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Products: What is the best u-value obtainable with double glazing?

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      CommentAuthorfostertom
    • CommentTimeJan 16th 2009
     
    In http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=113
    Posted By: richySealed units bother me, they have a limited life span and more often than not end up in land fill because splitting them to seperate the spacer bar from the glass is a lot of hassle.
    richy being a joiner (window maker?)
    • CommentAuthorsquowse
    • CommentTimeJan 17th 2009 edited
     
    as far as the original question goes - system 3 give a u value of 1.1 for their 4-16-4 warm edge argon filled units. they also do various other options with krypton etc;
    the factory is in east manchester and you would have to deal through one of their agents, eg alexander windows in stockport, but i expect they have others in different parts of the country.

    http://www.system3.ltd.uk/ultran.htm
    • CommentAuthorralphd
    • CommentTimeFeb 3rd 2009
     
    Don't just look at U-value, look at SHGC as well. A south-facing window with a 1.1W/m^2 rating and a 30% SHGC will have a higher net heat loss than a window rated 1.3W/m^2 and a 55% SHGC. Put windows with a good hard-coat LowE on the south and soft-coat on the rest.
    http://www.arkema-inc.com/index.cfm?pag=84

    Pilkington/LOF EA2 is the best lowE glass I have found for passive solar design.
    A good insulated spacer like edgetech's SuperSpacer premium is important as well (not the best one, but the best that seems to be commonly available in manufactured windows).
    Airspace is important as well. I've seen some manufacturers using only 11mm argon fill. Optimal space for argon in a double-glazed is ~14.5mm.
    Krypton has an optimal space of 9-10mm which is OK in a triple but in a double your spacer is so thin that in a typical window the improved insulating properties of Krypton vs Argon are offset by the extra losses in the spacer.

    With wood frames spacer choice is extra important; lower glass edge temperatures vs center cause condensation. To keep a healthy 40-45%RH indoors means condensation will form when the window edges drop below 6C. The wood will get wet, and then promote fungal growth. When the temperature drops to -10C, the sightline of virtually all double-glazed windows will drop below 6C.

    Going with a 16mm argon fill is probably justified; COG U-values will be worse than 14.5mm, but glass edge U-values will be better.

    Check out LBNL Window 5.2; it's used by a lot of North American window manufacturers for performance rating.
    http://windows.lbl.gov/software/window/window.html

    -Ralph
    •  
      CommentAuthorfostertom
    • CommentTimeFeb 3rd 2009
     
    Posted By: ralphdA south-facing window with a 1.1W/m^2 rating and a 30% SHGC will have a higher net heat loss than a window rated 1.3W/m^2 and a 55% SHGC
    Only if it's south facing *and* sees a lot of direct sun during the heating season. If other buildings, trees etc significantly obstruct that low-elevation sun, then forget about the solar-gain contribution to the nett energy flow. In that case, even if south facing, Uw (whole window U-value) is all that counts; g-value (solar gain, related to SHGC) is irrelevant because direct gain doesn't (much) hit the window.

    Beyond that, I hope we're not counting summer (outside heating season) solar gain into the equation?

    If it's either of these cases, we're looking at a crummy pseudo-scientific scam that the UK cheapo-window industry perpetrates on us, with govt connivance (if it's good for business it's good for UK plc)
    • CommentAuthorralphd
    • CommentTimeFeb 4th 2009
     
    Posted By: fostertom
    Posted By: ralphdA south-facing window with a 1.1W/m^2 rating and a 30% SHGC will have a higher net heat loss than a window rated 1.3W/m^2 and a 55% SHGC
    Only if it's south facing *and* sees a lot of direct sun during the heating season.


    Ok, open exposure south facing. Whether you are in Europe or North America, there are virtually no locations where the SHGC of an open exposure south-facing window is irrelevant.
    •  
      CommentAuthorfostertom
    • CommentTimeFeb 4th 2009
     
    Posted By: fostertomI hope we're not counting summer (outside heating season) solar gain into the equation?
    • CommentAuthorralphd
    • CommentTimeFeb 4th 2009
     
    Posted By: fostertom
    Posted By: fostertomI hope we're not counting summer (outside heating season) solar gain into the equation?


    Of course not. I'm a scientist, not an evangelist.

    The Canadian Energy Rating system for windows is based on net heat gain/loss over the heating season. Virtuall all hard-coat lowE windows are a net heat gain with an open south exposure in most northern climates (until you get close to the arctic circle).

    Check out the LBNL therm software. Easy to test windows in a house design and see how it affects energy use. It only has US climate data, but you can add your own. I just used a northern US city with temperature & insolation that is close to mine.
    • CommentAuthorSnikom
    • CommentTimeFeb 4th 2009
     
    HI there

    Newbie question

    I am about to build an extension (BCO willing) which has specified large folding doors opening onto our garden from the corner of the room, one door will face north and the other west. The western door will recive some sun but is not looking out onto open land.

