Window Installation Tips for a Deep Energy Retrofit
Use ‘Dudley boxes’ for a hassle-free way to install new windows in walls that are being retrofitted with exterior rigid foam
In May 2011 we began a deep energy retrofit of our old, cold, drafty house in Saugerties, New York. Because the house was poorly and cheaply built in the 1840s (apparently from scraps and salvage), we were leery of opening up the walls from the outside, lest we find that the clapboard siding and incomplete sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. were all that had kept the house from collapsing.
Because we were happy with the interior after 25 years of piecemeal renovation, we couldn’t open up the walls from the inside, either. Instead, we opted to wrap the house with two staggered 2-inch-thick layers of rigid polyisoPolyisocyanurate foam is usually sold with aluminum foil facings. With an R-value of 6 to 6.5 per inch, it is the best insulator and most expensive of the three types of rigid foam. Foil-faced polyisocyanurate is almost impermeable to water vapor; a 1-in.-thick foil-faced board has a permeance of 0.05 perm. While polyisocyanurate was formerly manufactured using HCFCs as blowing agents, U.S. manufacturers have now switched to pentane. Pentane does not damage the earth’s ozone layer, although it may contribute to smog. insulation, carefully taping the seams to form a water drainage planePath that water would take over the building envelope. Concealed drainage-plane materials, such as building paper or housewrap, are designed to shed water that penetrates the building’s cladding. Drainage planes are installed to overlap in shingle fashion (weatherlap) so that water flows downward and away from the building envelope., screw furring strips through the insulation to the studs, and finishing with fiber-cement siding.
We replaced all the doors and windows. For windows we selected triple-pane, argonInert (chemically stable) gas, which, because of its low thermal conductivity, is often used as gas fill between the panes of energy-efficient windows. -filled, low-eLow-emissivity coating. Very thin metallic coating on glass or plastic window glazing that permits most of the sun’s short-wave (light) radiation to enter, while blocking up to 90% of the long-wave (heat) radiation. Low-e coatings boost a window’s R-value and reduce its U-factor., fiberglass-framed units from Inline Fiberglass. We ordered the windows with factory-applied aluminum nailing flanges (which Inline refers to as “brickmold” flanges). These were not replacement windows that replace the sash in an existing frame. They are the same complete units used in new construction. The old window frames weren’t in good enough shape for us to even consider “replacement” glazingWhen referring to windows or doors, the transparent or translucent layer that transmits light. High-performance glazing may include multiple layers of glass or plastic, low-e coatings, and low-conductivity gas fill., nor were the window-to-wall connections sound enough to work up as part of a tight shell.
Historic renovation specialists tackle their first deep-energy retrofit
Our carpenters, Dan Dudley and Bob “Woogs” O’Keefe, had previously specialized in historic renovations. If it hadn’t been for the economic downturn and the slowdown in their high-end work, I doubt they would have considered tackling our modest project. Luckily for us, they took the job and brought us not only their broad experience but their creativity, insight, and common sense.
That none of us had ever done this before didn’t faze anyone. We didn’t see this as a pioneering effort and we expected to find well-documented procedures and techniques each step of the way.
Where can we find step-by-step instructions?
But when it came time to install the windows, we were shocked to find little usable information on how to actually do it. What little we found was mostly idealized cross-section drawings of a completed retrofit, with energy-nerd analysis of resulting benefits. All the building science and how-to articles seemed assume replacement of not-so-old windows in relatively recent construction, with standard framing in good condition.
We found no discussion of how to replace old, deteriorated windows in old, shoddy, irregular construction after re-skinning the exterior with thick rigid foam. That is, nothing of use to the carpenters working on our house.
So we figured out our own simple, repeatable, method that gracefully handled the challenges we found in installing thick foam and new windows in old construction. These challenges included:
- Irregular walls which have racked, sagged, twisted, bowed, and gone lumpy from unknown causes (we resisted the temptation to explore any more than necessary).
- Walls of varying thickness due to additions and modifications.
- Old windows not uniform in size, construction or installation details.
