  How concerned should we be about this?

Thanks so much in advance.  This site is a wonderful resource.

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  1. rockies63 | | #1

    Since you have very large picture windows facing east and west where they don't gather a tremendous amount of heat during the day (except maybe the west ones which could lead to overheating and glare in the evenings) could you decrease the size of those windows?

    After all, the very best window has the same insulation value as a really lousy wall. If the windows were smaller you'd have much less heat loss through them in the evenings and at night and that would certainly lower your heating bills.

  2. AKDigs | | #2

    Scott, thank you for the reply.

    The two large picture windows facing west capture the view of the inlet. The lot and the view are the primary reasons we bought this house. I am willing to sacrifice some money to preserve the view. We don't mind the overheating from those windows at all. We simply open a couple windows and the house cools down very quickly. It rarely gets above 70 degrees here.

    The two large picture windows facing east are actually going to be a remodel from what we currently have. We're planning to replace a sliding glass door with one of the windows and french doors with the other. Accordingly, we'll be bringing up the base approximately 19-20", so they should be more efficient than what we have now. The view to the east mostly oversees my gardening area and the forest, both of which we enjoy seeing from the inside. Yes, we could reduce the size of those windows though and still have a good view. That's something we will consider.

    Thank you for the suggestion.

  3. Expert Member
    BILL WICHERS | | #3

    You will likely see more benefit to using triple pane in the large picture windows and saving money with double panes in the double hungs. This is because the greater glass area compared to the frame area is where triple pane is of most benefit -- triple pane wins at center of glass U values, but the U value of the frame isn't much different for triple or double pane windows. Basically this means that triple pane windows get exceedingly better in regards to their overall U values as the amount of glass relative to the amount of frame increases, which is most noticeable in large picture windows.

    You may also want to consider replacing your double hung windows with casements to gain the better air sealing that the casement mechanism allows.

    I'd let the manufacturers recommend where to place the LoE coatings. I've never seen a benefit to trying to change the coating surfaces away from the recommendations of the glass manufacturers. Cardinal has an excellent "Technical Glass Guide" that goes into detail for all of their coating and glass IGU combinations that is available here:

    I would absolutely use i89 on any double pane windows. I also think you're making the right decision using LoE-180 with your concerns for best VT and SHGC. Any of the other coatings are going to reduce both VT and SHGC compared to LoE-180, and the U value improvements in many cases (especially with triple panes) isn't huge. With your triple pane windows, try to get 1-3/8" thick IGUs too for best U value. Don't settle for 7/8" triple pane IGUs since you lose most of the benefit of a triple pane's ability to increase U values with such small air spaces.

    Regarding the wind concerns, you may want to consider laminated glass for storm resistance. Laminated glass will be MUCH stronger than a slightly thicker pane of tempered glass would be. Laminated glass will also help to reduce sound transmission.


    1. AKDigs | | #5

      Thank you Bill. Our definite preference is to have triple pane windows throughout. The problem is, according to the manufacturer's website, we can't get picture windows in triple panes, which is why we're kicking around the 5mm x 2 idea for the double panes. I appreciate the suggestion for laminated glass though. I'll see how much that would add to our cost.

      I also appreciate the tip to check on the 1-3/8 thick IGUs. I've sent an inquiry to the dealer to see whether they have dedicated 1-3/8" frames for their triple pane windows or whether they just use the same 7/8" frames that they use for double panes. If they don't have the larger frame, maybe it isn't worth getting triple pane windows inside that smaller frame. I guess I'd have to check the ratings on that.

      If they do offer the larger 1 3/8" frame for triple pane windows, do you think it would be better to have one coat of LoE 180 and one coat of LoE i89 than it would be to have two coats of LoE 180 (without i89)?

      Sorry for all the questions. Until we made a decision last winter that we needed to replace our windows and started getting quotes, we thought this would be a simple process. We got quotes from 4 different vendors here and all of them tried to steer us toward windows with very low SHGC and VT. It doesn't seem like they've read much on cold climate windows.

      1. Expert Member
        BILL WICHERS | | #11

        It's odd that they'd offer triple pane in windows other than picture windows. The extra weight of the triple pane IGU can require beefier hardware, but in a picture window that's not an issue. I'd think picture windows would be easier to make with a triple pane option than any operable window would be. It may be worth checking with some other manufacturers to see if you can match the style you want from a supplier that can also provide the glass option you want.

