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Solar glazing

Drew Baden | Posted in Plans Review on

With all of the excellent advice I’ve been given, I decided to join GBA today to show my appreciation and support.

In my opinion, the value I’ve gathered thus far exceeds the amount of the membership cost.

At any rate, many of you have seen my plans that I’m creating and now I’d like to discuss my window options. I have decided to utilize the attic space as a bonus room and tossed a couple of windows up there.

I have my eye on Intus windows but I’ll also price thermotech since they’re geographically nearer to me. I’m still sourcing a manufacturer for the doors.

The following table lists the dimensions for the windows all around:

Elevation Room Width Length Type
Intus West Office 98 48 Partial tilt/turn
Intus South Office 48 48 Fixed
Intus South Kitchen 46 42 Fixed
Intus South Kitchen 46 42 Tilt / turn
Intus South Dining 36 48 Fixed
Intus South Living 96 48 Partial tilt/turn
Intus East Living 48 48 Fixed
Intus East Closet 40 22 Fixed
Intus East Master bed 78 22 Tilt only
Intus North Master bath 36 22 Fixed
Intus North Guest bed 30 42 Tilt/turn
Intus East Attic 48 48 Tilt/turn
Intus West Attic 48 48 Tile/turn

If my math is correct, my southern exposed glazing amounts to 87 sq. ft. which is 4.88 % of the 1778 sq. ft. 1st floor living area.

The windows I have my eye on are the Eforte line. My goal is to keep as many of the windows fixed as much as possible to control cost and U value, but I’d also like to have enough operable windows to create a nice breeze on those beautiful days without having to rely on a screen door.

These windows exhibit the following properties:
FIXED: U=0.12, R=8.33
OPERABLE: U=0.13, R=7.7
SHGC can range from 0.25 to 0.62.

A few questions:

Do I have enough solar glazing on the southern elevation to produce a bit of heat gain? From the articles I read on this site it appears I’m in the ballpark.

The representative at Intus informed me to just go ahead and use the standard glass (I believe SHGC = .36) for ALL of the windows in the house to realize excellent U values. He also informed me that I won’t need to worry about overheating by doing this and I’ll save on cost. Is this feasable? Should I design a higher SHGC on the south side? East and west?

I’m looking for some guidance here. I’m not sure if I’d want to add any additional windows to the south side so maybe I would be better off with a little higher SHGC on that elevation?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Drew,
    Please tell us your location or climate zone.

  2. Drew Baden | | #2

    Martin,

    I'm in 5A. I'm building on 3 open acres of farmland.

    This is from Dana's input on another thread: Windows: U0.24, with SHGC greater than 0.50

    But.. .I'm unsure to which elevation he was referring. Assuming south only.

  3. GBA Editor
    Martin Holladay | | #3

    Drew,
    For years, the usual advice has been to choose low-solar-gain glass for west-facing windows (and sometimes, for east-facing windows as well), but to choose high-solar-gain glass for south-facing windows.

    The SHGC of north-facing windows is irrelevant, so the most important specs for north-facing glass concern U-factor (as low as possible) and VT (as high as possible). Of course, these two goals for north-facing glass are incompatible, so compromise is necessary.

    For more information on glazing specs, see All About Glazing Options.

    In recent years, however, I've come to realize that the importance of solar gain through south-facing windows has been overemphasized. For more information on this topic, see these two articles:

    Study Shows That Expensive Windows Yield Meager Energy Returns

    Reassessing Passive Solar Design Principles

  4. Stephen Sheehy | | #4

    Drew- We used Intus Eforte UPVC windows in our new house in Maine, zone 6. Most of the glazing is on the south side, which offers the best views as well as some solar gain. To keep things simple we opted for the same specs in all windows:

    SHGC-.49
    VT-.71
    U-.12/.13

    We ended up with about twice the glass on the south side as you contemplate on your south side, and our interior space is about 1650 sq. ft., a little less than your first floor.

    The biggest grouping of windows consists of six panes that total about 11'x11.' There is a 3' roof overhang above them. We haven't experienced any overheating since July when we moved in.

