Guys Inside and Out: Understanding Window Ratings



How would you like to have the “windows” of 13th century Rome in your home in those recent rainy months?

They were simply a hole built in the roof to allow natural light and ventilation.

Just 200 years ago, many homes minimized their windows and used everything from flattened animal horn and fabric, to greaseproof paper and wood, to close the opening in bad weather.

As late as the mid-1970s, single-pane windows with aluminum frames were the “stock” item in supplier warehouses and only the more expensive homes had glass and wood frames. double glazing.

Indeed, windows have come a long way in a relatively short period of time.

Data shows that up to 70% of energy loss in a home is through windows and doors, 90% of which is through glass.

Until the 1970s we weren’t culturally concerned with energy efficiency, let alone saving energy, so who cared if the heat loss was so great that frost would build up around the ledges? window during cold periods?

It was such a common occurrence and a contributing cause of window sill decay that we used to ‘upgrade’ buyers to synthetic ‘marble’ ledges knowing we wouldn’t have to replace them. five years later due to dry rot.

After the country went through a few oil embargoes that skyrocketed our home energy bills, we became more aware of the need for a better product.

In the early 1990s, national energy codes began to impose minimum energy performance for various housing products, and it was from this requirement that testing standards evolved to allow us to compare various fenestration products. (windows and doors).

Groups such as the National Fenestration Rating Council, NFRC, have become the go-to resource for window and door manufacturers looking for a competitive advantage with energy efficient products. Nationally, the Energy Star program has evolved to create minimum energy performance standards for a variety of products, from computer screens to windows.

Today, you can simply go online to one of these groups to check the energy efficiency of a window or door that you want to specify.

Meanwhile, some groups were trying to limit the amount of glazing, reading windows, that could be incorporated into a home. According to their proposals, you would never have a room with windows on all three walls. Fortunately, the Home Builder’s Association and other groups have succeeded in fending off these “windowless” advocates.

The compromise was to require energy performance tests and a minimum evaluation for any fenestration product used in new or replacement construction.

If you look at a new window, you will see a label on the glass showing the U value, a solar heat gain coefficient, SHGC, a visible transmittance, VT, and an air leakage index, AL. These are the minimum requirements for a new product. Some units may also have a condensation index indicating the resistance of the unit to obtain condensation on the glass.

Most of us recognize the R-value of thermal insulation materials as representing the resistance of materials to heat transfer. A higher R-value equals better insulation. The U value is used to indicate the amount of heat flow that occurs through a material or set of materials due to the difference between indoor and outdoor temperatures. A lower U value therefore equates to better energy performance. It is represented by a fraction of 1.

The 2018 International Energy Conservation Code, IECC, requirement for windows in this part of the country is U = .30, equal to an R 3.333. In the 2015 edition of this code, it was U = .32. There is a product on the market today that claims a U value of 0.05, which equates to an R 17!

SHGC is an estimate of the amount of solar radiation from the sun entering the unit. It’s also expressed as a fraction of 1, so we want a lower number like 0.32 for that. It can help keep summer spaces cool and minimize damage to furniture and people from UV rays.

VT is an indication of the amount of actual light the window will allow. Again, it is expressed as a fraction of 1, so higher is better for many homes that seek a lot of light from these windows.

AL is used to indicate the amount of air that is actually flowing through the unit. Expressed as a fraction of 1, you want a lower comparative score.

There are also films to consider when evaluating effectiveness. You may have heard of low-emissivity films that reflect the sun’s harmful rays and can also reflect heat back into a home’s space in winter. Some manufacturers will use more than one of these films to improve window performance.

In most of these multi-pane windows, there is either air or a vacuum between the pieces of glass. But we’ve found that if we fill that space with Argon or Krypton, we can slow down the heat transfer even further.

In the end, the best window that is improperly installed will not perform well. Excellent installation technique and practice, combined with a solid written warranty, can help you make a difficult and hopefully once-in-a-lifetime decision about the best next window for your home.

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