Superinsulating My Old House, Phase 2

Phase 1 was an R-35 face-lift of the west façade. Phase 2 looks to the north and east.

November 02, 2009
November/December 2009
A version of this article appears in the November/December 2009 issue of Home Energy Magazine.
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I’m undertaking an energy retrofit of my old home on Rodney Street in central Helena, Montana. Phase 1 was an R-35 face-lift of the west façade (see “Superinsulating My Old House,” HE Jan/Feb ’09, p. 20). Phase 2 was installing 4 inches of expanded polystyrene foam under the 15-year-old steel siding on the north and east walls. This meant removing the siding, storing it, installing the foam, and reinstalling the siding.

The north wall is 26 feet high. It abuts on the sidewalk between my house and my neighbor’s house. The sidewalk is only 4 feet wide. The close proximity of the houses was a logistical problem that made installing the foam and reinstalling the siding the easiest part of the job. Just reaching the high part of the wall was the biggest challenge in phase 2.

The original brick veneer crashed down off the north, east, and south walls in the 1935 earthquake. Workmen cleaned up the brick, nailed  2 x 2  furring vertically to the plank wall sheathing, and installed wood siding over the  2 x 2s. The full 2 x 4 framed wall was blown with loose fiberglass from the inside in the 1970s. In the early 1990s, my teenage nephews and I filled the 2 x 2 wall space with cellulose before installing Tyvek and the new steel siding. Duh … why didn’t we install the foam then? No money. Poor planning.

To calculate the existing R-value of the north wall, I did an area-weighted average U-factor computation. Assuming that about 14% of the wall’s surface area was structural lumber, I came up with an area-weighted R-value of  approximately 16.9 for the existing wall.

There was a 5-inch ledge where the foundation had once supported the brick. We installed the foam on that ledge without increasing the footprint of the home. This was important, given that my neighbor’s house was only 4 feet away. The 4-inch 2-lb polystyrene foam adds approximately R-4 per inch, for a total of R-16. Adding R-16 to R-16.9 gives R-32.9.

The 4-inch-thick polystyrene foam was made locally with embedded strips of ⅝-inch oriented strand board (OSB) set 16 inches apart for fastening the foam to the wall, as well as for fastening siding to the foam. Snow-Foam Strip-It is the local company’s name for this product, but I’ve also encountered the names Drywall Backer and Styro-Stud. Dow has a foam board product called Wall Mate, which has slots for 3 x 3 furring strips. The 4-inch foam cost me $33 per sheet, or about  $1 per square foot.

As you might suspect, the 4-inch foam creates fastening challenges at corners and around windows and doors.  I bought a foam cutter to cut the foam accurately and square. I also bought a foam-grooving attachment, to install additional wood nailers flush with the surface of the foam.  We installed plywood strips 4–6 inches wide at the inside and outside corners to serve as nailers for the steel corner trim and the steel siding.

We used a level to install the first foam sheets plumb.  We fastened the 4-inch foam through the fastening strips to the flat wooden siding underneath with 5-inch screws. We predrilled the holes through the OSB to avoid splitting the OSB strips, and to space the screws consistently about 2 feet apart.

We sealed spaces between the sheets with one-part urethane foam. If you have to leave any gap at all between sheets, it’s best to leave at least a ¼-inch gap so you have room to fill that gap completely with the one-part foam.

We installed new Duxton triple-pane low-e windows with insulated frames and sashes into the old window frames to avoid interior demolition, and because the existing frame and casing were beautiful as they were. We filled the weight pockets of the old double-hung windows with fiberglass stuffed in through the weight pocket access panels. We foamed the top part of the weight pockets above the stuffed fiberglass through the pulley hole, after removing the pulley.

I ordered the Duxton windows with and without fastening fins. The first order was finless and proved difficult to install in our out-of-square window frames. We set both the finned and the finless windows on flat shims, which rested on an extension of the old finished windowsill.  Creating a perfectly level sill really helps. Lacking a level sill, you can use different thicknesses of flat shims. Windows should never hang on their fastening fins but should be supported by flat shims underneath the bottom corners and the center vertical supports.

