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Balancing greenness, budget, and double vs triple glazing on south side

user-954155 | Posted in Energy Efficiency and Durability on

We are planning a well insulated and sealed passive solar home on the east side of Vancouver Island BC near Vancouver (49th parallel). We plan R-40 walls, R-60 roof, R-20 under slab and about 8.4 percent of the floor area for south glazing.

The main house is 1,600 sq. ft. inside on 2 storeys with an attached suite of 900 sq. ft. Does anyone have any example energy modeling or real-world energy use examples for 2x vs 3x south glazing for this area like this Minnesota example? It takes into account the higher solar gain from double glazed.

This model seems to indicate little energy savings for 3x glazing over 2X glazing with SHGC. And on the coast its more temperate though less sunny in winter. With only south 2x glazing (3x on other sides) and night insulation it would be even better. However, I read other posts about up to 35% energy savings from Thermotech triple glazed,

Gienow triples are R-4.8, SHGC 0.57, ER 35 for south and R-5.1 for other sides

The double glazing we are looking at would be about R-3.5 and SHGC of 0.65 and ER 28

I would like to buy metal-clad wood windows, but getting the high performance triple glazed for all windows ($29,971 for 37 windows from Gienow) puts them over our limited budget. I prefer not to go PVC (16,381) for the environmental reasons (dioxins, PCBs, furans, vinyl chloride etc). Therefore I would like to explore the idea of getting double-glazed metal clad for the south (high SHGC) and possibly east windows and the remaining in triple glazing.

About half of the windows are on the south now although they are bigger than the others. It seems that the higher SHGC on the 2x glazing would compensate somewhat for lower R value especially if combined with night insulating coverings. Also we will eliminate five north windows to reduce the cost and that will help balance thermal performance. We could go with plain wood windows for north side as they are under the cover of a porch.



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  1. GBA Editor
    Martin Holladay | | #1

    There are lots of energy modeling programs out there. They range from simple Web-based models like this Window Selection Tool from the Efficient Windows Collaborative to the Passive House Planning Package (PHPP) software.

    Double clear glazing will obviously give you the highest solar gain, but will lose more heat at night and on cloudy days than low-e triple glazing. If you are planning to diligently open and close insulated shutters on the south side of your house, that radically changes energy modeling assumptions -- so these calculations get complicated fast.

    In general, the ER rating system takes all of this into account, but it necessarily fails to distinguish between climatic variations from Vancouver to Quebec. Financial considerations further complicate the equation.

    My guess (everybody hates it when I guess) is that in your mild climate, the cost of triple glazing will be hard to justify from an energy-savings perspective.

  2. user-954155 | | #2

    Thanks Martin. We're planning staggered stud wall on 9.25" plate filled with cellulose with 1.5" xps on outside, then rainscreen. Do you think its better to leave a more breathable exterior wall (minus foam) or minimize condensation / thermal bridge at plate etc and add the extra R value with the foam? No vapour barrier on interior, just airtight drywall and regular latex paint, so it can dry to interior.

  3. GBA Editor
    Martin Holladay | | #3

    Don't back me into a corner with a "What's better? What's worse?" question. Either type of wall will perform well if you get the details right.

    I will say this: I strongly recommend that you don't use XPS sheathing, since the blowing agents used to make XPS have such a high global warming potential. If you want foam sheathing, use polyisocyanurate instead.

  4. user-954155 | | #4


  5. PLnQUWAkNS | | #5


    I'm struggling with similar problems on my planned build this summer. Only in my area (upper midwest) I'm having troubles finding much available locally with high SHGC numbers.

    The leading contender for me are vinyl double pane windows with a U factor of .3 and SHGC of .2 (Gerkin). I wanted higher SHGC numbers for my south facing windows and also wanted to explore triple paned windows for better U factor so I priced out Marvin windows (wood clad and lots of coating options) and they are 3 times as expensive as the double paned Gerkins. As much as I'd like to go that route it's just not worth it for me. I'll just deal with less solar heat gain in the winter.

    It seems that jumping from vinyl to wood is the biggest price increase so if you're looking at wood anyway maybe the price difference from double to triple pane isn't that great. When I priced a .3/.2 (U factor/SHGC) vinyl Gerkin window as opposed to a .3/.2 wood clad Marvin window the wood clad was 2.5 times more expensive. Going from a double pane wood to a triple pane wood wasn't near as big of a price jump.

    I've been using Rescheck for heat loss calculations (take the UA number times the temp differential between inside and outside to get BTU/hour loss) and going from a .3 to a .2 U factor window in my 1000 sq. ft. single story gains me about 1000 BTU/hour. Maybe $150/year in energy savings for most people? I heat nearly 100% with wood that I cut and split myself so there's no real $$$ savings for me, just less work.

    That's ignoring higher SHGC on south facing windows though.

    I'm in climate zone 6, BTW.


  6. user-869687 | | #6


    If you live in a climate region where high SHGC windows at the south will save you energy, then don't stand for what these window companies are offering to sell you (if that's 0.2 SHGC). Let them know that you will not purchase windows that don't work for your well designed house. If these companies already purchase glazing from say Cardinal, then they can order 179 coating rather than 366 when that's what the customer needs.

  7. PLnQUWAkNS | | #7


    There are a couple companies available in the area that do offer higher SHGC numbers (.5) but, for me, they're prohibitively expensive considering my energy usage.


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