        A double pane IGU using i89 will perform almost as well as a 7/8" triple pane IGU. A 1-3/8" triple pane IGU is going to get you the best performance though, which is why that's what I'd look for, especially in any windows with a very large glass area. I'd go with the cardinal recommendation of two 180 coatings and then i89 on the inside. As I mentioned before, I don't see much reason to deviate from the manufacturer's recommendations on where to place the coatings.

        Many window manufacturers aren't thinking about SHGC. That's something that has been discussed on GBA many times. You may want to look into some of the Canadian window manufacturers too -- they tend to be more understanding of northern cold climate concerns.

        No problem asking questions! You'll be looking at your windows for a long time, it's best to ask as many questions as you can now to make sure you make the right decisions so that you'll be happy with your windows. I like to tell my consulting customers that my job is to make sure they never say "I wish we'd done x" at some time in the future.


  4. Jon_R | | #4

    Condensation is unlikely, but use this (which shows the condensation conditions):

  5. rockies63 | | #6

    If you are wanting to use French doors you might consider making them outswing. Any wind on the side of the building will actually press the doors more tightly against the frame, making them more weather tight. If you really want a weather tight door you might consider an outswing "garden door" which has a center bar between the two doors. Only one side opens as a full door, the other side opens part ways for ventilation and has a screen.

  6. AKDigs | | #7

    Scott, we're getting rid of both the french doors and the sliding glass door on the east side of the house. We never use the doors and they're very cold and drafty. We're replacing them with large picture windows, which are significantly less expensive than replacement doors would be. Sorry if I didn't make that clear in my earlier post. I do think we're going to take your earlier suggestion to reduce the size of the picture windows on that side of the house though.

  7. PAUL KUENN | | #8

    I would consult with cold weather climate housing up in Fairbanks.
    They use a local window builder and they are always willing to give good recommendations. Keep in mind any double hung windows will leak air at center (where they meet) so that completely destroys any value in the double or triple glazing.

  8. AKDigs | | #9

    Thank you Paul. I'll see if they're willing to talk with me about our windows. We did talk with Capitol Northerm Windows (recommended by the Cold Climate Housing Research Center) a while back and, while their windows are excellent, they are way outside our budget.

  9. Expert Member
    Peter Yost | | #10

    You should have a good way to measure your RH; see this GBA blog:

    Depending on how much cooking and baking you do, your gas cooktop and oven can be a significant source of household moisture (see this GBA blog:

    In your climate, try to keep the interior RH in winter at 30% or if you can tolerate, 25%.


  10. Expert Member
    Dana Dorsett | | #12

    >"At the moment, we have a gas powered forced air furnace, which contributes to the dry air inside our home. "

    The only way a gas furnace can appreciably contribute to the dry air issue is if the duct system is out of balance &/or the house is leaky. A furnace doesn't remove or add moisture to the air, only heat, but duct imbalances create room to room pressure differences that use "the great outdoors" as part of the return path in a leaky house.

    >"(3) We’re a bit concerned about condensation on the large picture/double pane windows since their CR is significantly lower than the double hung/triple pane windows. "

    Do you have condensation on the indoor surface of your current windows very often? At the reported indoor RH & temperature numbers it seems unlikely. If the single pane Pella's are leaky it's reasonable to expect condensation on the storms, but probably not the interior side glass. At ocean-tempered Anchorage weather the number of condensing hours in a typical year with a LoE180 (#2) +i89 (#4) window are few or not existent (except possibly during Polar Vortex disturbance extremes.) Without chronic condensation it isn't very risky at all, even though with i89 on surface #4 of a double-pane the surface may run a few degrees cooler than your current windows.

    Your mean wintertime outdoor temp is about +20F, condensation on a LoE 180 + i89 glass doesn't get going until -15F at higher indoor RH than what you'r reporting. According to the chart on page 22 (p25 in PDF pagination), at -20F outdoors the indoor RH needs to be 28% or higher (at 70F indoors) for condensation to form on LoE-180 + i89 glass even for the thinnest spacing option, 32-34% for thicker spacing.

    So while at the extremes you may have a few hours of condensing at a time, it won't be chronic condensation. When that condensation occurs the performance gained by the i89 on surface #4 goes away (water is very high-E), but since it almost never happens those few hours aren't going to make an appreciable difference in the heating bill.

  11. tundracycle | | #13

    @Paul, double hungs have improved. Loewen has a pretty good system (check head & sill details):

    I think still not to the level of casement, but perhaps close enough?