    When you think about what windows you want openable, give some thought to where you might place furniture. Many of your windows are 48" wide and they'll stick out into the room if you open them fully. We are pleased with the mix of openable and fixed, but if you aren't sure about a specific opening, you might want to make it openable, because you can't change your mind later and you can always keep it closed. On the other hand, the fixed windows have more glass area for a given rough opening and are cheaper. The slightly better U value shouldn't be a big factor, since all of the windows are really tight and perform much better than most windows.

    One reason we chose Intus was the availability of a local dealer (Performance Building Supply of Portland ME) who was helpful if we had questions and even came out to spend several hours showing our GC how to install them in the center of the double stud wall opening. While we have not had any problems with the windows, it is nice to know there is a local representative available.

    We installed Intus doors for the two primary exterior door openings.

    We really are very happy with the windows and doors.

  5. Reid Baldwin | | #5

    Is your spouse familiar with how tilt/turn windows operate? When I discussed them with my wife, she was put off by the fact that they open in instead of out. As Stephen mentioned, that impacts furniture placement. It also impacts window treatment options.

  6. Rick Miller | | #6

    To Stephen Sheehy:
    I have a question for you that should also help Drew (since this is his post).
    You said you have about twice his proposed southern glazing, or about 174 sq. ft., and that you have about 1650 sq ft floor area. That's about 10.5%

    Do you have any energy analysis results from software that gives you your estimated solar fraction (percentage of winter heating supplied by passive solar)? If so, I would be interested in that value, and I believe it would be helpful to Drew.

    Passive solar techniques work, if done properly. I have about 9.3% solar glazing (only the actual glass area was considered) facing about 154 degrees true azimuth, climate zone 4, almost 5. I couldn't fit any more window area on my south side, because it was a retrofit. Now that I have lived for a couple years with 9.3%, I have no doubt whatsoever that I would have went with as much glazing (percentage-wise) as Stephen has, if I had the wall space.. There has been no overheating in my house, but if I did have 10.5%, I would probably have found a way to add a little more mass. The article I wrote for this website is at

    https://www.greenbuildingadvisor.com/homes/making-old-tract-house-sunnier-and-more-efficient

    As noted in the article, an online calculator showed that I got about 54% of my winter heating from passive solar. Since then, the BEopt software (free download, and very sophisticated) gives a value of 50% of my winter heating coming from passive solar.

    Stephen, do you have any numbers on your passive solar heating contribution that you can share?

  7. Peter L | | #7

    Drew,

    So you are in a Zone 5A building zone. With Intus Windows you can get operable windows (tilt & turn) and the screens are installed on the outside. This has numerous advantages one of which is the ability to install aftermarket solar screen shades that serve as bug screens but they also block SHG. They sell them in markets like Phoenix and anywhere in the desert southwest. It's super easy to do as they just remove and then install the SHG fabric bug screen in the OEM bug screen frame.

    The advantage here is that in the winter you can remove the outside screen in the window and get maximum SHG. Then in the summer you can install the screen and not only reduce the SHGC but also stop the bugs from coming in.

    Are you getting the white frame PVC windows from Intus or the colored laminate frame?

  8. Stephen Sheehy | | #8

    Rick- I looked at one version of the PHPP spreadsheet for our house and it shows a projected solar gain of 2435 bth/h out of a projected heat load of about 13000 btu/h at a design temp of 5 F and the same gain at -5 F.
    I'm not sure how that would translate into a percent of winter heat provided by solar gain, but I assume it would be substantial, given that average winter temps are much higher than design temp.

    As I said previously, we never really overheated. It would get warm enough on a sunny January day that the mini-splits didn't run. I don't think it ever gets above 76-78 F. The concrete floors may help keep things comfortable.

  9. Chuck Jensen | | #9

    Drew, from the window list it appears there is not an egress window in the master bdrm. Might need a tilt/turn there. (oops never mind, i see the door) I used Intus Eforte uPVC in my build and they are impressive and heavy. Air sealing them was challenging because I don't trust foam or caulk with the expansion/contraction of the frames. Taping with SIGA Wigluv is the best way but them you want trim to cover the tape and be fairly water tight.

