Some pretty fancy curves that the drywall guys are bending 5/8″ sheetrock around. i didn’t think it bent like that!

There are five types of insulation going into the house:  sprayfoam polyurethane, straw bales (of course!), bluejeans cotton batts,  radiant barrier (bubble wrap and roof board), and cellulose.  Each has it’s job, and it’s strengths and weaknesses.

Spray foam

Spray foam: You can spray it almost anywhere, it sticks to any surface, it fills and expands in every crevice, and it creates a beautiful air-tight seal.  That, coupled with it’s high R value (“7” per inch) make it an excellent insulation – what’s not to love?

Insulation is mostly about “R value” or a measure of thermal resistance, which for the *real* geeks reading the blog has US units of ft^2*deg F*h / BTU

Unfortunately, it has fairly high percentage of fossil fuel content, and high embodied energy (see the blog entry “and then it was juuuust right”), it is expensive, and it gives off toxic fumes if your house should ever burn down – so we’d rather not use it anywhere a thicker layer of a lower performance insulation will do.  But where our insulation will be thin (like in the strawbale library ceiling) , or prone to air leaks (like around the edges of our roof vent baffles at right), sprayfoam is the right insulation for the job!

Strawbales in the library

Strawbales: A properly constructed strawbale wall is conservatively estimated at R 30 (with all the gaps filled with straw)… but given that it is 24 inches thick, that only comes to R 1.3 per inch – not exactly high performance insulation.  Still, it is a very well insulated wall simply because the walls are so thick.  When you take into consideration that is has zero (or even negative!) embodied energy since it is an agricultural waste product that it would otherwise take energy to destroy, it is about the greenest thing in the whole house.

Blue jeans between the floors

Bluejeans: In between the upstairs and downstairs floors, we’re using old recycled bluejeans cotton batts.  They have an R value of 3.7 per inch, are easy to attach into a ceiling, and have nice sound attenuating properties (we’ll have wooden floors upstairs, so you don’t want it to sound like elephants pounding around above your head).  Relatively low embodied energy, and recycled material!

Radiant barrier

Radiant Barrier: This is an insulating material that is more difficult to assess the value of.  Radiant barrier is being installed in the roof where it is an integral part of the roof board (it comes with a radiant barrier film on the back side), and in the East, South and West facing walls it is being used in the form of radiant barrier “bubble wrap” that will sit behind the cellulose wall insulation (see pic at right).   Some radiant barrier is marketed with “R numbers” ranging from 4-6, but that doesn’t really apply – it isn’t much of a conductive block, it rejects heat gain from radiation in infra red wavelengths.   It is pretty much useless unless it faces an airgap of some sort, hence the film on the roof board that has the attic space as its air gap, and the bubblewrap in the walls which maintain an airgap with the integral air bubbles.   How good will it be in improving the heat rejection of the house during the summer and the heat retention in the winter?  We haven’t attempted the calculation, and we’re not quite sure where to start.  Honestly, this is one of those “gut feel” decisions that could be useless, or could be the most important aspect of the insulation on our walls…. we need a guest post from a real heat transfer specialist.

Blown Cellulose Insulation:   In the insulated attic spaces outside of the library and in the exterior walls, we will be using wet-pack blown in cellulose insulation which is mostly recycled newspaper plus flame retardant.  It has an R of 3.7 also, but because it is blown in, it fits around pipes, switch boxes, wiring and odd-shaped areas better than cutting and fitting bluejean batts.  So while it isn’t as good as stopping air leaks around your insulation as sprayfoam is, it is much better than the batt forms of insulation in complicated areas with lots of perforations (in a Princeton study there was a 24.5% reduction in air infiltration for blown in vs. fiberglass batt).  It is largely recycled content, and ends up with less than 1/4 the embodied energy of fiberglass and 1/25th the EE of spray foam!  No picture yet – the cellulose installed this week is all up on the other side of the drywalled ceiling.  Next week there will be pictures from the walls.

