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Window Choices In Log Homes continued

2. Glass Types - It is important to get at least Low E double glass in virtually all US climates. Low E (Low Emission) refers to the ability of glass to transmit less energy, in either direction, because of thin reflective surfaces applied to one side of the glass. Modern high efficiency glazing boosts R-values to greater than R3. They can go higher, and not all R ratings are the same. Low E window glass from different manufacturers has different performance ratings. Low E glass offers another important benefit by greatly reducing UV radiation inside your home, allowing your furnishings to last much longer without fading and degradation.

There are many factors in the construction of a piece of Low E double glass such as the spacer bar construction, choice of sealants, the depth of the space between the glass, and what is contained in the space between the glass. Many of the better glazing systems fill the space between the glass with argon gas instead of air because it is a better insulator than oxygen. There are even better systems available, such as heat mirror technologies which use mylar sheets suspended between two sheets of glass to give the effect of triple or quadruple glazing, with R-values as high or higher than R5.

Be aware that windows can be rated for R-value in different ways. One of them is the R-value at the center of the glass, the other is the average of the whole unit. They are called respectively: "Center of Glass R-value" , and "Total Unit R-value". The best data uses both. Look for "Center of Glass" R values at or above R 3.5, and "Total Unit" values at or above R 2.5. Don't trust claims of unit values in excess of R-5, as only a very few top quality manufacturers claim values that high - even for fixed glass. Furthermore, center of glass R-values are the same for most sizes within the same product line and glass type while R-values for total unit are higher for large units than for smaller units from the same product line. If you have a large amount of glass (more than an area equal to 8-10% of your floor space), or are in a relatively more severe climate, hot or cold, get the highest possible R-values. For the most extreme weather conditions, consider buying a line of windows that offers factory storm units. Don't skimp here!!

Tempered glass will be required for safety in certain bathroom windows and tall windows or glass doors near floor level. Tempered glass should be standard right from the manufacturer in all sliding and swinging patio doors and deadlights.

3. Air-tightness - Windows can leak a lot of air if they are of poor quality, or if they are poorly installed. Windows are rated for leakage with a simple volume measurement, such as .03 CFM/Ft, (about three- hundredths cubic foot of air per minute per foot of sash). Generally speaking, casement and awning casement windows leak as little as one third as much as doublehung windows do. Even fixed windows such as picture windows and circle tops have leakage ratings. Check the specs. Look for CFM/Ft rates of less than .05 for casements, less than .15 for doublehungs, sliders and patio doors. This is a key factor in the tightness of your home. Another specification to check is the NWWDA class of window. It relates to the ability of a window to resist wind driven rain. This rating is important in high rise buildings especially where wind driven rain is a real problem. Minimum ratings for residential structures start at DP15. Focus on the air leakage and R-value ratings instead.

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