  10. Drew Baden | | #10

    I'm using as many fixed windows as possible and will consider the obstacles when tilting inward. The windows I'm eyeballing are the same Stephen references with a SHGC of .49, however I think I'll modify my plan to increase glazing area on the south or simply add additional windows here.

    I like hearing that the mini splits don't kick on in January. I just got off the phone with my HVAC representative and he is suggesting a 3-ton, 3 head hyper heat unit. 1 head for each bedroom and 1 in the open dining/kitchen/living room area. This is for ~1778 sq. ft. With the comment of mini splits not running, I wonder if I could get away with a 2 ton unit? Maybe that should be another thread but I'll take comments if you got 'em. Stephen, I'm curious how many tons you implemented? I'm shooting for walls r30 and ceiling r50-60.

    Peter - do you have a source for the aftermarket solar shades? I like this idea. Again, I question a 2 ton vs a 3 ton using techniques such as this.

    I better start stocking up on the Siga Wigluv but I might need a home mortgage to buy a couple rolls! ;) I viewed a couple of videos on this stuff and it appears that this is an item where the addage "you get what you pay for" applies. Good stuff here.

  11. Stephen Sheehy | | #11

    Drew: We installed two Fujitsu ductless minisplits: A 12RLS3H in the great room and a 9RLS3H in the master bedroom suite.

    I attach a floor plan (not quite final, but close-no wood stove and door into guest/TV room was moved) that shows the unit locations. Our installed insulation values are a bit higher than yours, i.e. walls about R42, Ceiling about R70. Air tightness measured at .59ach50.

    For the most part, the unit in the great room heated the whole house, although we occasionally turned on the bedroom unit. We keep all internal doors open. We're in Maine, zone 6.

  12. Stephen Sheehy | | #12

    Drew: Double stud walls, with two 2x4 stud walls 24" oc and 5" space between. Exterior wall is structural.
    If you have specific questions, e mail me: stephen at tidewater dot net.

  13. Drew Baden | | #13

    Hey thanks for sharing Stephen! I love seeing other's floorplans to harvest ideas from. Can you explain what wall method you used? ie; double stud, REMOTE, et cetera...
    update: looking again at your plan, it appears to be double wall.

  14. Expert Member
    Dana Dorsett | | #14

    "I just got off the phone with my HVAC representative and he is suggesting a 3-ton, 3 head hyper heat unit. 1 head for each bedroom and 1 in the open dining/kitchen/living room area. This is for ~1778 sq. ft. With the comment of mini splits not running, I wonder if I could get away with a 2 ton unit?"

    A three ton 3 head H2i multi-split can heat a CODE MIN 1778' house, and would be overkill for a high-R 1778' house in zone 5. Having one head for each bedroom is also extreme overkill. If it's a heat distribution issue pushing you that direct, use a ducted mini-split. A 1.5 ton Fujitsu -18RLFCD could probably carry the whole house load on it's own, but if not, a mini-ducted unit for the doored-off rooms and an appropriately sized wall-coil type for the open space would do it.

    Multi-splits can only modulate down to about 6-7000 BTU/hr @ 47F, which is a problem, since that could easily be on the order of 2x your heat load at that temperature. If a mini-split isn't modulating, it's bad for both efficiency and reliability. You MUST do room-by-room heat load calculations to get it right, and DON'T leave it up to the HVAC guy. Most will do at best a pro-forma load calculation based on code minimums/maximums and end up oversizing by more than 2x for a high-R house.

    Any of the RLFCD series mini-duct units can drop back to 3100 BTU/hr @ +47F, as do all of the RLS3 & RLS3H wall coil types. The Mitsubishi- FH09NA wall coil can drop back to 16-1700 BTU/hr @ 47F, which makes it VERY attractive for low load open spaces. I can't remember off the top of my head how low the Mitsubishi KDxxNA mini-ducted units can modulate, but if we need to stay Mitsubishi-only that information is on line.