Whole House Fan

Whole House Fan

In the attempt to make the house as energy efficient as possible, there are quite a few heat exchangers set up around the house to take help keep all the warmth and coolth in it’s proper places.  Some are active, some are passive, and they run the gamut of passing heat from air to air, air to ground, water to water and air to water.  Here is a run down of our systems:

Path of air through house

Heat Recovery Ventilation in action


Heat Recovery Ventilation

HEAT RECOVERY VENTILATION (or HRV): In the picture at right where you see the ducts hanging down, is the space in the garage where our HRV unit will go (the square insulated duct is the air intake).  In the winter when it is cold outside, the windows and doors are shut tight, and our super insulated house will do a great job of keeping all the warmth inside where we want it.  Unfortunately, it will also be keeping lots of stale air and indoor pollutants in.  To keep indoor air fresh, many building codes call for an air exchange rate of .35 air exchanges per hour.  It used to be that, in leaky drafty old houses, this happened naturally through the poorly insulated walls and around windows and doors, but with our tight construction, we would fall way below this if we didn’t actively ventilate the house.  Simply turning on a vent fan, and blowing 1/3 of our nice warm air outside every hour would mean a lot of energy going out with that warm, stale air.  Enter the HRV – an exhaust system that draws warm air from the potentially stinkiest areas of the house (like the bathrooms and kitchen).  It then passes that air on its way out the house through an air to air heat exchanger where it passes it’s heat to the fresh (but cold) incoming air.  You can even get units with HEPA filters!  The warmed clean air is then vented into bedrooms and living space, setting up a stable airflow through the house, and scavenging about 75% of that heat that would otherwise have been lost.  In the Spring when we open the windows again, we’ll turn off the HRV until next winter.

Earth Tubes

Earth Tubes

EARTH TUBES: In the summer evenings, when the air inside the house has spent the day heating up, most of the time we will be able to simply open the windows and turn on a whole house fan (the monster fan you see as the main picture on this blog entry).  Because this fan blows into the attic space, it also pushes the super heated attic air out, further cooling the upstairs bedrooms.  On those awful hot days when the outside air isn’t much better than the inside air, however, we’ll keep our windows closed when we turn on the whole house fan, and the earth tubes will come to our rescue.  These tubes are embedded deep in the concrete foundation, and run all the way around the house.  As we draw our ventilation air in through this air/ground heat exchanger, we will be cooling the air we bring in.  The picture at right shows the point in our great room where the (currently capped) tubes will bring air in behind a vent screen. (see the “Earth Tubes” blog entry from 6/20/09 for more pics and details)

1" Copper Pipe

1″ Copper Pipe in the Wine “Cellar”

Heat Exchanger

Heat Exchanger

THE WINE CELLAR:  The ideal wine cellar is a cave that stays between 55F-58F year round.  Around here in Mountain View, our deep ground temperature is 62F on average.  It is OK to store wine as high as 65F if you can keep it constant (high temperatures and fluctuations will rapidly oxidize the wine, and even a constant 62 is still going to age wine faster than ideal), but digging a deep cellar in this seismically active area would have been very expensive, and putting our wine in a wine refrigerator seemed to fly in the face of green design.   So we decided that if we can’t put our wine cellar in the ground, we’d bring the ground temperature into our wine cellar in the form of the domestic water supply.  This is another way to use the “coolth” stored in the ground.  Every time someone turns on a tap in the house, 62 degree water comes in and passes through an air to water heat exchanger inside our heavily insulated wine cellar, cooling the air.  It will be interesting to see how well this works.  At right you can see where our 1” copper water pipe passes through the wall, and it will be stubbed out and connected to a bank of hydronic baseboard heater  heat exchangers.  We have run wiring to the room so we can monitor the temperature and see how this passive cooling system works.  We’ll monitor it for about a year before we put any really expensive wine in there (not that we’re going to be able to afford any expensive wine after building the cellar for it!)

