THE SHELL GAME: Constructing a building’s exterior for optimum performance

Shell exterior of building - The shell game

figure 1Once you’ve analyzed the weather and the landscape around a building, your next line of defense towards making its indoor spaces economically comfortable every hour of the year is insulating its lowest floor, outer walls, and roof: what is known as the building envelope. This cutaneous construction may contain three kinds of insulation:

  • Batts – fluffy masses of spun glass that are fitted into voids in the construction.
  • Sheets – lightweight rigid sheets such as styrofoam and urethane that are fastened to walls and roofs and laid under concrete slabs.
  • Fills – lightweight granules or foams that are usually poured or sprayed into construction voids.

A big question with insulation is how thick should it be? Before speculating on the answer, let’s state some facts:

  • Insulation is a one-time cost that reduces a lifetime cost.
  • A prevailing myth in building construction is that every added inch of insulation is slightly less effective and after perhaps 6 or 8 inches or so you reach a point of no return where any further thickness won’t pay for itself. Not true. First, the point of no return is more like 20 inches. Second, the tipping point will not occur at a conceptual “added inch of insulation” but where the accumulating insulation suddenly requires a different and costlier construction to hold it. Due to continuing rises in energy prices, we will soon need to put many more inches of insulation in today’s building envelopes, and this will require new methods of construction.
  • The greater the temperature difference between the inside and outside of a building, the faster the heat flows through the building envelope (inward in hot weather, outward in cold); while the thicker the insulation in the envelope, the slower the heat flows.

Dealing with water

But all the above facts are befuddled by five little letters: water. Here’s the problem. Air contains a certain amount of water, usually as a vapour suspended in the air, measured as humidity. If the temperature of air goes down, its humidity goes up, until it reaches 100 per cent. If the temperature keeps going down, the water begins to drop out of the air because it can’t hold any more, and this moisture deposits on nearby surfaces.

Now when heat flows through an exterior wall wherever there are any seams, cracks, and pores in the construction, part of the migrating heat is carried through these openings by the air which, remember, also contains water. As the air flows through the wall and its temperature lowers, the air cools and its humidity rises until it reaches 100 per cent; then the water begins to drop out of the air and deposit on the wall’s construction. In the old days when energy was cheap and nobody thought much about insulation, all the air flowing through the envelope not only deposited moisture in the construction as the air cooled, the flowing air also carried away the deposited moisture.

But during the energy crisis of the late 70s, somebody got the bright idea that the way to stop a lot of heat escaping from a building in winter was to wrap it in an airtight membrane known as a vapour barrier. This stopped the heat flowing with the air—but it also stopped any flowing air from carrying away the deposited moisture. If this water cannot escape, it will rot the wood, rust the nails, dampen the batt insulation which ruins its ability to insulate, and act as cajun seasoning to a host of moulds, mildews, carpenter ants, and termites that love the nourishment of lignin—until after a few years a building’s outer construction will begin to stink, look awful, and fall apart all at the same time. In this respect we’re like B’rer Rabbit and the tar baby: de more we try to get unstuck-up, de more stucker-up we get!

If the shell of a building you live or work in is clad in Tyvek or one of its filmy kin, it is likely rotting around you right now and you don’t even know it yet. As the price of fossil fuels continues to rise and buildings continue to be clad in moisture-trapping vapour barriers, this situation has become perhaps the most insidious economic issue confronting American buildings today. Hear what Joseph Lstiburek, Ph.D., says of these thermodynamics:

Less energy flow (i.e. heat) from the inside to the outside means the materials on the outside of the building are colder in the winter. The colder the materials on the outside of the building become, the wetter they become and the wetter they stay. This is not good. Think of it this way. For every 100 units of energy you save on the efficiency and on the cooling side, you will need to give back about 20 units of energy to be dry. You are still 80 units ahead. The problem is that if you are greedy and want the entire 100 units, your building fails and your occupants become uncomfortable.

