Window selection discussion

[Pat]

OOOOWEEEEE, I'm tying my brain in knots trying to optimize window selection parameters. A little knowledge is a dangerous thing and boy am I DANGEROUS!

I understand the meaning of the terms used to describe window performance:

U-factor is the "heat loss factor", the inverse of R-Value

VT is visual transmittance, how much "daylight" a window provides (and allows fading of fabrics, etc)

SHGC is solar heat gain coefficient. This is the percent of sunlight hitting the window that makes it through. This differs from VT as it includes the IR (heat rays) as well.

AL is air leakage. Important but with decent new windows not as important as U-factor and SHGC. I, of course, want to make your ears pop when I slam the front door, so low AL windows are GOOD.

This is where things begin to get more difficult. We are located near "the line" between the Central Zone with significant heating and cooling loads and the Southern Zone with most significant loads being for cooling. Any given year is unpredictable and we may have weather more alligned with the Central Zone or the Southern Zone so you must design to be OK in either situation.

I hope to kick off a discussion that would be of general interest to folks building a house, remodeling, or updating their windows through retrofit. I'll use my house-under-construction as a concrete example but would be happy to read about other folks and their ideas and chalenges regarding their new house, remodel, or retrofit.

A good web site for background info is:

http://www.efficientwindows.org/ where you can get window selection info for any state. I think the US DoE is behind this site so it is your tax money at work for you.

If my new house were a simple rectangular econobox with standard roof overhangs it would be a lot easier to pick windows. I have windows shaded by porch overhangs and windows with just a bit of roof (eave) overhang. Some are on the south side and some on the North. Some face NW, some SW, some SE, and one is in a west facing gable end with no effective overhang. I am thinking that different window technology might be optimum for different window situations of orientation and coverage (shading).

Ordinarily I wouldn't mix window technology in one room because of the different VT and appearance would make it look a bit odd. I'm guessing that windows on the north side could have different technology from the south side, even in the same large room and probably not be noticed. You sure won't see them both at the same time from outside and being on opposite sides of the house (north and south) I'm guessing that the same windows would look different. The light of a north facing window is cooler and more blue in color (higher color temperature say 7000 Kelvin) while a south facing window has warmer light, maybe 5-6000 Kelvin (just guessing not measuring.)

So, the same windows on both sides would actually look different, especially on a clear sunny day but not so different on an overcast day. I'm guessing the different window technologies would have similar differences and if the different windows weren't side by side would probably not be noticed.

I'm just guessing here because I haven't read where this topic was covered but I'm thinking that with windows well shaded by porch overhangs on the south side that I can use windows with higher SHGC and hopefully higher VT to get better daylighting as long as I have a good U-factor to keep the windw from loosing to much heat in winter. Then for windows that arent shaded, say above the porch roof in dormers, I should have a lower SHGC to cut back on solar heating in summer.

For the north side whether under a porch roof or not, there is not much radiant heating so SHGC shouldn't be a big deal so good VT should be easy to get and low U-factor is still desired so you don't loose too much heat in winter.

I know there are windows that are electrically adjustable from transparent to translucent for privacy and they are quite pricey. Maybe one day there will be electrically alterable or otherwise adjustable U, VT, or SHGC capability. Meanwhile, I have to build with what I can buy now.

Anyone have any thoughts?

[img]/forums/images/icons/smile.gif[/img] Pat [img]/forums/images/icons/smile.gif[/img]

[slowzuki]

Wow, that is a long post. I'm building a shop this coming year and all my experience, reading and intuition has this to say:

Fixed windows always perform better than an operable one given a fixed price. If needed, install a smaller window and include an operable vent below or above depending on your house config.

Correctly designed overhangs work better than any fancy coatings. The tough time of year for overhangs is late fall when you can still get hot days + sun is low enough for solar gain. Drapes can help here.

Correct drapes and using them far exceeds any window coating performance.

Thats it, all I know [img]/forums/images/icons/wink.gif[/img] I hear your pain as it is a retrofit, so wading through coatings is your only hope!

I live in Canada so the slant is on winter performance here. It gets really hot here in the summer too, but the average is very low.

Ken

[Pat]

Ken, I did three consecutive winters in Minot, ND (25 miles south of Canada near Winipeg) and if your winters are anythig like in Minot, I know where you are coming from.

For the larger holes close enough to the floor to reach, I will likely use fixed windows flanked by opening windows, probably sliders. In the sunporch I will have fixed windows above the 6'6" level and sliders below. I raised up a short wall in the sunroom from 12 inches to 18 on advice of my contractor so the tops of the 5 ft high windows wouldn't be right at eye level and obsure the view (I missed that little detail).

I really don't like a lot of curtains and drapes. It seems contradictory to go to the expense of windows and then cover them up. A blank wall will outperform even a fixed window if thermal and infiltration performance takes first priority and if you cover it with enough cloth they perform the same light wise but the blank wall has superior infiltration and R-value characteristics. Insulated interior shutters for night use or MAYBE thermal drapes suit me but we'll see what my wife negotiates.

This is new construction and the window holes are all cut and framed just waiting for the right time to put in the windows, hence my elevated interest. I hear yoiur overhang comment, I guessed just right with overhang and Window height for my mom's 10 by 33 ft sun porch. I just came from my house site and the other day took some pix of sun angles in AM and symetrically in the PM: all looks good so far. If anything the porches will shade a fair bit more than optimum as regards solar control but the sun porch should be terrific. I'll probably have to have somecurtains or drapes over the dormer windows up above the porch roof in the vaulted great room with the control cord dangling 15 ft or so to control them. I don't like the electric ones all that much.

I found a good (FREE) program for doing window calculations and comparisons. Is is called RESFEN for, I guess, Residential Fenestration.