- Existing rough openings apparently framed without use of squares, levels, rulers, tapes, or plans. Even windows which once lined up were now cocked, skewed, racked and misaligned.
Typical extension boxes take a lot of shimming
Instructions from the window manufacturers for new windows assume that the rough openings (ROs) are close to perfect. You order new windows’ net or heel size 1/2 inch smaller than the RO and shim to fit, screwing through the window frame to the RO studs.
Typical retrofit detail drawings for rigid exterior foam insulation add an “extension box” built in place by nailing strips to the jamb studs. Then you shim the window to the extension box and screw through the side of the window frame to the extension box (or all the way through to the studs).
That couldn’t work for us, as some of our existing ROs had four different side lengths. Many were cocked more than 1/2 inch, and some were out of line with the neighboring ROs by more than an inch. Dan and Woogs recoiled from the idea of so much shimming and reframing, not to mention a stick-at-a-time extension box for each window, and we decided there had to be a better way. In addition, we were concerned about the lack of a positive seal between extension box and the studs, and between the casing, rigid foam, and extension box.
Cut loose from the rough opening
Our key insight was that if we built an extension box that had enough strength and integrity to stay square and planar during the installation process, we could ignore the RO and not try to correct it.
To do this, we built extension boxes sized to the window, not the RO, with a wide, integral plywood flange. The plywood nailing flange is built from 3/4-inch plywood, so it has the same thickness as our furring strips.
We positioned the windows on the wall as if the wall were perfect. We screwed the box through the flange to the building face, into the edge of the RO studs, instead of through the side of the extension box to the side face of the RO studs.
In addition to being much simpler and faster, this technique provides a blast-proof air seal and a deluge-proof connection to the rigid foam drainage plane.
Measure the new windows. There’s a trade-off here. The more clearance between the new window and the old RO, the easier it will be to correct for alignment and position problems. But smaller windows mean less glass, less light, less solar gain and less view.
We settled on no less than 1 inch of clearance, all around, but if I had it to do over I’d make it 1 1/2 inch to make it easier to apply spray foam. So net or heel size should be 2 to 3 inches smaller than the RO. If you are replacing a bunch of windows that are the same size, a smaller heel size will make it easier to line up the windows later.
Although our old windows were double-hungs, we chose casements for our new windows. Sure, casement windows are substantially more energy-efficient, but the advantage that is relevant to Dudley boxes is that casement windows let in more light and offer a better view. Since using Dudley boxes can reduce heel size — especially if replacing a row of misaligned windows — switching to casements recovers or even increases the glazing area. (We also prefer the modern, European look of a plain casement mounted close to the outer wall plane.)
Make the Dudley boxes. (Although the box design was very much a three-way collaboration, I think Dan contributed the most and deserves to be immortalized by having these named after him.) The sides of the box are made of 1/2-inch CDX plywood. The face flanges are 3/4-inch CDX plywood (to match the 3/4-inch furring strips on the rigid foam insulation).
The internal dimensions of the box are crucial. The box should be between 1/4 inch and 3/8 inch larger than the heel size of the window. Tighter isn’t better. Ideally, the gap will be wide enough that you can run the tip of a spray foam applicator along all four edges after installation and completely fill space between the window frame and extension box.
If the extension box is too tight, you risk voids in the spray foam, compromising the thermal barrier and air seal. The depth of the extension box is determined by wall thickness. You want the inside edge of the box to be 1 to 2 inches shy of the inside wall plane. Don’t try to make it flush to the inside wall; the extension box isn’t directly tied to the interior casing.
The width of the face flange should be around 8 inches. If this seems too wide, keep in mind that in addition to securing the window to the wall, it will be part of the drainage plane and will be the nailer for siding.
We made the Dudley boxes on site with a table saw and nail gun. We nailed and glued the sides together, roughly squared them up by diagonals, and then applied the flanges, again with glue and nails. The 1/2-inch side pieces are nailed to the 3/4-inch flanges. (Fortunately, it doesn’t really matter if some errant nails stick out a bit.)
