I think all windows are the largest net heat gain & loss in any insulated wall. But am likely to install thermal shades to reduce heat loss in cold months. to supplement energy efficient windows. Over hanging pergolas and trees will be the summer buffer.

Another benefit of the cellulose is its reduction of air infiltration an fire retardant and insect resistance . Plus i’m creating rain screen exterior walls to for their many benefits. Again thanks for your information. Tim

I can’t comment on your home’s character as I’ve never seen it, but I would say that thicker walls probably make a bigger difference than you think. Uninsulated walls not only have really low R values but probably huge air infiltration. Why do you say it is surprising how little difference that makes? Have you tested it? Of course, doing both is the best, but I’m just looking for best bang for the buck here.

If your windows are facing south, even poorly insulated windows can have a net heat gain due to the solar gain during the day.

Cellulose certainly is much more sustainable. And if you have enough volume in the walls you can do a fantastic job with it. The one thing I would recommend is reducing infiltration. Easily 30-50% of your entire heating bill is probably due to draft conditions.

Best of luck with this project! It sounds like you’re asking all the right questions.

The sustainability of cellulose verses foam appeals to me as does the huge difference in cost. Settling of the insulation is either due to poor workmanship and myth.

Regarding windows, you need to balance cost, R-value, and your home’s character. If your windows have real character (old wavy glass, beautiful double-hung frame detailing etc), you might find that a combination of reducing air infiltration in walls & ceilings, combined perhaps with custom-made storm windows, might give you the improvements you need without compromising the character of the house.

If the windows truly need to be replaced, I still urge you to consider the character of the house. Despite a slight performance weakness, double-hung windows might still be good. While fiberglass is probably the best frame option, wood or aluminum-clad wood might fit the house better.

Beyond all that, the better the R-value of the window the better. Triple-glaze is great, but it also makes for a heavier window. The third pane of glass adds virtually no insulating value by itself; the benefit comes from creating an additional air pocket. So you might also want to consider a double-paned window that has a thin polyester film suspended in the middle. Much lighter and essentially the same R-value. Also consider argon or krypton gas filling, as well as low-e coating.