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Which U-Factor

I need to replace old windows that were installed in the 1980's (double hung wood windows) and 1990's (double hung vinyl). My home was built in 1900. I know that the lower the U-factor is the better to keep heat in but in a home that doesn't have great insulation would it really be worth it to get u-factor .17 instead of u-factor around .28? The windows I have right now are .6 u-factor. I can't get a straight answer from window companies. The cost is a big difference. I live in MA. Help!

Thanks, Newbe

Re: Which U-Factor

this should help you out, it breaks down what it means. even still there are contradictions in what is better. finehomebuilding magazine did a article on just this a few months back


Re: Which U-Factor

U-factor is the inverse of R-value. U=1/R. A u-factor of .17 would be the same as an R 5.9. A u factor of .28 would be about R 3.6 and the u .6 would be R 1.7

The u.28 windows will be about twice as efficient as your current windows and the u.17 will be about 3.5 times better.

Without a detailed heat loss assessment of your house, it would be hard to calculate which is the more economical choice based on total owner cost. Total owner cost would be the cost of the windows plus the cost to replace the heat lost through them over their lifetime.

As a rule of thumb, windows and doors account for about 20% of your total heating bill. Most of that 20% will probably be the windows. Lets say that your total heating bill for the year is $1000. That means that about $200 is going out the windows. If you go with the u.28, you could save up to $100/year. If you go with the u.17, you could save up to $143/year.

The u.17 could possibly save you as much as $860 over the next 20 years.

Now adjust these figures to your situation and compare those to the cost difference between the windows to see if the more expensive windows will be worth it.

With minimal insulation in older houses, the rule of thumb for heat losses would be about 20% windows and doors, 20% infiltration, 20% walls and 40% roof/ceilings. Could that difference in cost be better invested somewhere else to get more return on investment? Would you get a better rate of return on upgrading the insulation, caulking, more efficient heating system?

Re: Which U-Factor

jkirk and Keith,

Thank you both for the helpful information. After further research based on the information you gave me, I am now able to make a confident and comfortable decision for our situation.


Re: Which U-Factor

Insulation value also doesn't tell you much about air leaks. Better quality windows may have the same R value rating, but will seal out air better and continue to do so for a longer time.

I know I'd put my 90 year old single pane wood frame windows up against many newer vinyl replacement for tightness. I can maintain a dewpoint indoors of 50F, when the dewpoint is 72F outside. Keep in mind there's no vapor barrier in the house and I have a LOT of windows. 42 mostly large windows in a 3200 sqft home.

IF cost is an issue. IF you have some common size window frames, you could install and re-use several storm windows for a short term solution, then replace a couple at a time and play musical storm window.

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