    @Zephyr7, I believe the issue is the thickness of the glazing needed for larger picture windows. Double results in a quite heavy unit. Triple is not only heavier for the glazing but adds additional weight for the extra frame structure needed. Our builder put a quite large Loewen triple in another house and said the cost, weight, and installation were significant.

  12. AKDigs | | #14

    Dana, the information you provided was extremely helpful. Thank you for spelling it out for me. I'm a few levels below novice on this topic so I sincerely appreciate all the details you provided.

    Regarding our furnace and ducts: we most certainly do have duct imbalances and we have a very old furnace. After we complete the siding and window project, I anticipate we'll have to face that issue too.

    Dana, Peter, and Jon, regarding our indoor humidity levels: we actually struggle to get winter time indoor levels above 30%. We only achieve that if we run a humidifier constantly when outdoor temps drop below freezing. So, given the information you provided, my condensation concerns have been alleviated. Thank you!

    Scott, we discussed reducing our window size on the east side of the home last night. We've decided to make them 50% smaller. That should help quite a bit.

    Zephyr7, I heard back from the vendor and their triple panes are 7/8". You prompted me to look in to this more and I thank you for that. I wouldn't have known to do it otherwise. We're rethinking whether it makes any sense for us to do triple panes in a double pane frame now. So confusing!!! We're also reassessing whether we could use a combination of 1/2 picture window and 1/2 casement instead of double hungs since most of the openings are approximately 40x60.

    Paul, after finding out that this particular manufacturer uses the same frame for double pane and triple pane windows, I went to talk to Capitol Northerm Windows(they're the window company recommended by the Cold Climate Research Center in Fairbanks). Their triple pane whole window U values are around .16 - .18. They're using Edgetech spacers (Super Spacers?) and the 1 3/8" frame, plus a thick exterior seal around the IGU, where it meets the frame. Their windows are far superior to any others I've seen in Anchorage. They're using LoE 270 on their glass though, which looked a little dark. They said I could special order LoE 180 but it might cost more and it would take longer to get here. I'm pretty sure their estimate is going to come in much higher than what we've set aside in our budget but they did tell me they might be able to work with us given the number of windows we're replacing (18 total). They don't do installations though. We'd have to get a contractor to do that for us. I'll wait and see what those numbers look like before we move forward.

    I really can't thank you all enough for taking time to help me work through this. I really had no idea how much engineering went into windows until I started down this rabbit hole...

  13. Expert Member
    Dana Dorsett | | #15

    >"Regarding our furnace and ducts: we most certainly do have duct imbalances and we have a very old furnace. After we complete the siding and window project, I anticipate we'll have to face that issue too."

    For yuks, even before swapping out the windows, run a fuel use heat load calculation from some of last winter's bills. The 99% outside design temp at Fort Richardson is -13F, at Elemendorf AFB it's -8F, and at the airport it's +9F, so pick the nearest one when making that calculation.

    That will give you a baseline by which you can measure the improvements as you address the windows, air leaks, etc.

    Replacing the furnace is an opportunity moment for right-sizing it, which is CRITICAL for comfort when heating with ducted hot air. The fuel use load numbers on the pre-improvements fuel usage will puts a very firm stake in the ground on the maximum capacity you'd ever need, and you can re-calculate as you go along. Most existing furnaces are more than 2x oversized for their loads, which automatically oversizes the air handlers, increasing the outdoor air infiltration & drying issues.

    By the time you've cut your window losses in half and tightened up the place it'll probably be 3x or more oversized (it might already be) , and might not be much more comfortable despite the reduction in the heating bill, all due to ridiculous oversizing. It's more comfortable to be slightly undersized and running a 100% duty cycle when it's -10F outside than 2x oversized and running only a 50% duty cycle. But people tend to be more worried about being too cold than suffering through the hot-flash followed by the chill all season long, with remote rooms at the end of the duct run never being adequately heated. Oversizing is what gives hot air heating a bad rap.

    See if any of Nate Adams' videos sound familiar:

    Believe it or not there are now air source heat pump solutions that can be fully specified at -13F, and depending on where your load numbers come out in the "after upgrades" picture that can be more comfortable than an oversized fossil burner. You'd probably be the only house in Anchorage heating that way, which could be a problem if/when it needed service, but it's something to contemplate when the time comes.

    But at the very least tracking the load via fuel use as you fix up the leaks in the house & ducts and address duct balance issues you'll know when the time comes what output capacity will work and what's going to be too oversized for comfort when replacing that furnace. There are lots of good 2 stage and modulating options out there that simply didn't exist in the 1980s.