    Run two sets of load numbers, the load at +47F, and the load at your 99% outside design temp. Without both sets of load number you run the risk of buying too much equipment, oversized to the point that it's less efficient and less comfortable due to on/off cycling. Let us know you need help figuring out U-factors for unconventional and or high-R assemblies to run those load numbers.

    1. Drew Baden | | #22

      "Run two sets of load numbers, the load at +47F, and the load at your 99% outside design temp. Without both sets of load number you run the risk of buying too much equipment, oversized to the point that it's less efficient and less comfortable due to on/off cycling. Let us know you need help figuring out U-factors for unconventional and or high-R assemblies to run those load numbers."
      Dana - I am VERY close to building. I don't know how to run load numbers you suggest despite multiple attempts on spreadsheets or otherwise. Are you interested in making a buck on the side? If you're pro-se, that's all the much better but I wouldn't have a problem sliding you a little green for your time.

      I plan on 9' walls. The windows in the east and west in the dining and living have been eliminated leaving only the southern facing windows in those rooms. The door in the master has been eliminated.

      Values in parenthesis are in (U-value/SHGC) format

      QTY. SIZE. W x H
      ---------------------
      2 103 x 70 South - Living/Dining - Comprised of mullions equally divided two operables and one center fixed. Operable (.16/.38) Fixed (.16/.44)
      3 35 x 60 East - Bedroom - (.15/.24)
      1 34 x 40 West - Kitchen - (.15/.24)
      1 58 x 20 North - Bathroom - (.15/.24)
      1 56 x 64 North - Bedroom - (.15/.24)
      2 34 x 38 basement - not sure which elevation yet - (.15/.24)
      1 42 x 79 South - Front inswing entry door - (.18/.35)
      1 120 x 72 East - Slider Door (.15/.28)

  15. Drew Baden | | #15

    Thanks for the feedback Dana. I called my HVAC rep (Dale) and reminded him that my house will be well sealed and super insulated. He forgot that I mentioned this a month ago--but the last time we spoke I told him I was just entertaining the idea and not to invest much time in me... yet. He has lots of clients, so I understand the miscommunication.
    When I spoke with him yesterday and told him my projected R values he quickly understood and asked for my window dimensions, window performance metrics, and wall/ceiling R values.
    I've invited him to the chat here and he seemed interested, so we'll see if he pops in. However, I don't want to waste his time (or yours) on calculations just yet without having a solid layout of my windows. Having said that, I'll be chatting with an architect soon to add some appealing flair, work on the roof lines, possibly add more or less windows, and hopefully finalize my layout. From the feedback thus far, I believe I will increase the south facing glazing.
    On another note, I received a quotation from Intus and the windows are coming in at just over $8k. That's without shipping charges. I'm told shipping will come in around $500/palette. I'm still waiting on a quotation from Thermotech, but I really like the Intus Eforte line.

  16. Jonathan Lawrence CZ 4A New Jersey | | #16

    Drew,

    The pricing on the windows looks very good. There was an Intus rep at the NESEA show in March and they were passing out sell sheets for a Efforte window, any color you want as long as it was white, any size you want as long as it was 15sf, and any style you want as long as it was fixed for $475. The tilt n turns were around $650 I believe.

    Also, I like your most recent house plan. You have come a long way since the first one which would have made for some difficult air tightness detailing. Are you aiming for passive house metrics on the house?

  17. Drew Baden | | #17

    Good stuff there by Intus at the show! :)
    Thanks for the kind words on my design. I owe a bunch to the collaborative group here in guiding me to much of that thought.

    I'm not aiming for a passive house.Just something well sealed and well insulated. I want to challenge myself to [email protected] but not sure if it's possible with my budget. I also have my mind set on a minimum wall value of R30 and roof/ceiling at R60 or higher. I'm not sure if I'll insulate the ceiling/energy truss or implement a hot roof design merely because I haven't gotten that far yet.