The insulation cage

insulation cage
Figure 2 – insulation cage

Is there any way to notably reduce the heat flowing through a building envelope without trapping moisture in it? There is! It is the insulation cage, a wall construction that contains thick insulation, allows enough air to flow through the construction and carry away any deposited moisture, and is relatively easy and economical to build. Its construction appears in figure 2. This 12-inch thick thermal armour has two 2×4-inch stud walls, each 3.5 inches thick, with a 5-inch airspace between—and no water-trapping vapour barrier around it. The outer and inner stud walls are built separately (the studs of each should align); and since each wall is light they can be built quickly. This was a major problem with the 12-inch superinsulated walls developed in the late 1970s: each took a huge amount of labour to build. I know, because I built one once—and Lord, I never spent so much time doing so little good. I found that while three men could easily raise a sixteen foot length of 2×4 stud framing with a window in it, the same three men could barely raise a six-foot length of 2×12 nominal stud framing with a window in it because it was so much heavier. The logistics of this are as follows: If you can lift 100 pounds and you need to lift 90 pounds (i.e. raise a 4-inch wall) you can do it; but if you need to lift 270 pounds (i.e. raise a 12-inch wall) you can’t do it and you must find another way to get the job done; but if you need to lift 90 pounds twice (i.e. raise two 4-inch walls), you can do it. This is why the insulation cage is easy to build, because it is not one heavy 12-inch wall so much as it is two light 4-inch walls.

The insulation cage has another big thermal advantage: the 5 inches between the two stud walls eliminates another heat loss known as perimeter heat flow. In standard stud framing, heat conducts through the studs between the insulation and the plates along the wall’s tops and bottoms; and since heat conducts through wood about four times faster than through fiberglass batts, nearly 40 percent of all the heat flowing through standard stud framing flows through the pieces of wood between, above, and below the insulation. This doesn’t happen in the insulation cage. As a result, only three-tenths as much heat flows through this construction as through a normal 6-inch stud wall. This means every thousand dollars of heating bills out of your pocket becomes three hundred dollars!

The insulation cage has other advantages that every carpenter will appreciate. The 2-inch-nominal cap plate normally nailed on top of the two stud walls to hold them together is replaced by a 12-inch-wide strip of £-inch plywood which alone has five advantages:

  1. The plywood is lighter and easier to cut than a 2×12.
  2. The plywood’s laminated 3/4-inch thickness is stronger laterally than a 2×12 is across the grain.
  3. A 3/4-inch plywood cap won’t shrink vertically over time as will most 2-inch nominal lumber.
  4. Four 12-inch wide pieces of plywood can be cut from one 48-inch wide sheet with no waste.
  5. Where walls meet at 90-degree corners and tee intersections, the plywood can be cut into L and T shapes that will make these junctions so rigid they won’t budge if you bump a truck into them.

As for the five-inch airspace between the two walls, it alone has four construction advantages:

  1. The space dampens noise transmission through the walls.
  2. It makes the walls more impervious to fire because half the wood is 5 inches from the other half and the batts between them won’t burn.
  3. The space allows electricians to lay the wall’s wiring on the floor between the two walls without wasting time drilling holes in every stud which also weakens them and without wasting more time pulling the wires through the studs which also can damage the wire’s cladding.
  4. Electric outlets mounted in the inner stud walls will no longer be notorious infiltrators of air because the boxes’ backs are shrouded with several inches of insulation.

How to build an insulation cage

  1. Build the outer 2×4 stud wall including its sheathing as it has always been done, but leave off the cap plate.
  2. Build the inner stud wall as you built the outer one. The studs in the two walls should align. With this framing nobody needs to learn any new construction techniques or how to use any new tools.
  3. Nail the 3/4-inch plywood cap plates onto the two walls.
  4. Fit two layers of batt insulation between the studs as follows. (1) fit 10-inch nominal batts (actual thickness = 9 1/2 inches) all the way in between each pair of double studs and staple the batts’ paper flanges along the back edges of the inner studs. (2) fit 4-inch nominal batts (actual thickness = 3 1/2 inches) in front of each 10-inch batt—to create a snug 13-in-12 inches of superinsulation. The slightly compressed batts will slow heat flow better because they eliminate airspaces that often remain around their corners in standard installations.
  5. Apply all interior and exterior finishes as with normal framing—except for those awful vapour barriers. Instead, cover the plywood sheathing with easy-to-apply tarpaper, which protects the framing from rain and allows it to breathe through its seams. Tarpaper also costs about 4¢ per square foot compared to 11¢ for Tyvek.

