Energy Efficiency

The last couple weeks have been fits and starts. Basically finishing up the exterior insulation and installing the strapping.

I used 3/4" plywood "straps" screwed through the foam into the studs. The straps serve as attachment points for the siding, which I will put on next.

Finishing the foam was a laborious process. All the seams of the foam had to be taped. If I couldn’t tape them for some reason, I had to fill the gaps with spray foam. This was important to make sure there were no air leaks (or at least as few as possible).

To give an idea of how meticulous the air sealing has to be, here is what I did to seal around the sewer cleanout. I first sprayed expanding foam, then waited for the foam to cure before I trimmed it and finished with tape.

I had to use 6" long screws to attach the strapping. It was a little nerve-wracking to drive these screws through thick foam, but I only missed a stud three times. I had to use a corded drill; the cordless drivers could only drive a few of these at a time before the battery power gave out. I almost overheated the drill, too.

At the top and bottom of the walls I had to put insect screening. Once the siding is on, there is a continuous air space between the siding and the foil-faced foam. The concept is called a “ventilated rainscreen“. The siding can be a little leaky; any moisture that gets behind will run down the foam and out of the wall, or evaporate. This improves the durability of the building by reducing water intrusion and extending the life of the siding and, in the case of the wood parts, its paint or stain.

The air gap also allows the reflective foil facing to serve as a radiant barrier. This is pretty important in a hot climate like ours. It reflects a lot of solar energy away from the building in the summer time.

This is the aluminum insect screen at the bottom of the wall. It wraps around the strapping. When the siding goes on, it prevents bugs and other critters from crawling into the air space between the siding and the foam.


I started installing the foam insulation on the exterior walls this past weekend. I took a couple pictures this morning but it was difficult to get good shots. The glare from the sun reflecting off the foil facing was blinding!

One layer of foam. A second, thicker layer will follow.

There will be two and a half inches of polyisocyanurate foam insulation board on the outside. It’s predominantly in two layers. The first layer is an inch thick, and is tacked to the framing with cap nails. The second layer is an inch and a half thick, and is temporarily held in place with long screws and plywood washers until we install the 3/4″-thick plywood strapping. We use 6″ screws to secure the strapping to the framing.

Two layers installed, with all seams covered with foil tape to prevent air and vapor intrusion.

Later, once all the strapping is installed, we’ll attach the siding to that. We will put corrugated steel siding on the north and south walls, and redwood lap siding on the east and west walls.

So far I’ve got the north wall insulation complete, most of the east wall, and the first layer on the south wall.

The foam insulation is fairly expensive, but there are many advantages of the foam insulation over conventional batts:

  • The continuous foam on the exterior eliminates “thermal bridging”, which is what happens when you insulate only between wood studs. Wood studs are terrible insulators.
  • The foam has a foil facing which serves as a radiant barrier. Put it in the sun and you can feel the heat reflected back at you. This is invaluable in a hot, desert climate.
  • Foam has a higher insulation value per inch than batt insulation. If it’s thick enough, you can eliminate batt insulation entirely and leave the stud bays open, thereby facilitating utility maintenance in the eventual remodel.
  • Batt insulation is difficult to install properly, especially if you have pipes or wires in your wall. To get the rated insulation value, batts have to be installed darn near perfectly.
  • Foil-faced foam is waterproof and impermeable, so it serves as both a moisture barrier and a vapor barrier. If installed on only one side of a wall, it makes the house more resistant to moisture. Just be careful not to create a “moisture sandwich” by installing impermeable surfaces on both sides of the wall. By choosing to use polyiso on the exterior, I can’t use a waterproofing membrane on the walls of our shower.
  • Foam insulation board doesn’t itch or release cancer-causing fibers into the air for you to breathe. It’s extremely easy to cut, handle, and install.

I chose this design because I wanted thin walls, and I wanted to be able to run utilities like pipes and wires in exterior walls. I am able to have a high-performance structure framed with 2X4s instead of 2X6s; in a bathroom, those inches really count.

This next weekend, Caleb and I will finish the foam install and the strapping.

Our bathroom addition isn’t going for any sort of “Green” certification, but I am trying to make it as “greenish” as possible. I’ve definitely had some Green Building Fails, like the foundation using seven cubic yards of concrete. That’s seven cubic yards of carbon into the atmosphere for a piddly bathroom, compliments of yours truly. I definitely could have done better!

But it vastly exceeds insulation requirements for our climate, is sited for passive solar, is built using durable materials and methods, specifies no mechanical heating or cooling, specifies energy-efficient or low-water-use appliances and fixtures, provides a permanent greywater reuse system, and it even calls for a rain barrel. Perhaps it will look cool enough that future owners won’t decide it’s ugly and tear it apart for remodeling.

Most of those green features really don’t add much to the cost of the addition. Some even qualify for rebates and tax credits. But sometimes trying to find stuff is a real pain (try asking a local lumberyard about Forest Stewarship Council-certified lumber). Trying to determine what really is environmentally responsible versus what is simply “greenwashed” is a challenge, too.

Given that, I think our addition should qualify for some kind of Green award after all. Maybe if I give my kids some markers and green construction paper they’ll make one for me. (They’ll have to learn how to spell first.)

