Thermal Mass / Home Insulation Question

[geo.gs]

So I went back and read http://www.mrmoneymustache.com/2011/09/01/what-is-thermal-mass-and-how-can-it-make-you-money/ today. It got me back to thinking about ways that I could make my home more energy efficient.

Currently my house has plaster and lath walls. There is no insulation in between the 2x4 studs. On the outside of the studs there is 1x8 boards loosely hammered up, 1/8 inch of pink foam wrap, and then vinyl siding.

From everything I have read you do not want to add blown in insulation in between those studs as it causes mold problems in the long run and usually doesn't work.

I live in Wisconsin. Heating was extremely expensive last year (about $120 a month in both January and February) even though we had a warm winter.

My dad suggested tearing out the plaster walls, adding in fiber glass insulation, and then installing drywall. Sometimes I think that this would be a good idea (maybe I would use closed foam insulation instead of fiber glass) but I can't say that I am in love with the idea. Especially since I have read that the house would then not block/dampen out-side noise as well.

However, regarding thermal mass, would it instead be advantageous to tear down the plaster walls, fill in the 2x4 studs with more 2x4s by screwing and caulking them together, and then drywall over that? The idea is that the whole house would end up being basically a log cabin with an r-value of 8-ish (which is probably better than I have right now) and a whole lot of thermal mass from the solid wood walls.

I've tried looking up more information on the retro-fitted solid wood wall idea, but haven't really found anything. I'm guessing that either my keyword selection is bad or that the lack of relevant articles is an indicator that this isn't a good idea. Anyone have an insight to this?

[Jamesqf]

I'd suggest doing the same thing, but working from the outside of the house.  That is, you take off the existing siding (metal, in my case, and in need of replacement anyway) down to the sheathing, and build outwards from that.  In my case, it was 2" foam insulation, plus replacing all the old single pane, aluminium-framed, and leaking windows with double-paned vinyl ones.

Of course the first (and often the easiest) thing to do WRT insulation is the attic/ceiling.

[ShavinItForLater]

It sounds incredibly expensive and not likely to give you any better insulation R-value vs. putting in fiberglass insulation--probably worse.  You can fit R-13 or R-15 insulation in a 2x4 cavity.  Even if I was wrong, I have to imagine that any cost savings over traditional insulation would be more than outweighed (no pun intended) by the additional cost of solid 2x4 walls.

The ideas in the article are mostly with finished materials, where you have the choice of using heavy or light materials--but as MMM stated, he didn't do all that *just* for the thermal mass--it was just a side benefit.  Also, as I see it, thermal mass is really a complementary concept to insulation--they both slow the temperature change, but in different ways.  I still don't think any "dual effect" of solid 2x4s would overcome the cost.

I have a similar issue by the way, very old house with plaster and lath most places.  I would put in fiberglass in, at least under the attic floor if not the walls, but unfortunately I also have lots of knob and tube wiring, and as best I can tell it's not worth it to spend thousands of dollars to replace the wiring for the vastly smaller potential savings on energy costs.  I've started with the more cost effective fixes like sealing doors and windows and outlets to reduce "leakage", as well as CFL/LED bulbs and the like. 

I did start a project to staple some insulation to the attic roof where the attic is unfinished--it's a walk up attic with plaster walls and ceilings in 2/3 of the space--but ran into some ventilation issues because the stucco soffits have no vents, and I'm worried they'd sag and fall off if I cut holes in them--they've already been repaired once for sagging.  Replacing the soffits with wood, repainting, etc. started to look like more cost than it would be worth.  In the meantime I cheaped out and just laid some insulation over some of the attic floor, since we don't really use it except for storage (thermal mass!).

[gooki]

What your suggesting is more likely to create thermal bridging. Which is not productive in increasing your homes thermal efficiency.

[atelierk]

From everything I have read you do not want to add blown in insulation in between those studs as it causes mold problems in the long run and usually doesn't work.

Actually, my understanding is just the opposite. Blown-in cellulose (shredded newspaper) is treated with chemicals that not only make it fire retardant, but deters mold, rodents, and bugs. I would think that if someone's getting mold in their insulation, then they've got other, more serious problems that should have been addressed first - like how/why is moisture getting into the stud cavities in their walls?

My parents' house was built in 1962 and had 1" of insulation in the walls, and 3" in the attic. About 5 years ago they had cellulose blown into the walls (can't remember the resultant R-value) and attic (close to R-50 there). It has made a huge difference in their comfort and heating bills. It's worked fine. Not sure where you'd get the idea that it "usually doesn't work". It's a very common method for retrofitting poorly insulated homes here in NY State, and is one of many acceptable approaches for the state rebate programs that encourage energy efficiency.

[velocistar237]

However, regarding thermal mass, would it instead be advantageous to tear down the plaster walls, fill in the 2x4 studs with more 2x4s by screwing and caulking them together, and then drywall over that?

Plaster has more thermal mass than drywall or wood, and what you're proposing sounds like creating a giant thermal bridge. We went with the blow-in cellulose. It's three times as dense as fiberglass insulation, so it reduces the noise better. Insulation will save you more than thermal mass will. Blow-in cellulose plus foam board under your siding is probably your best option. If you want thermal mass, fill a box with dirt. Paint it black and stick it in the sun, or run a pipe through it and turn it into a rocket stove mass heater.

[Jamesqf]

Also, thermal mass is probably much more effective in the intermountain west, like Colorado or northern Nevada (where I live).  It's just a matter of daily temperature fluctuations.  Hereabouts we see summer daytime highs around 100F, and nighttime lows in the 50s.  With any sort of thermal mass & halfway decent insulation, opening the windows after sundown allows my house to cool off, and it will stay cool into the next evening on all but the very hottest days.

[geo.gs]

Everyone, thank you for the suggestions, insight, input, etc. It sounds like a solid 2x4 wall isn't going to be a winning idea. Guess I'll probably look into insulating the walls eventually instead.