    ASHRAE recommends 1.4x oversizing for the heat load at the 99% outside design temp, but if you don't use deep overnight setbacks even 1.2x oversizing will usually cover the Polar Vortex disturbance lows.

    1. user-723121 | | #19

      The modern 2 stage forced air furnaces with ECM fan motors are quite impressive. We changed out to 45k low fire, 66k high fire in 2006 and could not improve on that decision. With some attic air sealing, additional blown insulation and some foundation insulation our seasonal gas usage was cut by 40%. The old gas furnace was a single stage 150k giant with an estimated efficiency of around 75%. The new 95% furnace really added to the energy savings. We used to use a winter nightly setback to 62F but have since gone to a 68F for comfort. The energy penalty for this appears to be as follows, 2.3 Btu/sf/hdd with 62F setback, now 2.4 Btu/sf/hdd with 68F setback. The deep nightly setback did take a lot of extra capacity (2 1/2 hour runtime) to bring the house up to temperature on the coldest days, design temperature heat loss for this house is around 29k.

      Cycles per hour is something to consider, the Honeywell thermostat I used was factory set at 5 cph, I have it set at 3 cph. The furnace for this house at least does not have to run every 12 minutes. Even at 3 cycles per the furnace will only go to high fire during design temperature conditions.

  14. rockies63 | | #16

    I'm glad my suggestion to reduce the size of your windows on the east side of your house was helpful. As to whether you should go with 1/2 picture windows and 1/2 casement because your RO are 40x60 do you mean one window will be a picture and the other a casement or a picture over a casement (in the same 40x60 RO)?

    I'm also guessing that 40" is the width of your RO, in which case that's pretty wide for a casement. A casement will certainly be much tighter (in regards to air infiltration) than a double or single hung window but you might consider a piece of picture glass over a smaller awning window. Awning windows are also very tight when closed while still giving you the opportunity for fresh air when you want it.

    Another benefit to a low awning window is that you can set up a "thermosyphoning" effect by opening a higher window on the opposite side of the house . Cool air enters lower down through the awning and exits through the higher window across the house, helping to remove excess heat.

  15. AKDigs | | #17

    Scott, your idea is FAR superior to the one I had for a picture/casement combo. I like the idea of a larger picture (40x40) with a smaller awning (40x20) above or below it. I REALLY like that idea! DONE!

    Zephyr7, I double checked with the vendor and he told me that we can indeed get the picture windows in triple pane. My mistake. The problem remains, however, that they only offer a 7/8" opening for the IGU. From what I can gather from the NFRC website, it looks like the triple pane is still slightly more efficient than the double pane in the same space. So, we'll likely go for that option. I also read through Cardinal's Technical Glass Guide and it looks like we should go with LoE180 on either one or two surfaces. We can't put i89 on the #6 surface because the risk for thermal breakage is too high. The only comparative data on efficiency I can find from this particular window manufacturer on 180-clr-180 vs 180-clr-clr is with a 9.6mm air space between each pane. We're going to be stuck with 6.5mm between each pane if we go with 3mm panes and stick with this particular manufacturer. According to the NFRC website, the 180-clr-180 (with 9.6mm air spaces) has a slightly better U factor (.19) than the 180-clr-clr (.24) and the difference in VT and SHGC is negligible. I'll have to see what the difference in cost is for the second coating.

    Dana, we will definitely do the heat load calculation. Thanks for pointing us in that direction!

    1. Expert Member
      BILL WICHERS | | #18

      The picture/awning combo is nice — I have that combo on my house too. It works pretty well.

      I’d suggest checking out some other window manufacturers that offer more options. In your climate, it’s really worth going with high performance windows.


  16. rockies63 | | #20

    One thing to be careful of when putting an awning window over a picture window is to make sure that the dividing bar between the two sections isn't at your eye height when standing in the room. That's why you usually see the awning section at the bottom.

    Another consideration in the "3 panes vs. 2 panes" debate is that the internal temperature of the glass for the 3 pane unit will be higher than the 2 pane, thus making sitting near that window a lot more comfortable.

  17. Expert Member
    BILL WICHERS | | #21

    That’s a good point. For the OP, my windows have a little less than the lower third as the awning part, the upper a little more than 2/3 of the overall window is the fixed picture window part. You don’t really notice the divider this way, and these are large windows over 3/4 the height of the wall.


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