    I had planned to use low e glass are you guys saying that this will not gain any benefit?

    Thanks

    Chris R
    • CommentAuthorralphd
    • CommentTimeFeb 4th 2009
     
    Posted By: Snikom
    I had planned to use low e glass are you guys saying that this will not gain any benefit?

    You will benefit from lowE. Since they are not south facing go with the best U-value you can get for your money (SHGC is only important for windows with an open southern exposure).
    • CommentAuthorSnikom
    • CommentTimeFeb 5th 2009
     
    Thank you

    Thats what I originally thought but this thread got me confused :confused:
    •  
      CommentAuthorfostertom
    • CommentTimeFeb 5th 2009 edited
     
    Posted By: ralphdSHGC is only important for windows with an open southern exposure
    That's the key statement! - thanks for spelling it out. 'Southern' means SE to SW? Elsewhere, ignore SHGC (or g-value = that can be as low as you like - tho that also means reduced light transmission as well as IR heat) and go for max Ug (glass-only centre-pane U-value)) and Uw (whole window U-value).

    Posted By: ralphd
    Posted By: fostertom
    Posted By: fostertomI hope we're not counting summer (outside heating season) solar gain into the equation?


    Of course not
    Just checkin'!
    • CommentAuthorralphd
    • CommentTimeFeb 5th 2009
     
    Posted By: fostertom
    Posted By: ralphdSHGC is only important for windows with an open southern exposure
    That's the key statement! - thanks for spelling it out. 'Southern' means SE to SW? Elsewhere, ignore SHGC (or g-value = that can be as low as you like - tho that also means reduced light transmission as well as IR heat) and go for max Ug (glass-only centre-pane U-value)) and Uw (whole window U-value).


    Yes, though some lowE coatings have very low emissivity with visible transmittance within 90% of clear glass; LBNL Window 5.2 has hundreds of glazing types in its database with these numbers.

    For SW exposure low SHGC is better; otherwise you get too much solar gain in late summer (Aug/Sept) when you don't want it.

    -Ralph
    •  
      CommentAuthorfostertom
    • CommentTimeFeb 5th 2009
     
    Posted By: ralphdFor SW exposure low SHGC is better; otherwise you get too much solar gain in late summer (Aug/Sept) when you don't want it.
    You mean higher SHGC is OK for SE because it allows gain in the morning, whereas higher SHGC to SW wd just add more evening gain, after a long day of it, so best to resist that?
    • CommentAuthorralphd
    • CommentTimeFeb 5th 2009
     
    Posted By: fostertom
    Posted By: ralphdFor SW exposure low SHGC is better; otherwise you get too much solar gain in late summer (Aug/Sept) when you don't want it.
    You mean higher SHGC is OK for SE because it allows gain in the morning, whereas higher SHGC to SW wd just add more evening gain, after a long day of it, so best to resist that?

    Mostly yes. My estimate is that even for SE it's better to go with lower U-value, and the lowE coatings with the lowest U-values have the lowest SHGC.
    •  
      CommentAuthorfostertom
    • CommentTimeFeb 5th 2009
     
    And same for due S? just to get it clear. And for W presumably, but not E? and not E-N-W?
    • CommentAuthorralphd
    • CommentTimeFeb 5th 2009
     
    Posted By: fostertomAnd same for due S? just to get it clear. And for W presumably, but not E? and not E-N-W?

    :smile:
    •  
      CommentAuthorfostertom
    • CommentTimeFeb 5th 2009 edited
     
    No, I'm serious, just want to get your view quite clear! Always thought the split between insulating but solar-accepting glass and all-out insulating glass would be E-W - but you're suggesting it's more like SW-NE?
    • CommentAuthorralphd
    • CommentTimeFeb 5th 2009 edited
     
    Posted By: fostertomNo, I'm serious, just want to get your view quite clear! Always thought the split between insulating but solar-accepting glass and all-out insulating glass would be E-W - but you're suggesting it's more like SW-NE?


    Sorry, I thought it was intentional humor; "just to get it clear" sounds like a pun when you're talking about windows.

    I haven't tried to figure out where the line is between good/bad for high SHGC. I chose to have the side of my house with the largest wall area face 10 degrees east of south. On the south side I used hard-coat lowE with a high SHGC. Everywhere else I used soft-coat with a low SHGC and slightly better U-value than the hard-coat.

    -Ralph

    p.s. I had a tech from the window mfr come out with a glass tester to make sure the right glazings were in the right windows; he had to change 2 of them that were mixed up.
    • CommentAuthordelboy
    • CommentTimeFeb 6th 2009
     
    Isn't Passivhaus spec for windows at 0.8w/m2K?

    Don't know where you can get them though.
    • CommentAuthorhowdytom
    • CommentTimeFeb 9th 2009
     
    Try looking on the back of the greenbuilding bible(fourth edition) !

    Tom

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