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Frugal Happy: Our House Becomes a Giant Foam Box

A months long project transforms this energy hog into a comfortable, efficient space

Two layers of rigid foam insulation insulate the exterior walls. The first is 3-inch-thick polyiso installed in the stud cavities, a technique called cut-and-cobble. A second layer on the interior reduces thermal bridging.
Image Credit: Wen Lee
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Two layers of rigid foam insulation insulate the exterior walls. The first is 3-inch-thick polyiso installed in the stud cavities, a technique called cut-and-cobble. A second layer on the interior reduces thermal bridging.
Image Credit: Wen Lee
As built, exterior walls were completely uninsulated. After the renovation, exterior walls have two layers of rigid foam insulation plus plywood sheathing for structural reinforcement. All of the work was on the interior of the house. First, Chris cut sheets to width on a table saw. Then Chris cut the pieces for a precise fit in each stud cavity. He used a circular saw or a Japanese Kataba saw. After the insulation was installed between the studs, gaps along the edges were sealed with caulk or canned spray foam. A layer of plywood reinforces the walls, an important detail in seismically active southern California. After the plywood is a second layer of polyiso foam insulation, this one 1 inch thick, to reduce thermal bridging through the wall framing. Furring strips applied to the final layer of foam will create a service cavity beneath the drywall.

Editor’s Note: This post is one of a series by Chris Stratton and Wen Lee, a husband-and-wife team living in the Los Angeles area who are turning their 1963 suburban house into an all-electric, zero-net energy home. They chronicle their attempts at a low-carbon, low-cost, and joyful lifestyle on their blog Frugal Happy. This post was written by Wen.

It turns out that one of the most important determinants of how much energy your home uses is also one of the most invisible: insulation. I know, insulation is soooo unsexy. No one ever sees it or thinks about it.

But imagine being outside on a very hot summer day. Would you rather take shelter inside a box made of wood or Styrofoam? Same question on a freezing, snowy winter day. In both cases, of course you’d choose the foam box. Insulation makes a difference!

Thermal insulation is basically any material that heat can’t travel through easily (like foam). When a building is protected with insulation, it helps keep the inside temperature the same, even if the outside temperature changes. The more insulation a building has, the more stable the indoor temperature (and the less energy is needed for heating and cooling).

So that’s why we decided to turn the house into a giant foam box.

To be honest, this is hard, tedious work to do on your own. It took Chris three months to insulate just the walls of our common rooms. The ceiling — which we’ll cover in our next post — took another four months. It turned out to be bigger and more overwhelming than we could have imagined. But to us, it was worth it.

For the past 35 years, our house had only a thin layer of insulation in the attic and absolutely zero insulation in the walls or floor (see Image #2 below). Which meant that in the winter, whenever we turned on the heater, nothing would keep the heat in, so it would quickly escape out the building, and the furnace would have to keep running hard for the house to stay warm. The opposite would happen in the summer with the air conditioner. In other words, we were wasting huge amounts of energy. Not good.

Our solution? Cover all the exterior surfaces of the house with high-quality insulation (see Image #3 below). For our region, California code requires new homes to have an insulation value of at least R-19 in the walls and R-38 in the ceiling. Since Chris is so passionate about this, he decided to make our walls R-26 and our ceiling R-46. Take that, climate change!

As you can see, the new assembly is rather beefy and intricate! The wall thickness increases from 5.5 inches to 8 inches (yes, our rooms will actually be a tad smaller after all this is done). While it is quite intimidating to construct something so complex, Chris took it one layer at a time.

Phase 1: A 3-inch layer of foam

Before putting in any insulation, Chris cut thin long “sticks” (1/2 inch by 1/2 inch) and stuck them in the back corners of each empty bay. This created a small air gap, just in case any moisture (like rain) somehow manages to get into the wall assembly in the future. Pretty unlikely in our dry climate, but it’s important to allow moisture to dry out so there’s no risk of mold or rot in the walls. [Editor’s note: The house owned by Chris and Wen has no wall sheathing. For more information on the use of 1/2″ x 1/2″ sticks when insulating this type of wall, see “Insulating Walls in an Old House With No Sheathing.”]

My cousin Hank and our friend Ernie helped with this process — thanks guys!

Now, we were ready for the foam.

So… we needed a lot of foam. Over 100 sheets, actually. This type of insulation is called rigid foam insulation. The technical name of the material is polyisocyanurate, or polyiso for short. There are other kinds of insulation that are more commonly used — such as fiberglass and cellulose — but rigid foam is much more effective (and, uh, expensive).

After we removed the drywall, the walls were just a lot of empty bays. Chris’s goal was to fill each bay with a 3-inch layer of rigid foam.

The upside to rigid foam is that it’s one of the best insulators available. The downside is that, well, it’s rigid. While cellulose or fiberglass insulation is squishy and easy to stuff into different shaped spaces, rigid foam doesn’t have that flexibility. It has to be cut to fit.

Which meant Chris had to carefully measure the dimensions of every bay (which were all just very slightly different), and cut a custom piece of rigid foam into the exact size and shape of each one. [Editor’s note: This insulation method is called the “cut-and-cobble” approach. For more information on this technique, see “Cut-and-Cobble Insulation.”]

Chris used a table saw to cut each 4-by-8-foot sheet into smaller pieces (see Image #4 below). He wore a respirator because cutting the foam filled the air with a cloud of fine polyiso dust. It’s kind of like snow, except it never melts and will probably make you sick if you breathe too much into your lungs.

