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mincus 02-10-13 08:48 AM

Insulation ideas for 2nd floor
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!

hamsterpower 02-10-13 04:52 PM

First idea- I think the rocks idea would only work for walls heated by the sun. But if you use you outside walls it ruins the insullation you do have. If you windows are big enough and placed right, an interior wall in direct sun light might work. But this also slows the heating as much as the cooling. not sure if there would be a net-gain.

Second idea- if you intend to stay that long and you are not worried about payoff period, go all out the first time. Take down the drywall and the fiberglass. Seal the walls with an inch or more of sprayfoam. Build another 2x4 wall with offset studs inside. Finish with cellulose and new drywall. I have done just that in two of my four rooms on the main floor of my house and there is no perceivable loss of space and it's SO much warmer and quieter. I hope to do the next room this summer.

It is not as easy as it sounds to move the electric to the new wall if you leave up the drywall. You will likely need to extend the wires, that means unaccesable junctions in the old boxes, that is against code and bad form.

mincus 02-10-13 09:19 PM

I think that because they would be slowly gaining heat throughout the day, the rocks would still work. They have 8-10 hours to collect heat, even if not from direct sunlight. I know most thermal mass material you read talks about having it in the sun, but obviously that's not always possible. Some of the rock filled walls would get some sun, but not all. And no, I was not going to use an exterior wall :) I think the only reason there would be an overall gain is because I use heat pumps. If I was using a gas furnace, it runs at the same efficiency day or night, and it likely wouldn't make much difference. However, my heat pump is far more effective during the day than at night. My overall idea is to build heat up during the day and be able to keep it through the night. This makes my heat pump run less frequently, and is easier on it by running it more during higher outside temps.

I do like your wall idea. However, I was planning on doing each room individually over the course of a couple years. So, doing spray foam would be hard, as would blowing in cellulose (renting the machine each and every time, etc). Wondering if I could use XPS foam, cut slightly smaller and sealed on the edges with canned spray foam. I know I've heard this technique for sealing rim joists. Would that work if I pull out the old fiberglass?

In doing this, are you renting a machine each time? What about the spray foam?

With regards to the electric boxes, I am reasonably sure that the wire in and wire out both go up to the attic for each of those boxes....although now that I think about it, maybe I'm wrong....seems like it would be a waste of wire. Assuming that is the case, moving the box out would not be hard at all. It would just be an inch or two higher on the new wall. I still have to investigate this more to make sure. Otherwise, I could just put in extra outlets and move the position of the current ones. This would of course require taking out the drywall, like you suggest. Now that I get to thinking, removing the old drywall is sounding better and better. It would allow me to put in the foam (increasing my r value for those first 3.5 inches), not worry about wires, tie my new wall directly to the old one at top and bottom, etc.

Any idea about how good XPS is at reducing sound compared to cellulose or fiberglass?

I would never hide a junction box. Had several of those in my old house and it made me quite angry!

MN Renovator 02-11-13 12:08 AM

"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." Awesome to know. I've got 7 inches or so of cellulose (R25) and am looking to 16-20 inches(~R60-R75) and it's good to know it's very noticeable. I figured it would but it's nice to hear of a good confirmation that it's not just a small barely noticeable difference.

The wall inside a wall sounds like a ton of work. Moving the electrical outlets out isn't easy because the wires are too short and I wouldn't want a whole pile of wire splices in my house's branch circuits, that's a whole bunch of twisties in junction boxes to trust. My own opinion on the matter instead of tearing down this drywall or constructing new stuff would be to install a 2" layer of rigid foam on the outside of your house. IMHO that's easier because it is less invasive, can be done when you are doing your siding anyway, and also cuts the thermal bridging and you can seal the gaps between the rigid foam sheets and create a more airtight enclosure that way.

Rocks in the walls, not sure you'll get that much thermal mass doing that, usually thermal mass is done with huge chunks of concrete fashioned into the house, such as a kitchen island. I'm not saying rocks in the walls won't make a difference but it might not be as nice as you think.

Best of luck, you've got a great start.

Blue Bomber Man 02-11-13 08:24 AM

Id suggest looking into the mooney wall concept for your exterior walls, it eliminates like 98% of thermal bridging in a wall and drastically increases effective R values.

It works great when you are remodeling and is very cost effective.

I believe has quite a bit on the subject.

Good luck!

hamsterpower 02-11-13 09:01 AM

I like the sprayfoam method over the foam sheets because there is little to no waste. I buy a 600sf fast rise kit from Tigerfoam for each room and after all the prep. I just keep spraying until the tanks are empty. I think it works out to equal or even cheaper than all those little cans to seal around the sheets.

Renting the cellulose machine from the nearest box store usually costs $20 or less and I can return it the same day. I know SF will disagree but I've had good luck with getting a nice dense fill on my walls.

I could save more money by doing two rooms at a time as my rooms are small and I end up with about 4 inches for sprayfoam on the walls. But I am happy with the results.

mincus 02-11-13 07:37 PM

MN: You live in minneapolis and only have 7 inches?!? Woah!

Not ready to replace the siding, so I didn't want to do the foam on the outside. The house is pretty big, so new siding would add a huge cost to the project.

Blue: I've looked into the mooney wall, and think it's a great concept.

I really like the idea of taking out existing fiberglass, and doing a couple inches of foam on the back, against the outer wall. This would give me the airtight seal, but leave room for cellulose or fiberglass. That would only leave me about 5 inches to blow in cellulose. Would I be able to pack it well with only 5 instead of 7 inches? I know you dense pack it, but does anyone worry about settling over time? It looks like in the mooney wall they have a depth of around 5 inches. Do the horizontal 2x2's in the mooney wall help to prevent it from settling? I don't want to do all this just to realize there's not insulation in the top foot a few years down the line!

hamster: So, do you dense fill with cellulose on top of your spray foam? If you put in 4 inches of spray, how many inches of cellulose do you have?

Thanks for the ideas guys. I'm starting to narrow down my options!

hamsterpower 02-12-13 04:00 AM

I do the best I can at dense pack with the tools I rent from Lowes. We do worry about settling, that's why we try to dense pack. I leave a little gap between the outer original studs and the inner new studs so I get 3-4 inches of foam and 4-5 inches of cellulose. > R40

In one of the other threads SF told us how to use mesh netting to hold the cellulose, before the drywall goes up. I will try that next time so I won't have to patch new walls were the cellulose goes in. I am confident that I got firm, continuous fill with the cellulose but the mess would make it easier to be sure.

kabutomushi 02-22-13 06:42 AM

If I were doing an inner wall, I would take the old drywall and insulation down for several reasons:
1. I could inspect for rot, air gaps, etc.
2. I could install tyvek or similar air barrier.
3. I could pull staples on wiring to move sockets to the new wall.
4. I could get rid of the old vapor barrier and any potential complications from it.
5. I could choose to use foam insulation, or could at least use fresh, insulation.
6. I could install insulation properly--I have seen a lot of badly installed stuff.

S-F 02-23-13 01:52 PM

A couple extremely important things to remember when making a double wall in a retrofit: You will still have band joist issues. If you have decent replacement windows you will still have more air leakage through the band joist than in all windows combined (usually). Also there is the enormous thermal bridging issue there as it a band of 8"-10" wood wrapping the house on all sides. You also have one of these under the first floor which brings along the same kind of air leakage and thermal bridging issues. This is one of the main reasons that outsulating has become so popular. It's dead easy in a retrofit. To properly insulate and air seal in a retrofit from the inside you would need go gut the house. Or at least gut one part of the house at a time.

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