Any experience w/Southwall V-Kool?

Constantin

.... in a post last week, Geoff McDonnell made me aware of Southwall and it's interesting array of window films both for OEMs and (apparently) <a href="http://www.southwall.com/products/v-kool.html">after-market applications</a>.

Given the huge potential that these film could have in reducing the heat load of my home, I wonder if anyone has used them, and to what effect. I am considering them for the historic windows in my home, the storms in front of them, to be exact.

The V-Kool technology seems like a very good way to keep 50% of the IR out of the house while only slightly tinting the view from the inside. One detail that is unclear to me is whether the film is applied to the outside or the inside of extant windows. Any thoughts?

GMcD

Films

These films DO work in a retro-fit quite well. A lot of marketing in the Pacific NW also includes the seismic aspect- keeps the windows intact in a 'quake. Normally they are applied to the inside surface of the glass since window-washing on the outdoor lite will scratch the film. It's in the hands of the applicator- sometimes if they are not applied "just right" you can notice a slight grain or "texture" to the adhesive if you are close up focused on the glass, instead of looking through it. Still, the best solar gain control is to use exterior shades so you can use clear, un-tinted glass to maximize the natural lighting.

Constantin

Thanks, Geoff!

Well, I ran the numbers last night, and to say I'm stunned would be an understatement. Since the new windows in the home are already low-emittance, I decided to just "apply" the V-Kool product to the old windows.

Said old windows are reconditioned (wavy-glass and all that) units with modern Harvey Tru-Channel storms in front of them. Since V-Kool rejects 98% of IR and has a shading coefficient of 50%, I went into HVAC-Calc to recalculate the storms to have "low-e, 50% shading, reflective" properties.

Here are the results of the before and after. Is this a typical result, or are my settings in HVAC-Calc too aggressive?

Another question that comes up is to what extent this product helps with our net energy equation. Presently, the house seems to heat itself quite well... yesterdays top temperature was 50°F, the thermostats were set to 60°F, and the interior temperatures were never under 64-65°F. Thus, the insolation seems to heat the house quite well. If I apply these films, I can basically kiss that heat gain goodbye, unless I find a way to mount these films only in the summertime...

How do I find the best balance for energy efficiency between limiting insolation in the summertime and maximizing insolation in the wintertime?

GMcD

Net energy equation

Well, it's not that simple. First of all, you are in a heating dominated climate so any winter solar gains are beneficial. It really matters when you look at the energy sources used for your cooling equipment vs heating equipment. Typically the heating is provided (in many areas of North America) by much cheaper energy sources than the electrical energy used for cooling appliances. So the net gains by using solar control systems are mainly financial, in that you will save a lot more energy$$ by reduced electrical energy vs the loss of some cheaper heating energy. In a pure energy world where the cost of heat energy is the same as cooling energy, you have to run some iterative models at different SC values to see the best compromise depending on the yearly solar hours vs heating degree days.

Another thing to keep in mind- the published figures for the performance of glass and films is usually based on "centre of glass" values which provides the "best" numbers. The actual real-life performance is the "overall" performance of the whole window unit which includes the framing system. I use the freeware from LBL off their website - Window 5.2 - to do apples to apples comparisons of windows and glazing systems to get realistic values. One example I recently ran was some triple glazed aluminum framed stuff with a thermal break in the frame. The manufacturwers data indicated a U value of 0.26 (centre of glass). The actual true "overall value", taking the framing system into account, was a U=0.34. A significant difference if you've sized your heating/cooling equipment tightly!

I'm still a fan of adjustable exterior shading, or at least fixed exterior shading that keeps out the high summer sun, but lets in all the low winter sun. The window performance has to be climate specific and exposure specific, that gets architects all bent out of shape.

Constantin

You raise some very good points...

Boston is heating-dominated by a good margin and the solar gains in the wintertime are welcome indeed. Between the floor heating and the sun, I think we'll never get our cat off the floor again....

Anyway, I whipped together the following spreadsheet that shows the cumulative effects of degree days, fuel source, etc. I assumed a COP of 2.5 for a 19 SEER AC, which may or may not be conservative. Overall, we still heat a lot more than we cool...

As adding exterior shading is not a possbility in a historic district w/o reopening the process with the historical commission, I am going to step back and hope that the tall decidious trees around the house will give sufficient shade in the summertime to make the house AC happy.

Constantin

You raise some very good points...

Boston is heating-dominated by a good margin and the solar gains in the wintertime are welcome indeed. Between the floor heating and the sun, I think we'll never get our cat off the floor again....

Anyway, I whipped together the following spreadsheet that shows the cumulative effects of degree days, fuel source, etc. Overall, we still heat a lot more than we cool...

As adding exterior shading is not a possbility in a historic district w/o reopening the process with the historical commission, I am going to step back and hope that the tall decidious trees around the house will give sufficient shade in the summertime to make the house AC happy.