Month: April 2021

A: Your question on sound deadening has given me the perfect opportunity to test out the sound meter I have recently bought. Using a white-noise generator on my iPhone I measured the sound deadening of the four different window inserts (see pages 129-134 in the book). As I thought, the flimsy Window Inserts one is the worst, but it is not the glass Innerglass that is the best, it is acrylic-plastic sheet Indow. This is what my perception was, but I thought it was just due to the road noise being higher on the side where the Indow is. Cutting the noise level by 20 decibels is a big reduction. If noise reduction is more important to you than thermal insulation, I would consider the Indow or the Alpina one, which is considerably cheaper and has similar noise reduction and thermal performance.

I found that Alpina was not the best value for money, the Innerglass one pays back faster. It is slightly more expensive but it is a higher R value. The table with these results is on page 133 in the book. Here is the table on the sound deadening results:

A: I have seen them used in churches, but the window will often need two or three window inserts to cover it completely.

Q: Do you know about the availability of high efficiency windows that will be acceptable for homes in historic districts with strict regulations to adhering to historic accuracy?

A: I think probably the best way to deal with this is to add the window inserts you can see in Chapter 3 of Zero Carbon Home. These are invisible from the outside so historical commissions usually are OK. If you use the glass ones and get the low-E version, you can add about R2 to an existing window. If the existing window is R1 you now have an R3 window, which isn’t great compared to modern low-E triple-glazed windows at R4-5 but it is 3-times better (meaning it loses one third of the heat and dollars) than a single-glazed window. They are also far cheaper than replacing windows and you can fit them yourself, you don’t need to hire a contractor. 

Q: How are you calculating the R value? are you using the center of glass U value only? should you consider the frame? 

A: The R-values I measured for the window inserts are taken in the center of the glass or plastic. As you can see from the infrared photographs in Chapter 3 of Zero Carbon Home, the frames are all considerably warmer than the glass or plastic sheet. The frames also have far smaller area than the glass or plastic sheet. Hence, I think that the center of the glass measurement is a good approximation of the overall performance of the window insert. 

Q:  Just had a contractor tell me that triple glazed windows are a waste of $ in the desert southwest – – true?

A:  Replacing any windows is expensive. Windows are expensive to buy and expensive to install. The full cost of installing any windows is unlikely to pay for itself with energy-bill savings, at least not for decades. However, in our home the additional cost of triple-glazed windows above the cost of double-glazed windows paid for itself (with the additional saving from triple-glazed windows above the savings from double-glazed windows) in about 5 years for a return on investment of 19%. I think something similar would be true in Texas. See Chapter 3 in Zero Carbon Home.

A: Thick, lined, pleated curtains that go to the floor can add between R3 and R6 to any window. If you add these curtains to a triple-glazed low-E window (which is about R4 or R5) your total insulation will be almost as good as a wall, which is often about R10.

A: Window inserts are sometimes called interior storms because they do look a bit like exterior storms, however the function of window inserts is quite different to exterior storms. Window inserts provide insulation, sound deadening and draft sealing. Exterior storms protect the wood frame from weather but provide little insulation and little draft sealing.

A: In a hot climate you need to keep the heat out. In a cold climate you need to keep the heat in. The answer is the same in either climate: insulation and triple-glazed windows. There are examples of how much money you can save by installing double and triple-glazed windows in the book in chapter 3. In more remote places like Australia, the choice of windows is often less than we have in the US or EU. This often makes triple-glazed windows hard to find. I even found it hard to get them in the US, my windows came from Canada. If you cannot get triple-glazed windows I suggest looking for the best low-E double-glazed windows. If you use a low-E 272 coating on the outside and a low-Ei89 coating on the inside you will get about R4 insulation with only the slightest greenish tint. This is as good as a triple-glazed window with i89 on the inside.

A: No. We have R4 triple-glazed windows on the lower floor and R5 on the upper floor. They are completely clear to the eye. There are photographs of four different types of low-E glass in the book in chapter 3, so you can see exactly what they look like.

A: The optimal air gap is about ½”. This is supported by both academic research and experiments I did on my own with small double-pane windows that I made in varying thicknesses.  Almost all double and triple-glazed windows use this spacing. If the spacing is larger the ½” the insulation value goes down because then convection currents start to move the heat from one side to the other.