Make sure the flange pieces are cut square, and tight joints between flange elements will keep the box square. Be generous with the waterproof yellow glue, as a good seal of the extension box joints could pay off in reduced air infiltration if your spray foam seals aren’t perfect.
Install the rigid foam insulation. We found it easiest to install the windows as we installed the foam. Sure, the ladders and scaffolding were already in place, but more importantly we applied full sheets of foam over the window openings. Since we were living in the house during the renovation, we didn’t want to darken whole rooms and then come back days later to do the windows. So we did each wall in two passes: downstairs, then upstairs.
First, we took out the old windows. Then Dan and Woogs inspected each RO and replaced any rotten framing. (There was more than we expected).
Then we applied both layers of foam, staggering the joints where possible, taping all the seams (double tape on horizontals), and securing the foam to studs with furring strips and 6-inch or 8-inch HeadLok screws.
Our usual practice was not to cut the window openings before applying the foam. Sometimes, however, a partial sheet of foam would suggest covering part of a window opening, so we weren’t absolute about this. Again, it’s important not to cut the foam back to the RO at this point.
Lay out the new window positions on the foam. Here’s the crucial innovation of this technique. First, we transferred the vertical and horizontal center of the existing RO from inside to outside by sticking an awl thru the foam from the inside and then transferring the distance to top and bottom to the outside. Where walls had multiple windows in a row, we did all the windows at once.
Using a laser level, we established the top line of the new windows by determining the lowest top point of the existing ROs and going down 1/4 inch to 1/2 inch. That’s the top cut line through the foam.
We measured down from that top line exactly the outside height of the Dudley box. That’s the bottom cut line. Next we established the vertical center line of each window and measured out from it exactly half the outside width of the Dudley box. Those are the vertical cut lines.
When that’s done, you have the cut lines for the Dudley boxes marked on the foam. It’s time to step back and eyeball those black rectangles. We say, “If it looks right, it is right.”
Fit the Dudley box. Cut through the rigid foam on the lines. (We used a 6-inch drywall taping knife sharpened on one side edge.) Each opening should be exactly the size of its Dudley box.
Test fit it. It should be a press fit, and it might take some effort. Don’t push it all the way in yet; just make sure it starts okay. Then remove it and apply three parallel beads of caulk on the foam around the opening. If you apply one fat bead it won’t squeeze flat, but three smaller beads will squeeze easier and merge together. (This caulk is just one element of the air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both. connecting the windows to the wall. It is not the primary drainage plane water barrier.)
Secure the Dudley box to the studs. From the inside, measure from each edge of the Dudley box to the nearest stud, header, or sill plate. Transfer the measurements to the outside face of the flange. Secure the Dudley box to the framing with sturdy screws thru the flange and foam.
Do not screw through the sides of the Dudley box to the RO framing. Even if you shimmed perfectly, the box would be likely to distort. It’s important not to stress and deform the box.
Don’t worry about strength; the screws through the flanges are plenty strong enough to take the weight of the window, and you’ll be adding even more support later. As with the furring strips, the long screws to the studs put the foam under compression, with the result that the Dudley box is strongly anchored to the wall.
Mount the window. Drill the aluminum window flanges (the brickmold flanges) with countersunk holes spaced according to the manufacturer’s specification. (This is usually 4 inches from corners, 8 to 12 inches apart along edges). We did this on the ground before hoisting the windows to the scaffold.
Securing the window takes two people (at least, depending on size): one inside and one outside. Lay beads of caulk on the inside of the aluminum flanges. Again, this caulk is part of the air barrier, but it does not come into play as a water barrier unless the primary drainage plane has failed.
Position the window in the Dudley box, with one person inside and the other outside. The inside person puts shims between the bottom edge of the window frame and the Dudley box. The Dudley box won’t deform because it is evenly supported by the foam where the window bears. If the box has not racked or twisted, a few shims should give an even gap all the way around the window.
Check the window for plumb. If the opening in the foam was cut plumb and square, the window should sit perfectly in the box. If the box was sized accurately, the gap between box and frame should be even on all sides.