This weekend we installed the three clerestory windows and the door.

Clerestory windows

The shed next door proved to be a handy platform for installing the windows. I built a sturdy scaffold from extra 2x4s and plywood and leaned it onto the shed roof.

The windows are of the awning type, which means they are hinged at the top and open with a crank. The screens go on the inside, though they aren’t installed yet. I will install them after construction is complete so they don’t get torn. They are aluminum-clad wood windows and very energy efficient units. Installing them was pretty easy, except that the self-stick flashing doesn’t stick to tarpaper very well. Nothing a few staples couldn’t fix.

Door viewed from shower area

The door is really pretty. The light that comes through in the morning is very cool. I can’t wait to take my morning showers in here!

The door is extremely heavy. It is a fiberglass door (we didn’t want a wood or steel door in a shower area) but the weight comes from all the glass. It is triple-paned, which was necessary to meet energy efficiency requirements. Since I was careful to make sure the stud on the hinge side was plumb during framing, the door was not difficult to set. The hardest part (other than wrastling its bulk into place) was installing the lockset, which required some modifications to the stock bore.

Well, as of today, the addition is “locked up”… literally. Hooray! A major milestone achieved. Next week: lots of foam insulation.

I took last week off, and Caleb and I made rapid progress. We finished framing, sheathed the walls and roof in plywood, got most of the tar paper up, insulated the roof and one wall, and installed the metal roof. Whew!

A view of the framing from the northeast.

I used pressure-treated lumber to frame the shower area. It is regular lumber treated with stuff that bugs and fungus don’t like. Why do this, you ask? The foam insulation I plan to install on the outside of the house is vapor-impermeable polyisocyanurate (building science nerd-speak for fancy foam board). Because the foam board doesn’t allow any moisture to pass to the outside, the building has to dry toward the interior. Since the interior is a humid shower, I wanted some insurance against rot and termites. We didn’t use plywood around the shower, either. Confused? Good. Just know it’s awesomely over-thought.

This is the roof deck shortly before we finished nailing on the plywood. We tried installing the plywood on the roof before installing the plywood on the walls, but the structure was very wobbly. The plywood really provides incredible strength. We were using a nail gun that shot smaller nails than are in the code book, so we had to use extra nails. There are thousands of nails in this little room!

View from the southwest, with plywood shear panels in place.

This is what the room looked like when we called for inspection. The inspector needed to see that all the plywood was nailed on properly and in the right locations. We mostly passed. Which is to say, we slightly didn’t pass. One panel didn’t provide enough shear bracing for the west wall (it’s the one right in front of you, to the left). Long story short, there was some confusion about what I thought I permitted and what was actually allowed. But the inspector was cool and said we could continue, we just needed to show that we had provided adequate shear bracing before we put up drywall.

The roof sheathed and covered with tar paper.

I was glad to move on to roofing. After shooting thousands of nails I was really tired of plywood. The roof is pretty unusual: the insulation is on top of the roof deck. Four and a half inches of polyiso foam over felt over plywood. And steel on top of all that.

The roof covered with polyiso insulation.

The thick foam provides a little more insulation value than is required by code in our area. There will be another six inches of rock wool insulation between the rafters below, making a fairly superinsulated roof. Cozy!

Roof with purlins and metal roofing going on.

We installed the galvalume roof ourselves. A local metal roof manufacturer measures the roof deck and fabricates all the pieces, then delivers them. They provide some basic instructions and loan you a few necessary tools. Installing the roof was pretty easy. There was some cursing over the waste vent pipe flashing, though.

We finished the roof as it was getting dark, so no pic yet. But it’s purdy.

This past weekend I tried to get a head start on framing. So without expecting any laborers friends or neighbors to be able to help, and knowing I’d be out of town the following weekend, I ordered a truckload of lumber and had it delivered. Meantime I got started on sill plates and flashing.

I installed copper termite flashing under the sills. It is supposed to make termites easier to detect by forcing them to tunnel out into the open. It's also pretty.

Between the termite shield and the sill is a foam gasket to stop air leaks.

Some of the anchor bolts were awkwardly placed. Some are where studs should go. This one was a tight fit under the stack cleanout. I couldn't get the sill plate to fit between the top of the bolt and the cleanout. I had to use a grinder to cut off the top of the bolt.

I had to chop off the overhang because the new addition is taller than the room we're attaching it to.

I managed to frame a little over one wall. My neighbor helped do the short wall; the tall one I had to do solo.

I spent a lot of time doing the stud layout. It was made more difficult for two reasons: pipes and anchor bolts. Some of the anchor bolts are where studs should go, as is the greywater pipe for the washing machine. I can’t move the pipes or anchor bolts, so I either have to adjust the stud placement or notch the studs to fit over or around the anchor bolts.

Since the south windows are eight feet up, the view is of trees and blue sky. The window placement is great for passive solar heating in the winter, too.

This is the view from the back patio, with the bedroom to the left. The building on the right is the shed, so you can see there is a sort of outdoor hallway to get behind the new bathroom. I'll put a gate back there to create a sort of secret garden. Don't tell anyone.