It came from this site: http://windows.lbl.gov/software/

Other S/W there as well, all free, compliments of the American Taxpayer.

[img]/forums/images/icons/smile.gif[/img] Pat [img]/forums/images/icons/smile.gif[/img]

[gsganzer]

Make sure the fixed windows will meet the building code regarding egress in case of fire. The building code usually stipulates the number of openings required in a room that will allow escape. I know that some of the casement windows won't open enough to allow you to climb through the opening.

[Chillimau]

As far as the frame construction, extruded, peltruded what ever they want to call it, fiberglass would be my choice. However, I don't know how cost effective this material is as of yet. I believe it will be cost effective soon, but soon is today for you! Fiberglass expands and contracts at a very similiar rate as the glass with temperature changes. Vinyl on the other hand is the most temperature sensitive and is the most popular currently.

I spent Christmas in a 2 year old home with vinyl windows. What ever the brand of windows are in that home, I wouldn't take if they where given to me. Vinyl frames, creaked, popped, leaked, whistled, wiggled. Winds outside ranged from 15-30 mph, nothing extraordinary, but a good wind in Oklahoma. You could see the curtains moving from the infiltration and it was nice laying in bed with the head board under the windows listening to the windows creak and pop and have the covers up around your ears. And temperatures outside where in the low 50s! Made the whole house uncomfortable. So far, I am extremely unimpressed with vinyl windows.

Either Fine Homebuilding or Journal of Light Construction recently had an article that a Canadian firm wrote about windows. I don't remember the exact numbers, I'll try to find them, but many brand new windows would leak water into the wall cavity and given a few years all windows tested leaked water into the wall cavity. So, when the windows are installed, proper flashing is critical. Although, they only tested windows available in Canada, many of the same windows and manufacturing techniques are used for windows in the United States.

I would buy windows with the expected effective life span (10-15yrs) in mind. The windows can probably used 25yrs or more, but weather stripping, glazing seals deteriate and eventually is it cost effective to replace the windows due to lost performance over time. In other words, if you stay in the house long enough, windows are not a one time purchase. And technology and materials will probably be better 10-15 yrs from now.

One other thing is to see how easily the glass section can be replaced when a rock from the weed eater, lawn mower or aspiring ball player cracks the glass. Some windows, you will need to buy the entire sash to fix a broken pane.

YMMV

Gary

[gsganzer]

Pat,
Check out Marvin windows, if you haven't already. I looked at them when I was looking for replacement windows and was very impressed. They have a model that's wood construction inside with a fiberglass cladding on the exterior.

Unfortunately, none of them worked for my 35+ year old windows and openings. Custom made would have killed my budget and couldn't be justified for the cost return. If I was doing new construction or had more standard openings, I would have used them.

[Pat]

gsganzer,
The fixed windows I am intending to use are in places like 15 ft above the floor in the great room and above 6'6" in the sun room which has three doors, and in other locations where there is no safety consideration. You need a second route of egress for any room where you sleep. It can be a window but doesn't have to be. All windows need not be egress windows. It is good to exceed the minimum safety standards but you don't have to go WAY overboard. Up in Canada, the source of the fixed window stated preference, they often have a second means of egress in the form of an escape hatch way up by the peak of the roof to get out when the building is snow bound.

Down here in sunny south central Oklahoma we don't have that problem. There is another consideration... If glass is within 18 inches of the floor and other reasons you need tempered glass to meet the safety code. I'm not fuly up to speed on all this stuff but my GC is.

[img]/forums/images/icons/smile.gif[/img] PAT [img]/forums/images/icons/smile.gif[/img]

[Pat]

Gary, I got a recomendaton from my HVAC guy for a window place in Goldsby (near Norman). He has used their windows in two of his own houses. I talked to them today and they have about anything you could want in any price range. One I am considering is vinyl with 40 yr guarantee. It has glass breakage coverage and free replacement for (I forget how many years) and then prorated out to 40 yrs. It has some aluminum reinforcement embedded in critical locations for added strength such as in the locking mechanism area.

I think F/G or GRP in UK dialect is good but pricey. Wood is not an option as I WILL NOT INTENTIONALLY INCREASE MY MAINTENANCE. This essentially leaves aluminum, clad, and vinyl. There are good vinyl windows but they aren't builder grade cost competing versions.

So am I crazy or what? I'm considering jalosie (sp?) windows for the sun room. Wouldn't have to take them down in summer and in the inbetween seasons you could keep up with weather reverses withoiut carting windows back and forth. Sure ot as high performance and plenty of infiltration in a strong blow but it is a sun porch not a bedroom.

[img]/forums/images/icons/smile.gif[/img] Pat [img]/forums/images/icons/smile.gif[/img]

[Pat]

gsganzer, I have seen most all of the quality national brands and compared the waranty and price to brand x. Frequently the BIG NAME windows are not a good deal unless you NEED to be able to name drop. Of course you have to devote a lot more time to researching the lesseer known brands and sifting out the junk. Federal Energy Star standards have begun to make posted specs more meaningful but still yo have to be careful and read between the lines.

We have over 90% of the window holes roughed in and can fir down a bit if required. Different manufacturers supply different actual dimensions when ordering the SAME stock dimensions.

Luckily my GC had no faith in the V-Buck plastic window framing used with ICF construction as the installers didn't brace sufficiently and the rough window openings are all hour glass shaped. We still have room to shim down, luckily.

[img]/forums/images/icons/smile.gif[/img] Pat [img]/forums/images/icons/smile.gif[/img]

[Chillimau]

Pat, Here is another window disscussion that sound interesting and has some comments worth researching.

JLC building science forum - low e glass