Shower Heat Recovery

Shower Heat Recovery

Ross Koningstein’s Shower Heat Recovery Data

SHOWER HEAT RECOVERY:  When you take a shower, think about all that nice warm water going down the drain.  That is a lot of heat!  It turns out that if you use a water to water heat exchanger, you can use the draining shower water to preheat the incoming cold water.  This system is made by GFX, and you can see the cold water coils at right wrapped around the shower drain.   Ross Koningstein who instrumented his house in Atherton and is measuring the effects of all of his green tech actually put thermocouples on his input water and measured the flow.  The graph I stole from his website is at right, and since he took the data, I figured I didn’t need to repeat the experiment.  From his measurements, you can reduce your hot water use by 20% after the first minute of the shower.

That’s enough heat exchangers for one house.


Exterior View of Strawbale and Window

It has been a long time between blog posts, but there will be a few catchup.  Catherine has been spending her weekends finishing up all the detailing on the straw bale library interior.  We left the exterior finishing to the experts because it has to be waterproof!

Bamboo is tied through the bales to lock them in place, courtesy of “Boa Constructor”.

After the bale raising party, there is still plenty of work to be done before you have a finished bale room.  The walls of rough bales need to be completely locked into place, and to have their surfaces prepped for the final finishes (stucco on the outside, plaster on the inside).

Locking the bales in place to the foundation starts with the “imbalers” or pieces of rebar that were embedded in the foundation onto which the first row of bales were placed.  The sill plates also have 10d nails in them that act like velcro on the underside of the bales.  Between the sill plates,  the bed for the first row of bales also includes a layer of gravel to allow drainage should (heaven forbid!) any water ever get inside the bale wall.  The key to the longevity and structural integrity of a strawbale construction is keeping the water out!


Lots of details on top of the first course of bales

Side view of bale courses

As the walls were built, the bales were notched into the wooden frame, and the corners were locked together with alternating bales at the corners (like brick laying), and then further locked in with large rebar “staples” that were pounded in on each layer.  On the first row of bales, the electrical wiring is also run to electrical boxes for outlets around the room.  Old-school strawbale construction has you pounding rebar pins vertically down through the stack of bales to tie them together, but for in-fill construction, this is difficult because you have a roof in place above your walls.  Instead, the bale walls are locked together with bamboo poles on the inside and outside that are tied through the wall and tightened to make the bale walls monolithic structures.The top row of bales are held in place on the outside wall by an exterior beam that runs around the perimeter of the room.  The top row of bales is then notched and “persuaded” into place against this beam which prevents them from falling out.  These details were all complete on the day of the bale raising, then the process of completing the walls on the interior began.

2×4’s for the bookshelves

Deep windows

The final step for locking the walls in to place, is to use a chainsaw to notch the bales at regular intervals around the interior wall so that vertical 2x4s can be installed to lock the bales in on the inside.  (Standing on a ladder, cutting straw with a chainsaw is fun for about 10 minutes.)  These vertical 2x4s (which visible in the pictures at right with green spray paint on them – all these pictures can be opened to see larger versions) not only help prevent the bale wall from toppling into the room during an earthquake, but they also provide mounting points to help prevent bookshelves from toppling in an earthquake (this is, after all, the library).  In addition, they provide the internal framing for attaching the lath needed for completing the window details.

When finished, straw is tightly packed under the lath

Catherine pounding in straw!

Window and interior wall finishing is a bit more like sculpture than carpentry – that is, if your preferred media are straw and expanded metal lath.  To take the raw end of strawbales, and make them into a smooth firm surface that can be a finished plastered surface, you need to staple metal lath next to the window opening, then bend it around and staple it to the interior 2×4. You then proceed to ram loose straw into this uneven space with improvised tools until you have a nicely finished curved opening to the window that can be plastered, and won’t crack if you then lean against it.  This is especially important in the large window seat!

The window area — ready for plaster

Close-up of the straw packed in

The main interior walls are a bit easier as they are already relatively smooth.  Once you have found and filled in all the little chinks and gaps in the straw at the top of the bales and around the edges, you can attach lath over the entire surface.  There is some interesting constructs like corner keepers to make out of various types of lath.  Several weekends were spent fabricating all the details and installing them with about 2000 staples to keep everything in place.  Now the entire room is finally trued up, nicely finished and has a surface ready for plastering.  Whew!

Close-up of corner