After using the table saw, Chris manually cut the foam to precisely fit each bay (see Image #5 below). For this he used either a circular saw or a Japanese Kataba saw. Then he would install each cut piece into its corresponding bay by gently pounding it in place with a mini sledgehammer and a block of wood.

Because we vaulted our ceiling, often Chris had to make diagonal cuts at exact angles, which was a pain to get just right. It was a slow, painstaking process. Chris completed five to 10 stud bays each day. Slowly but surely, the walls started filling up with shiny foil-faced foam.

Chris finally finished all the bays: woohoo! But wait — there’s more. Time to air seal. I didn’t know this before, but Chris explained to me that air sealing is a critical, often overlooked step when insulating a building. Without air sealing, you just end up with a foam box full of holes and cracks. So Chris used caulk or canned spray foam to seal every seam around each bay (see Image #6 below) Tedious, but important!

Phase 2: structural reinforcement

The next step was to cover the walls with a layer of 1/2-inch plywood (see Image #7 below). This actually had nothing to do with insulation, but instead was about earthquakes.

Our house was constructed in 1963 and had very little seismic supports built in, which is scary since California is riddled with fault lines. Adding a layer of plywood over the building frame helps provide lateral (sideways) support and keep our house from collapsing during a big earthquake.

Thankfully, Chris’s big brother Charlie visited from Cincinnati and stayed with us for a month, and was a huge help with the house project. With Chris and Charlie combining forces, construction moved ahead at a faster clip.

Phase 3: More rigid foam

Now that we have impressive plywood walls, what’s next? Why, another layer of rigid foam, of course.

You might be wondering why on Earth we’re adding more rigid foam. Didn’t we already do this? Yes, mostly. But remember that Chris only put the rigid foam inside the bays. Which means that the framing around each bay actually did not get insulated. All the wood studs and blocking are uninsulated, so that heat can still leak in and out from those places. (This is called thermal bridging.) It’s like we have a beautiful thick wall of rigid foam, but it’s full of cracks.

To address this, Chris and Charlie added yet another layer of rigid foam on top of the plywood, except this time only 1 inch thick and continuously covering the entire wall (see Image #8 below).

And yes, they air sealed this layer of rigid foam too. Now there will definitely be no leaks! The most airtight giant foam box house evarrrrr.

Phase 4: Furring strips and service cavity

Insulation is now complete: hooray! But wait — there’s more (again).

We can’t put drywall (the final, finished layer) directly onto the rigid foam, because we need some space behind the drywall to put things like electrical wiring. Chris’ solution was to install horizontal pieces of wood, called furring strips, over the rigid foam (see Image #9 below). The furring strips (old ceiling joists we ripped in half) also serve as anchor points for electrical outlets that will be installed later on.

Chris and Charlie used a laser level to make sure the furring strips were perfectly horizontal and parallel.

Fully insulated with 4 inches of polyiso rigid foam, seismically supported with plywood, and air sealed not once but twice — our walls have never looked so good! Ready for wiring, and then the final drywall.

It’s just a shame that all this awesomeness will be hidden inside the wall for no one to see. The only proof that Chris spent three months painstakingly insulating the walls are, well, these photos. That’s how it goes with insulation.


  1. Jon_R | | #1

    It would be interesting to see a cost comparison to putting only 1" of rigid foam in the wall, putting foam strips on the studs and then filling with damp spray cellulose. Perhaps with the addition of diagonal steel strapping for strength.

  2. Andrew_C | | #2

    RE foam strip alternative?
    I suspect that the potential seismic activity led to putting the plywood directly onto the studs. Diagonal strapping is good for reinforcement, but I'd like whole wall sheathing...but I haven't run any calculations.

  3. Expert Member

    Unrelated Question
    Can a wall sheathed on both sides with plywood be successfully dense-packed, or does it need to be bounded on one side with a more pliable material like mesh or membrane? Plywood can take an awful lot of holes without measurably affecting it's structural integrity.

  4. Expert Member

    Chris and Wen
    Just reading this blog makes me tired. Good work!

  5. RedDenver | | #5

    sandwiching plywood with foam
    Isn't covering the plywood sheathing on both sides with rigid polyiso a bad idea? If the plywood ever gets wet, it can't dry in either direction.

  6. GBA Editor
    Martin Holladay | | #6

    Response to Jason D
    It's a little hard to tell from the photos what type of facing there is on the interior layer of polyiso. But even if the facing is aluminum foil -- which obviously makes the rigid foam a vapor barrier -- there really isn't any mechanism whereby the interior plywood would ever get wet. I wouldn't worry about it.

  7. engr496 | | #7

    Would this assembly perform well on a 1920's home with brick veneer, 1" air gap, tar paper, and true 1"x gapped board sheathing on exterior side of 2x4 framing? I was originally considering using 1/4" edge-caulked fiberboard directly on the board sheathing as an air barrier, followed with rock wool in the cavities; I suppose I could still utilize this method to allow drying to the inside, even if I follow up with a layer of rigid rock wool atop plywood to combat thermal bridging. Eventually, I will most likely paint the brick, but for now it is original.

    There are no weep holes in brick. The brick does open to the attic space, but is covered over with rim joist in the crawlspace. We are in Nashville, TN.


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