Put one screw at each corner of the aluminum window flange, into the plywood flange of the Dudley box. Check the window for smooth operation and troubleshoot or adjust as necessary. (I think we had to slightly adjust 2 out of 28 windows.) Screw the rest of the aluminum window flange to the flange of the Dudley box.
Disregard the manufacturer’s instructions to screw through the window frame to the studs (or extension box). The factory-applied aluminum flange is plenty strong enough secure the window to the wall, and you’ll be adding even more support shortly.
Flash the window. The idea is to continue the drainage plane formed by the foam’s outer foil layers over the plywood flange and over the aluminum window flange. We used 6-inch self-adhesive butyl tape in the following sequence:
- One piece at the bottom over the aluminum window flange to the edge of the plywood flange, vertical over the aluminum brickmold;
- Two pieces over the sides: first over the aluminum window flange, then wrapping the edge of the plywood flange to the foam;
- Two pieces at the top, first over the aluminum window flange, and then wrapping the edge of the plywood flange to the foam.
We paid a lot of attention to corners. You can find many fine instructional flashing videos on YouTube, as well as a series of helpful videos here on the GBAGreenBuildingAdvisor.com site.
Foam the window. On the inside, spray low-expansion door-and-window foam (the yellow stuff, not pink or orange) in the gap between window and Dudley box. Once it has set, pull the shims and fill that gap with foam to eliminate the thermal break.
Try not to let too much foam boil up past the window frame; it’s a pain to trim later. Seal the junction between the Dudley box and the rigid foam and the studs with spray foam. You don’t need to completely fill the gap to the studs, just make it air tight where the box meets foam.
Note that all this spray foam provides more than ample support for the window and plenty of mechanical grip as well. We didn’t test to destruction, but my sense is that these windows won’t come out unless the whole wall disintegrates. We did test for air infiltration with a blower door and theatrical smoke; there wasn’t any.
Apply interior trim. You will find that the Dudley boxes greatly simplify the interior trim process. If you were rigorous about keeping the gap even between box and window frame, you can lay jamb material flat against the Dudley box and get just the right reveal with minimal shimming (none, usually).
Because the walls are now thicker by 4+ inches, we found MDF to be the best material for painted jambs on windows with a painted casing.
We converted all of the downstairs windows from extension-jamb-and-casing to drywall returns. With Dudley boxes, there's almost no shimming. You just lay 1/2-inch drywall flat on the Dudley box side, and the even 1/4-inch gap gives a perfect 1/4 inch reveal on the window frame. Smiling carpenters!
Upstairs, we aimed to disturb the existing walls around the new windows as little as possible. Dan and Woogs realized they could make complementary inside Dudley boxes — in other words, Dudley boxes made of window trim — and completely avoid the stick-by-stick window trimming process. To make this work, you have to be organized and accurate with tape and ruler. You measure from the window frame to the wall plane to get the box depth on each side (just as for extension jambs). We decided the flange (casing) width room by room to minimize wall patching. The “inside Dudley box” should slip inside the outer Dudley box with a satisfying “Whoosh!,” and a few nails are all that's required to secure it. Our inside walls are pretty lumpy in places, but surprisingly little caulking was needed behind the casing edges. In some rooms I didn't even have to touch up the walls — I only painted the inside Dudley boxes.
A faster, better method
Dudley boxes go together much more quickly than built-in-place extension boxes, so you’ll save a lot of time. If you have multiple windows the same size, as we did, you can cut the parts all at once on site and assemble as needed. Or you could make them in the shop and speed things up even more with a biscuit joiner or your rotary clamp machine.
Dudley boxes are easier to install. Once you have the window locations accurately marked on the foam, the rest is easy — no fussing with shims, levels, etc., and much less chance of warping or twisting, because the boxes hold their shape.
Finally, you get a better performing wall, because all connections are accessible with spray foam, and they are backed up with caulk. Try it on your next retrofit. If you don’t agree it’s a better way, I’ll be surprised.
Joel Schuman is a semi-retired furniture designer and software developer living in Saugerties, New York.
- All photos by Joel Schuman
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