Not only is the insulation cage economical to build, thermally superior, and impervious to rot, it is stronger than any 2×6 framing. I wouldn’t wish this on anyone, but if a tornado swept through a neighbourhood of houses framed with insulation cages, more homes would be standing afterwards.

Another advantage of the insulation cage is that instead of mounting baseboard heating units against a floor’s exterior walls where some of the heat will escape through the unit’s backs directly outdoors, the units can be mounted against interior walls where the heat escaping out the units’ backs will remain indoors. Then your energy bills will be another ten per cent or so less. Today’s baseboard heaters are located around a floor’s perimeter because then the heat spreads evenly from one side of the building to the other; but if the cage holds in the heat three times better, the heat between the walls will spread more evenly and it will make much less difference if the heaters are centrally located.

Finally the cage not only eliminates moisture damage and lowers heating bills when heat flows outward through the building envelope in cold weather, it also eliminates moisture damage and lowers cooling bills when heat flows inward in warm weather—because the thermodynamics of one is the reverse of the other.
In wood framing, some say the studs should be 24 inches apart instead of 16. Unlike what the 16/24 ratio implies, you will not reduce the number of studs in the walls by one-third because the reduction occurs only between the wall’s openings and its corners. In a typical 12-foot-long stud wall with a door or window, you will rarely save more than two studs. At the same time the wider spacing makes exterior and interior finishes flimsier and it offers less support for cabinets, shelves, and large pictures installed inside. Altogether the wider spacing is a fine example of being penny-wise and pound-foolish.

Another popular idea is to clad a building with several inches of rigid insulation. Though this insulation is thermally strong, it has a few weaknesses:

  • It is difficult to fasten exterior finishes to the insulation and the insulation to the wall inside. This is typically done by attaching a metal clip to the side of a furring strip to which the exterior finish is nailed, then inserting a long steel screw (up to 14 inches if needed) through the clip and the layers of foam into a stud behind, as sketched in figure 2. To secure this connection you must drive the screw blindly through all those layers of insulation into the stud’s centre—not to either side where it would often split the stud’s edge and form a weak connection. This is a mighty narrow target for a woodbutcher to hit 49 out of 50 times—unless the targets are timbers.
  • Since steel is a poor insulator, each long screw acts like a thermal soda straw that sucks heat through it several hundred times faster than through the surrounding foam insulation. In this construction this conductance can amount to a significant perimeter heat loss.
  • Some rigid foams burn and emit large volumes of deadly gas. Before using this insulation, take a tiny chunk outside, light it and take a whiff of the fumes. Then decide if you would like to enclose you and your loved ones and/or business colleagues in this material.

Rigid insulation does have a place in building construction. It is between an outer and inner masonry wall and under concrete floors. There the thermal barrier won’t burn, no perimeter heat will flow through it, and nothing can rot.

Insulation cage or spray-in foam?

The insulation cage also indicates the limitations of another insulation that has been promoted a lot these days: spray-in foam. This product’s installers—the gun guys—have to wear oxygen respirators and head-to-toe protective suits because as they apply the foam, expanding droplets fly into the air and stick to light fixtures, electrical outlets, floor registers, doors, windows, tools laying around—you name it. In one installation the owner decided to photograph the work for his records; the goo flying from the guns got in his hair and ruined his camera. Yet a knowledgeable person must monitor this work because:

  1. The gun guys must mix the foam’s ingredients precisely because off-ratio mixes have considerably lower R-values (R-value is a measure of heat flow through a material: the higher the R-value the better the material insulates).
  2. Installations of more than two inches require several passes because thicker applications will release too much heat in the foam and can char it.
  3. If too much foam is applied between the studs, the excess must be laboriously trimmed from the studs’ inner faces or the walls’ interior finishes will look lumpy.
  4. If too little foam is applied you won’t get what you paid for. If you decide to use this material, insist that the gun guys bring along an extra uniform for you or another knowledgeable person to put on before the motes begin to fly.

