Replace Windows or Install Insulation?

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Q: What is a better energy-saver: insulation or new windows? Our brownstone is very drafty, and we can only afford to do one energy saving application. Should we open our floors and put insulation or get new windows?

- Sent in by Alice

Editor: As a former student of Gail Brager, UC Berkeley professor and one of the world's leading researchers in building performance, we can answer this question with confidence.

Do the windows.

Here's the funny thing: overall, you would increase the insulation value of your house the most by adding insulation. But air takes a lot of energy to heat up or cool down, so when it leaks out through drafty windows, you have to use more energy to heat more air.

As a side benefit, if you replace the windows with new ones that are both double-glazed and that have a Lo-E coating, you'll increase the thermal comfort in your house. That means the glass surface of the windows will not get as cold on the inside, so you will be able to sit close to them in the winter without feeling a chill.

In fact, air infiltration —the building science word for drafts— results in 33%-50% of heat loss in the average house; if yours feels drafty, it could be even higher. (PDF with more than you probably want to know about this here.)

So, our advice is to go for the windows first: either replace them outright or carefully weatherstrip them and add storm windows, which might be a better choice if they are historic or in good shape otherwise. Then bank your monthly energy savings next winter and use it to insulate the house from the top down: attic, side walls, then basement.

Image via Flickr member Meg Zimbeck licensed under Creative Commons.