I had designed my home to be passive solar, and had centered this around having the house sit partially on a slab, and a partial basement.  The Bank wont allow this - Must be full basement so that total sq. footage can be a McMansion. Ok, fine -  So, the house was designed so the south side would have the appropriate amount of windows.  With the slab, I was in line for ~150 sq ft of window, w/o slab  - I'm looking at ~80 sq ft for tempering.  This house is a timberframe - thousands upon thousands of pounts of eastern white pine.  At night, a weathered timber is noticably warmer than outside air for a few hours after sundown if it was sunny all day and not windy.  A patio door will put me over the top for glazing on the south - just wondering if a timberframe will be able to contribute to help prevent overheating or even some solar mass considering wood's poor storage capacity.

Just curious on any opinions...

bigshow -

...Some simple chemical engineering will tell us how much energy the wood can store. At room temperature, and with 12% moisture content, the wood's heat capacity is 0.38 BTU/lb*F. So let's say your frame weighs 10,000lbs (your truck delivery driver can tell you) after it dries in 2-3 years, multiplying 0.38*10000 gives us 3800 BTU/F. Now, let's suppose it's winter, and your house has been sitting sitting at a cozy 70F for a number of days (ie, everything is in equilibrium). Then, you quickly open the windows, and your indoor air temperature quickly drops to 69F. If you had no other heat input into the home, the frame is capable of releasing 3800 BTU's of energy over some specified amount of time. That time is governed by the wood's conductivity constant.

The same principle affects all the objects in your home...furniture, lamps, blankets, etc. The capability of each object to store heat is governed by the heat capacity constant, and the rate at which it releases the heat is governed by its conductivity. After building a timber frame, and knowing it's cost, I would not count on the frame to be an economical way to store heat.

My suggestions would be:
1) Insulate the heck out of your walls and roof. Use SIPS or spray foam
2) Insulate your slab and foundation footers, especially if you have radiant heat in your slab
3) Insulate your rim joist BEFORE your electrician and plumber get in there. It makes for an easier job. You can even install an insulated rim joist cut from a SIP panel
4) Put in a high efficiency boiler or furnace. If you're installing a boiler, spend a few extra bucks and get a modulating-condensing boiler. You'll get even more efficiency if you use a high efficiency boiler that also supplies heat to an indirect fired water heater.

All the best.

olpjeb,

thanks for the reply - yeah, as suspected, the frame will do very little.

On your points - I have all those grounds covered: SIPS, Geothermal, etc...the tip about a ripped SIP as a rim is an idea I've never heard though! cool.

The simple model for wood doesn't work. IIRC the specific heat of wood isn't precisely linear due to the phase change properties of lignin (the 2nd most abundant component in wood behind cellulose) at relevant temperatures. This makes it difficult to predict the thermal performance of log homes, since the lignin content will vary with species & growing location etc.

Lignin may be the minor component, but it can account for over half the specific heat in temperature ranges in & near comfortable room temps. As a result, bulk wood inside the conditioned space tends to work pretty well as thermal mass in buildings- better than the simple model predicts. Per unit volume it's better than concrete, since it has both thermal mass as well as some insulative properties to stretch out the lag with delta-T.

This has been tweaked empirically & used to advantage in high wood-mass dual-envelope designs like the Enertia House:

http://enertia.com/Science/HowItWorks/tabid/68/Default.aspx