Adding thermal mass to stud-framed exterior walls

So I'm in the process of opening up my small house a little bit and moving the woodstove to a more central location, a utility room that is currently almost comically drafty and uninsulated. I'm essentially absorbing the utility room into my living space. Part of the process is redoing the exterior wall, resizing the window and door and trying to hold heat better. The wall is currently the root cause of most of the house's winter inefficiency, and I'd like to change that.

Caveat: given my cold Michigan climate, I'm not sure choosing thermal mass over insulation is the right decision to make, but given that the wall has southern exposure and the wood stove is going to be about 6 feet away, I thought it seemed to make sense in this case.

The wall is currently made up of studs on 16-inch centers, and not including the door, is only a few feet long. My original thought was to stack straw bales on the interior side and plaster over them, but that seemed like it would seriously encroach on the small living space so I had to come up with some alternatives. First I thought I'd build a small, gusseted frame on the interior side and blow in copious amounts of cellulose, which would work well, but because I like to make things difficult for myself, I started considering doing the same, but with light straw-clay instead of cellulose. The clay got me thinking about the possibility of using essentially no insulation though, and going with just solid mass. I thought this would allow me to store more of the heat, without making the wall overly thick and reducing floor space. Does this all seem like fairly sound reasoning?

Given that the wall is already in place, and there's no reason to take out either the sill plate or the top plate, I'm probably going to be infilling right into the studs. (1950's house so they didn't shirk on pieces of wood, which also means there's pretty extensive thermal bridging: another reason to go with thermal mass around them.) I see no reason why the foundation wall below shouldn't be able to hold significant weight, so I don't think I need to worry about that. The question is, what material will mesh well with the studs? My first thought here was to simply use cob, but I'm not sure how well the mixture would adhere to the wood. I also considered stones, stretching some kind of metal mesh across the front and filling behind it, though that would require first plastering up any leaks. I've thought about cordwood too, but again, not sure how well that would stick against the stud frame.

Any thoughts on this project? Perhaps it's misguided from the get-go, but I hope not.

As an addendum, the concrete floor of the room (over a small basement/crawlspace) will probably need to be further supported from below if the wall extends significantly into the room. I may be interested in installing a rocket mass heater... and have looked at several reinforcement options including just building up a foundation from below. Any thoughts on that?

This is tangential to your questions... But are you removing the interior walls of your utility room when you incorporate it into your living space?

I ask because much of the comfort provided by a wood stove comes in the form of radiant heat. You get the most benefit if it's in the same room people are in. If you are heating a little room and relying on the warm air spreading into other inhabited rooms (especially in a house with a lot of thermal loss to the outside) you're setting yourself up for burning a lot more wood.

Yes I am. I'm replacing the interior wall with three posts and a beam. If I don't try and build a rocket mass heater I will also install a half-wall/bench made out of cob to soak up some of the heat as well

Dan is correct air does not have enough heat capacity to transfer radiant heat in short wave (solar) or long to other mass by convection. Mass transfer heat to other mass by radiation, and to the human body the same way. How effective depends on the e-coating of windows, absorption of the mass material(s) and it's ability to store and reflect radiant heat, color. You can learn more here: http://www.healthyheating.com/Radiant_Design_Guide/Floor-covering-R-values.htm#Radiant Flooring Guide

The best method to add mass to 2X4 thin wall framing is to spray it with thin layer mix of 30% magnesium phosphate, 68% kaoline or bentonite clay, and 2% borate as a fire retarder. 3/8 mag board, Fiberglass seams with a 1:1 (mag phosphate and sand joint compound). Exterior thin coat of mineral silicate based stucco paint as water-proofer. Use cotton insulation it dries fast or use the spray above on dense packed cellulose. Aquarium grade nontoxic caulk. No vapor barrier, the MGO is non-conductive and a thermal break. Clay plaster or lime on the inside. Contact George Swanson he has the MGO and recipes or can find a supplier close to you: http://www.breathingwalls.com/drupal/drupal-5.14/

Other than that, mix a batch of hemp or straw in lime, little clay, create insulating mass at least 3" outside the outer studs, flush to the inner studs. Lets see, 3.5 studs + 3 = 6.5 wall. Add 2 inches of stucco and plaster = 10.5 wall, ouch! You'll need form work, and stuffing access at top. That should yield ~ r-16 with mass effect, CO2 absorbtion, insect and termite proof, sound dampening, etc..

Hi Kieran,

I think you may not have the room for a full mass wall. I would probably build a hybrid; rock wool insulation in the stud framed wall and inside of that a mass wall of CEB/adobe or scrounged brick. Support the floor with some diagonal bracing held up by a ledger on the foundation wall and tie the masonry to the framing with brick ties. If you need to air seal, make a mastic with BLO and lime, then plaster the inside of the wall..