    Just after updating this post earlier, the Thermotech quotation arrived in my e-mail. It came in at just under $6500, so a considerable savings over Intus. I actually viewed a Thermotech window in the past and remember that they incorporated a neat interior channel to fit drywall into to provide a nice finished look. That stuck in my head this whole time. Regardless, I'll really need to focus in on the performance of the two vendors to understand what I don't know. I mean, what's the real difference between u.18 and u.19. (There are just random numbers, not empirical data from vendors) Is this difference negligible enough to worry about? How does this really affect my entire project overall? Are the vendors fudging numbers? Are production techniques repeatable enough to hit the published numbers each and every time? After coming up with a solid understanding of this data and a definite window schedule, I'll start focusing on these metrics a little deeper. Right now, I don't think I need the uber-high SHGC on the south as long as I have plenty of glazing. I'd rather have the lower U values to retain what heat I'll have.

  18. Jonathan Lawrence CZ 4A New Jersey | | #18

    Drew,

    Technically speaking the difference between U.19 and U.18 is about R.3, so R5.26 vs R5.56. I would assume those numbers you were given are whole assembly, which is what you want to look at, and not center of glass. That is a pretty good window for your CZ and since you are not shooting for PH standards, but still a very well insulated home, then your money might be better spent on other areas rather than getting down to a U.12. Although I would defer to someone else here who has direct experience in your CZ. With quality windows like these, your biggest concern becomes making sure they are installed properly. Check out Hammer and Hand's best practices on their website for their current recommended install (backer rod with sealant, not spray foam).

    The high SHGC has lost a lot of its play in the PH movement in the States. As the quality of the construction and materials has gotten better over the years, the gains achieved by having free southern sun radiant heat in the winter time has become less important. There have also been issues with overheating - especially in the swing seasons. The Europeans have been able to make it work because they regularly incorporate external shading solutions. So if you do go with high SHGC on the southern side, make sure you have a shading solution. It can be very difficult to address after the fact.

  19. Drew Baden | | #19

    I'll have 4 exterior doors in my house one of which will lead to the garage.
    My question revolves around the other 3 that will directly interface with the external environment.
    I'll have one door on the south main entrance and 2 other doors on the east. The east side will have a door in the living area and another in the bedroom.
    Intus exterior door comes back at just under $3000. Ouch! If I choose this door, I cannot afford more than one.
    My question is, if I purchase only 1 top-end door, should I place it on the south for the excellent SHGC or would I be in a better position by placing it on the east for the better U value?

  20. Stephen Sheehy | | #20

    Drew: If you can only install one, I'd put it where you'll use it most, that is, the primary entrance. The Intus hardware is really nice.

  21. Drew Baden | | #21

    Thanks Stephen! Easy and simple answer. I was thinking too deep about performance on this rather than function.

  22. Drew Baden | | #23

    Dana-you wrote
    "Run two sets of load numbers, the load at +47F, and the load at your 99% outside design temp. Without both sets of load number you run the risk of buying too much equipment, oversized to the point that it's less efficient and less comfortable due to on/off cycling. Let us know you need help figuring out U-factors for unconventional and or high-R assemblies to run those load numbers."
    Dana - I am VERY close to building. I don't know how to run load numbers you suggest despite multiple attempts on spreadsheets or otherwise. Are you interested in making a buck on the side? If you're pro-se, that's all the much better but I wouldn't have a problem sliding you a little green for your time.

    I plan on 9' walls. The windows in the east and west in the dining and living have been eliminated leaving only the southern facing windows in those rooms. The door in the master has been eliminated.

    Values in parenthesis are in (U-value/SHGC) format

    QTY. SIZE. W x H
    ---------------------
    2 103 x 70 South - Living/Dining - Comprised of mullions equally divided two operables and one center fixed. Operable (.16/.38) Fixed (.16/.44)
    1 35 x 60 East - Bedroom - (.15/.24)
    1 34 x 40 West - Kitchen - (.15/.24)
    1 58 x 20 North - Bathroom - (.15/.24)
    1 56 x 64 North - Bedroom - (.15/.24)
    2 34 x 38 basement - not sure which elevation yet - (.15/.24)
    1 42 x 79 South - Front inswing entry door - (.18/.35)
    1 120 x 72 East - Slider Door (.15/.28)

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