What is a window u factor

What Does U-factor for Windows Mean?

When measuring the energy efficiency of a window, you will want to know the level of insulation it provides you with. This is done by measuring the U-factor which ascertains how much heat is transferred through a window. The lower the U-factor, the better the resistance to heat flow in and out of the window. This means you get a better quality window. The values for U-factor range between 0.25 and 1.25. The optimal measure for your vinyl window will depend on your location.

How to determine U-factor for Windows?

A U-factor rating generally refers to the entire window assembly including the glazing, the frame as well as the spacer. The spacer is a part of the window that separates the glazing panels and this often results in the U-factor level dropping a bit.

High performing double window pane set-ups will have a U-factor of 0.30 or lower. In fact, triple-pane windows go down to as low as 0.15, which speaks a lot for their energy efficiency. Some window manufacturers have begun to add coatings that emit lower as well as use gas in spaces to decrease the U-factor level.

A few people also take into consideration the center of glass factor — this refers to the rating of the glazing on the glass alone. Though not common, it is sometimes asked for as well.

Energy Efficient Windows for Various Climates?

Knowing the U-factor of your vinyl windows is beneficial whether you are in a cool climate or a hot one. Insulation is necessary for a home to keep it at ideal temperatures. Take a look at some of the major climates and the necessary U-factors to deal with it.

Cold Climates

These are climates that require heat to be retained in a home. In such cases, a U-factor of 0.30 or less is ideal for windows. If you have skylights in your home, then 0.55 would be the ideal level to look at. If you do not depend extensively on air-conditioning, you will want your windows to retain some of the heat of the sun during the day. In such cases, 0.32 U-factor is good.

Mixed Climates

These are areas where you will need heating and cooling in equal measure. For such homes, a U-factor of 0.32 or less is best for windows. Skylights should not exceed 0.55. To help determine what U-factor you are looking at for your home, examine your heating bills. The higher the bill, the more necessary it is to have a window with a lower U-factor. This helps increase energy efficiency as well.

Hot climates

Hotter regions are where you need a higher cooling factor. In such cases, you should look at a U-factor of 0.60 for windows. Skylights may be at 0.70. Such U-factors are also useful in colder times when you will increase the heating in your home, and this keeps the heat in.

U Factor Windows FAQs

How do I lower the U-factor of my windows?

Manufacturers offer numerous choices for lowering the U-Factor (a lower U-Factor means the windows keep more of the heat inside). They could utilize low-emissivity coatings on the window glazing (layers of glass), numerous layers of glass (glazing), gases between the layers of glass, or thermal breaks.

Is it true that a greater U-value is better?

Thermal transmittance is measured in m2K/W units, with a larger figure indicating better performance (in contrast to the lower figure desired for U-value).

What U value should I aim for?

The ideal insulating materials have a U-value that is close to zero; the lower the number, the better. According to current building codes, the elements in a new structure must have the following maximum U-values: 0.3 W/m2k for the wall. 0.15 W/m2k on the roof.

What is the u factor in window ratings?

To convert the U-factor of a window to its R-value, divide 1 by the U-factor. For example, a window with a U-factor of 0.25 has an R-value of 4, which is computed as 1 0.25 = 4. What is the U-Factor, and how does it work? The National Fenestration Rating Council created the U-factor rating system (NFRC).

U factor (also known as U value) is the most referenced metric of performance when it comes to replacement windows, and for good reason. The total unit U factor tests the entire, finished window assembly for thermal performance at a 5mph wind speed. Essentially, this rating tells you how well the unit will insulate, and specifically measures the rate at which heat can pass through, without regard to air infiltration. U factor is actually the inverse of the more commonly referenced “R factor” when it pertains to other building materials, however U factor is specifically used for fenestration (glass).  To convert, simply divide. For instance, 1/5(r factor) = .2 u factor. 1/.2(u factor)=5 r factor, or R5. Keep in mind that will this conversion generally holds true, it is not fully accurate as R factor measures the resistance to heat transfer, as opposed to the rate of that transfer.

Common Manipulations of U value

Unfortunately, despite the seemingly black and white nature of an independently tested performance rating, many manufacturers and window salespeople will manipulate these ratings to serve their purposes. Center of glass u factor (COG) is one such item. This rating is legitimately tested at the center of the glass unit which is going to be superior to the total unit U factor. Unfortunately, this rating really has no relevance to you the consumer, and is used purely to mislead and confuse. It will typically have a COG designation with it, however most people don’t really know what that means, and simply compare it to the total unit U factor of another product which is obviously not apples to apples in comparison. Another manipulation is to cite R factor, which while it can be converted, is not the proper rating for a fenestration product, and can also be fudged in various ways.

The NFRC rates windows by TOTAL UNIT U FACTOR, and therefore this rating should be used for comparison purposes to avoid manipulation.

How much actual difference does a change in U factor make?

Current 2015 Energy Star rating is .30 u factor or lower (lower is better). This rating is set to move to .27 for 2016. We are often asked, “Well what does that difference actually mean to me?” After all, the difference in R factor would only be a point or so. Well, the difference is actually a bit more profound than one might think. The general rule is that one-hundredth of a point in U factor equals a 3% change in performance. Therefore a change from .30 to .26 for instance, reflects a 12% increase in thermal performance of the window. Furthermore, when these differences are calculated into the average R factor of a wall assembly (how heat loss calculations are figured), it becomes even more profound.

Lastly, whether it is u factor, shgc, air leakage, or other pertinent window performance ratings, the number itself gives you even more information than just how that product is tested to perform. It is a significant indication of the quality of a given product. Great windows have great performance ratings, and poor windows have poor ones, with very few exceptions to the rule.

If you have any questions about this topic or any others regarding replacement windows, contact us today! (262) 784-4460

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