The advantage of the finless installation was that we could set those heavy windows from indoors. The new finless windows rested against narrow stops on the outside, and we installed ¾-inch round molding from the interior to hold the window in place and cover the gap between the new window and the old finished jamb. The width of the frame was a problem, because it allowed the exterior stop to overhang the frame by only about ½ inch, which forced us to customize the little stops to each window’s out-of-squareness. We then shimmed the finless window in four places with a pair of wood wedges on each side of the window and foamed the gap thoroughly all around, which cemented the window in place and sealed out air leakage.

The finned windows must be installed from outdoors, which is daunting even with a man lift. However, the 1½-inch fins, attached to the exterior edge of the old five-quarter (1¼-inch-thick) window frame, compensated effectively for the out-of-square frame. When you remove the old exterior window trim, you’re left with the edge of the old window frame, which may be flush with the siding or may protrude.

The window perimeters contain a thick band of structural lumber, which constitutes a large thermal bridge. One advantage of exterior foam is that it covers the structural lumber around the window to reduce this thermal bridging. We installed the foam board flush with the edge of the old finished window jamb and sill. We made a new sloping foam windowsill to cover the old wooden sill. Then we trimmed the sides, bottom, and top of the foam board window exterior with common 2 x 4s. We used 6-inch screws, countersunk 1 inch into the face of the 2 x 4, to fasten the 2 x 4 trim through the foam into the old wood siding underneath. The 2 x 4 creates a raised trim area that we tucked the J-channel behind, to terminate the siding. We extended the wood sloping sill with a custom-cut polystyrene block capped with the 2 x 4 trim.

The windows were finished with a J-shaped piece of custom steel trim, in an accent color that contrasts with the color  of the siding. The steel trim wraps around the 2 x 4 into the window opening, dead-ending into the new window. We had a local steel siding expert bend this custom metal trim. The width of the return varies a little from place to place around the window, and our poor planning left a bit more gap for caulking than I wanted. We should have measured in numerous places around the windows and taken the largest measurement. Then we could have scribed the trim to fit tighter to the window. Figure around $28 for labor and materials per linear foot for these window trim details.

Steel siding is ideal for use with stripped foam board, because steel is lightweight, it locks together, and it’s very durable. We needed siding in addition to what we had removed, because the new wall was bigger than the old wall, and because we damaged some of the old siding when we removed it. Even after 15 years, there was little color difference between the new siding and the old siding. 



The steel siding produced by different manufacturers won’t necessarily lock together. Luckily, I bought my siding 15 years ago at a local lumberyard, which still sells the same brand. The new steel siding cost around $200 for a 100 ft2 package.

We established a level line at the top of the siding starter piece. This starter piece consists of only the narrow top edge of a full sheet, which hooks into the bottom of the first full sheet of siding. We rented a transit, or builder’s level, to establish our marks for this level line. As we installed successive rows of steel siding, we made sure that the courses were completely hooked together, so that the horizontal rows remained level.

The ends of each sheet of steel siding are terminated with one of three pieces  of steel trim:  inside corner, outside corner, or J-channel. These trim pieces all need a wood backer, grooved into the foam, that is wide enough to back both the screws for the trim piece and the screw holding the ends of the sheet of siding.  For an inside or outside corner, figure at least $20 per linear foot for labor and materials.

OSHA wouldn’t approve our climbing apparatus, and I don’t recommend the methods you see here. However, I’m comfortable at heights and didn’t want to build custom wooden scaffolding, which was our only other alternative short of divine intervention. Lacking a skyhook, we extended the man lift to the hilt from the backyard and chained a 20-foot aluminum scaffold plank to its deck. We chained the other end of the plank to a scaffold rung on staging located beyond the narrow sidewalk in the front yard. My helper is the father of a young child, so he stayed on the ground while I installed the foam and siding up high. Please, please be careful when you are working at heights! 

John Krigger
is the coauthor of The Homeowner’s Handbook to Energy Efficiency and a teacher at the Web-based training institute Saturn Online.

For more information:
For more on Saturn Online, go to www.saturnonline.biz.
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