This work also requires far more electricity to operate the drum warmers, proportioning machines, and foam pumps than it takes a few labourers to install batt insulation with power staplers; and if the installation is off-grid, it requires renting a trailer-mounted generator for the day. Old buildings have another problem. If the foam is sprayed through holes drilled in the interior finish (the usual method, rather than removing the finishes to expose the framing), it is virtually impossible to fill every void between the studs below the wires running between the electrical outlets—and every little hole in the insulation drains heat the way the little hole in the bottom of a sink drains all the water.

Using spray-in foams can also create bureaucratic hassles. Some manufacturers prohibit applying foams at more than certain thicknesses, which makes some code authorities prohibit the same; then there go the advantages of superinsulation. Some building inspectors also demand a letter from an engineer stating that the framing is strong enough to support the foam (never heard this done with cottony batts); some officials have voiced fire and health safety concerns because most foams support combustion and emit toxic gases; and some foams void shingle and roof venting warranties. To top it off, for all these headaches you often pay two or three times what you’d pay for semi-skilled labourers to install fiberglass batts.

Insulation cages for old buildings

Insulation cages are great for new buildings. What about old ones? This is a serious consideration, because there is no way all the king’s contractors and all the king’s crews can tear down all the millions of existing energy-wasteful buildings in this nation and replace them with new ones that consume a third as much energy. So if we are to solve the energy crisis, we must find ways to make existing buildings energy-efficient. But how?

What other solution is there than to thicken the existing exterior walls? With not just six inches of insulation, but twelve inches. This can be done in three ways:

  1. Tear down the existing walls and replace them with thicker ones.
  2. Thicken the existing walls from the outside.
  3. Thicken the existing walls from the inside. The first option requires more work and more disruption of the building’s use during the work. The second, being outdoors, would be affected by bad weather. So let’s home in on the third.
Framing for rigid insulation
Figure 3 – Framing for rigid insulation

If a building has lots of wasted space inside which is made more efficient as described in chapter three, you would probably have plenty of room around the inside of the exterior walls to build a second wall up to a foot thick and fill it with as much insulation. This work could be done indoors one room at a time, so occupants could still use most of the building. One kind of building today would be perfect for this. McMansions! The bloated spaces in these wigwams of modern wasteful society could easily endure a little liposuction inside their skins, especially if the remaining interior becomes more sveltely comfortable. This construction could be performed as sketched in figure 3, as follows:

  1. Remove the interior finish from the exterior walls and the floors and ceilings 9 1/2 inches back in from the walls.
  2. Fit pieces of batt insulation into the voids between the ceiling framing above the wall and the floor framing below for 16 inches in from the wall.
  3. Erect a second 2×4 stud wall 9 1/2 inches in from the existing wall.
  4. Insert 10-inch thick batt insulation into the new wall.
  5. Refinish the new wall same as the old, according to taste.

As usual, all this isn’t as simple as this glib description suggests. For one thing, you’ll have to remove the electrical outlets and wiring in the old walls and reinstall them in the new ones (but any electrician can do this). A more serious matter is if any closets, kitchen cabinets, or plumbing fixtures are against exterior walls. You’ll probably have to tear them out and relocate them further indoors. Also, framing the second wall around existing windows and exterior doors would require a skilled carpenter.

As for thickening a building’s exterior walls from the outside, this may be a better choice if the spaces inside are already compactly designed. One way to do this is to:

  1. Strip the exterior finish from the outer wall but leave the sheathing since it helps support the building.
  2. Remove any Tyvek or similar vapour barrier.
  3. Drill 3/4-inch holes 16 inches apart vertically in the sheathing between each pair of studs. These “nostrils” will help the wall breathe.
  4. Build a 9 1/2 inch ledge at the wall’s base just below the first floor. This can be done several ways depending on the existing construction.
  5. Along the ledge’s outer edge, frame a 2×4 stud wall for each floor up to its ceiling or the roof’s eave.
  6. Insert 10-inch batts from the outside between the studs.
  7. Sheath the studs, add the exterior finish, clean up, and you’re done.
Superinsulating old roof eaves
Figure 4 – Superinsulating old roof eaves

