guest post by Doug Hansmann

We always planned to have a mass wall between the kitchen/dining room and the den.  It was designed to  soak up sun on sunny days and heat from the wood stove on cold nights and cloudy days.  But over time we began to realize that our mass wall would function fine on the kitchen/dining room side where the wood stove and most of the south windows are, but it wouldn’t be so hot on the den side.  Since the den is where we expect to sit quietly reading or watching a movie, the thought of this relative coolness was paradoxically, not too cool.

What to do?

Andy DeRocher at Full Spectrum Solar highlighted the issue as well as some potential fixes.  For starters, we decided to make the den its own heating zone so it could be controlled independently, but even so, with two outside walls and three windows, warming the den promised to be a challenge.

It occurred to me that the PEX tubing we were burying in the concrete floors could also be applied vertically in the mass wall, and I approached Andy with the concept.  Andy was familiar with installing radiant heat in floors, and even in ceilings, but he hadn’t installed tubing in a wall before.  It was different, but intriguing.

If this concept was new to Andy, you can imagine how it struck everyone else in the building process.  Speculation about the purpose of our novel array of rebar and PEX tubing ran wild. Was it a retro TV antenna?  How about  a communications portal with the Mother Ship?

The plan was to pour a 10” thick, 8-foot wide, 7-foot high concrete slab directly above the basement wall.  (Underhill’s basement is only 2/3 the size of the main floor and offered a great foundation for a mass wall between the kitchen/dining room and the den.)

When you step off the beaten path in construction, you have to anticipate problems and create solutions on the fly.  With the mass wall sitting directly above the basement wall, it will straddle the interface between a slab-on-grade den floor and a joist-supported kitchen/dining room floor.

Although the entire floor was poured all at once and looks monolithic, we cut in an expansion joint between the two rooms, anticipating the possibility for some movement over time at this interface, as the house settles.

Andy suggested encasing the PEX in foam pipe insulation where it comes out of the floor and up into the wall array and again where it goes back down into the floor.  This way there will be some flexibility in an otherwise very rigid but potentially crackable location.

After the floor was poured, raising the timber frame was next. (see last Friday’s post)  This meant laying out the timber frame very close to an extremely exposed and somewhat fragile PEX tubing framework.

Here’s Doug stabilizing the rotation in a beam with a cant hook during the timber frame assembly while sneaking an admiring glance at the embryo mass wall.

The timbers were successfully assembled and raised without bending, or worse yet, nicking the PEX tubing.

Can you find the PEX tubing in this photo?

Andy had pressurized the PEX tubing loops so we could monitor the system for leaks, and with all the heavy-duty timber framing going on inches from the skeleton of our mass wall, there were frequent visits to the pressure gauge in the basement to make sure the system hadn’t sprung a leak.

That wasn’t the only risk our poor PEX tubing was experiencing through this period.  PEX can be degraded by uv light.  Andy said PEX exposed to the elements would be fine for 30 days, but the clock was ticking, and we’d definitely had some hot, sunny days recently, including a freak one-day heatwave in the 90s on May 27.

As it turns out Mike Flynn’s crew was back on site the very afternoon that the timber frame was raised, and they poured concrete by the bucketful over and around the rebar and PEX, giving true meaning to the term “mass wall”.  The PEX loop in the wall was cheated over to an off-center position about 3 inches from the den and 7 inches from the kitchen/dining room. (Thanks for another great idea, Andy.)

All told, the PEX was exposed to the sun for only 2 weeks.

An issue with all radiant floor systems is that there is a lengthy lag time of several hours between when the hot water first begins to flow through the PEX tubing and when items above the floor, such as  your hands and nose, start to warm up.

But radiant heat’s built-in lag time also creates the potential for overshooting a comfortable room temperature.

Once again, Andy had a great solution.  The thermostat in this room will sense both the air temperature as well as the slab temperature, and use both to determine whether to call for more heat.  In order to plan for the floor temperature sensing, Andy and I worked out the dimensions of our outside wall thickness, and he precisely placed a copper pipe to be imbedded in the concrete for housing a slab sensor that will connect with the one in the wall.  This way the slab sensor can be easily changed over time, as necessary.  No jackhammers needed for the job.

Our architect, Della Hansmann, stands beside the sturdy thermal mass wall, which is still wrapped in a tarp to slow down the curing process and increase its strength.

While the mass wall fulfills several great thermal functions, it will also be anchored at its top to a beam that supports the loft floor above the den.

This sturdy interface between the main living level and the loft will help  stabilize the entire timber frame against racking (twisting forces that can cause damage to a structure in situations such as high wind).

Denise and I think this mass wall is going to be a big plus in so many ways, and here’s how we expect it will work.  On sunny days, the kitchen/dining room side of the wall will soak up solar heat and the den side will get heat from the solar hot water array.  On cloudy winter days and cold nights, the wood stove will be the heat source of choice, and the mass wall will store and stretch it out.  If the den seems too cool, we can put on sweaters, wrap up in afghans or maybe use a bit of propane to heat this single PEX tubing loop.

The PEX/rebar grid is all sealed in concrete now, but rest assured, communications with the Mother Ship are still fully operational.