Insulation ideas for 2nd floor

[mincus]

A couple years ago, I bought a two story house built in '84. I've done quite a few insulation projects to keep in my precious heat during the winter. I have two heat pumps, one for the 2nd level, one for the first. I've been pretty happy with them so far.

I've been experimenting the last couple of years with the best way to run them in the winter. The first year, I just kept the house at a constant 66. They ran throughout the day, and almost constantly through the night (very bitter winter 2 years ago). The next year, I decided to start running it a bit more during the day to build up the heat so it doesn't run as much at night when the temps are typically in the high teens and low 20's. It worked well last year, but the heat pump was typically running by 2:00 or 3:00AM.

Last November, I insulated the attic. I had 8-10 inches of blown in insulation up there. I increased that to 22-24 inches of cellulose. I insulated all openings with expanding foam. The front and back sides of the house aren't that deep due to the roof slope and the need to keep adequate ventilation from the soffit vents. Overall, the added insulation has made a pretty big difference. I slowly increase the temperature of the 2nd floor throughout the day to 71. I drop it down to 63 at 9PM. With the added insulation, and the mild winter we've had, the heat rarely kicks on overnight anymore. It is typically around 65 in the morning. Pretty good I think! The main level was always more temperature "steady"....not varying as much due to outside temps (more thermal mass, no exposure to cold attic temperatures, etc). However, that has now switched. The 2nd floor now seems to suffer less from temperature swings.

Anyways, I'm looking at ways to even better keep in the heat overnight upstairs. I am planning on staying in this house for at least the next 30 years. So, although I don't want to spend large amounts of money, payoff period isn't a huge deal to me.

I feel like I've gotten most of the "low-hanging" fruit. While the windows are original, they're pretty decent wooden double planed windows, that I don't think I want to replace yet. I have weatherstripped them. I have a separate idea to build insulated shutters for the windows...that's farther down the line.

I'm thinking of two main ideas for helping upstairs. My first idea would help store heat during the day.....adding thermal mass. I've researched this a bit, and thought about adding bags of rocks to the walls. I would have to cut holes in the walls obviously, but am pretty versed in patching jobs. I was thinking of just buying rocks from lowe's, drying them, putting them in ziploc bags and dropping them in. Any thoughts on this? Would there be too much pressure pushing out on the bottoms of the walls? Could that cause any issues over time? My goal in this is to be able to only raise the upstairs temp to 67 or 68 in the afternoon (instead of the current 71). With the added thermal mass, I could possibly store the same amount of heat, without the added delta T. This should decrease the amount of heat loss to the environment, while providing the same benefits I currently get.

My second project idea is designed to help keep the heat in and reduce sound. These two goals are about equally important in my mind (we live along a fairly major street and the road noise all night bothers me). Growing up, we had 2x6 walls. My dad always raved about them, and I wanted 2x6 walls in the house we bought. Unfortunately, we fell in love with this house (the wife didn't seem interested in my insulation reasons for buying elsewhere :-). Anyways, I was thinking of adding on to the upstairs walls. Again, I've done quite a bit of research and realize there are many different options. I want to keep it simple. Let me explain my plans and let me know what you think.

I currently have a sheet of drywall on top of a 2x4 wall. Insulation is fiberglass batts. There is a plastic vapor barrier between the drywall and insulation. My first thought is to leave the old drywall up. Put a new 2x4 wall up next to the old one. I would stagger the 2x4's to prevent thermal bridging and fill the gaps with fiberglass batts (perhaps a bit overstuffed). All I would really need to do is cut the carpet back, move the tack strips (and move the electrical outlets out to the new wall thickness.

If I did it this way, do I need to remove that vapor barrier and add a new one underneath the new drywall? I have done a lot of reading about vapor barriers and the dew point, but frankly I don't quite get it.

Space isn't a huge consideration, although I really don't want to add more than 4" to any of the exterior walls. That would start to make the rooms feel a bit small.

Would I be better to remove the current drywall, and build my new wall right up to the wood of the old wall? I know there's not much insulation value in that old drywall, but I figure if nothing else it helps reduce any kind of air infiltration.

Any thoughts or help would be appreciated. Thanks!