What about roofs? If a building has a wood truss roof, you can lay thick batts on the existing insulation in the attic to create 16 full inches of insulation. But if the roof has short eaves, a problem may occur where the trusses’ ends meet the top of the exterior wall and the eave of the roof. At this triple intersection a constriction would prevent installing the full depth of insulation. One solution appears in figure 4, whose construction would proceed as follows:

  1. Remove the lowest 2 or 3 feet of roofing and its sheathing up from the roof’s eave.
  2. Mount on each exposed rafter or truss strut a triangular cleat whose outer edge is high enough to allow a roof vent to fit above the new insulation to be laid in the attic. Each cleat should be the same thickness as the strut or rafter it rides on and is held in place by small 3/8-inch plywood gussets nailed to its sides.
  3. Resheath and reshingle the roof the same as before.

Now all this work may look too difficult to think of doing it yourself. It may even look too difficult to think of anyone doing it. But any carpenter worth his framing square can do it. Besides, think of all the money you’ll save someday when energy prices are five times what they are now. Then, remembering that ancient day when you haltingly took these pages to a local builder, you’ll look back at what you did and smile.

Shingle or shake?

Thickening existing interior walls from indoors
Figure 5 – Thickening existing interior walls from indoors

A note on asphalt shingles. These are usually replaced every ten to fifteen years and they create 10 million tons of waste per year. Enter Enviroshakes and Panelshakes. Made from waste wood fibres, old tires,used milk jugs and other recycled products, these shingles have a brownish-gray hue that weathers to a silver-gray similar to cedar shakes. They are easy to install, are maintenance-free, require no added treatment or preservatives, and are fire-, mould-, and insect-resistant. They claim to be competitively priced and last more than 50 years. To learn more about this product, visit

Looking forward

As the decades go by and energy prices keep rising, these constructions will become increasingly practical. In fact, if I wanted to stack my chips on the one “dark horse” idea in this book that presently seems the most far-fetched but fifty years from now will prove to be the most sensible, thickening existing building envelopes would be it. You may also think that thickening these constructions as described above will be a lot of trouble. And it will be. In fact, it might be about a third as much trouble as searching for a better place to live, moving out of your old abode, moving into your new one, and trying to sell your old energy guzzler to a less gullible public.

As if the past can point a quicker path to the future: out in south-central Colorado, at 8,000 feet above sea level where temperatures soar above a hundred degrees in summer and plunge below minus thirty in winter, is a town with a frontier command post of the same name: Fort Garland. Built in 1858 to garrison 100 soldiers commanded by Kit Carson, this fort (which today is a historical landmark that is open to visitors) has several long buildings whose thick exterior walls retard heat flow in this area of climatic extremes. How thick are these walls? Twenty-four inches. In the corner of each room is a little fireplace to keep the occupants toasty warm. Each windowsill is so wide one can sit on it and enjoy a view of several fourteen-thousand-foot peaks stabbing the azure a few miles away. Those oldtimers sure knew what they were doing well over a century ago. Certainly we can do as well in the near future.

[ background=”#b6c4b3″ color=”#000000″ border=”0px solid #cccccc” shadow=”0px 0px 0px #eeeeee”]After graduating from the Cornell School of Architecture in 1964, Robert Brown Butler has worked as a carpenter, contractor, and registered architect. Through the years Mr. Butler has received a variety of honors for his creative work. He is the author of The Ecological House, which introduced many ideas that are further advanced in this volume, among other books.This excerpt reprinted with permission from Architecture Laid Bare!: In Shades of Green, by Robert Brown Butler. © 2012, Robert Brown Butler.[/]

image 1: Pixabay

Written By
More from editor
Passive House: Interview with Katrin Klingenberg, executive director of PHIUS
Passive Houses are designed to maximize energy conservation through the precise construction...
Read More
Leave a comment

Your email address will not be published.