A few months ago I finished installing a set of six custom windows that I had designed and built. Except for the insulated glass units themselves, I built nearly the entire window set from scratch. I installed the windows in my strawbale shed. While still in the planning stages for the shed in late 2019, I had decided to make custom dual-pane windows instead of purchasing stock sizes. A decision that was somewhat ambitious at the time, it ultimately turned out to be a fun side project as it got going. The designs the windows were completed during the last part of 2020 and then actual building of the windows started in the late spring of 2021.
All-in-all, it took about a year working on them for 2-3 hours typically per week to build, finish, and install them. While I ultimately decided not to build the windows for my full-size house due to the time commitment involved (considering I work a full-time job), I really enjoyed the experience building these windows and feel that they are likely as well or better constructed than what can generally be purchased commercially. In this post, I am first going to cover the design features of the windows, then move into discussion of building them, and lastly, installation in the 10×10.
Windows are considered one of most important parts of a building envelope and rightly so. They generally offer the main avenues of heat energy loss or gain, water intrusion, air leakage, solar loading, maintenance requirements, and a significant percentage of the overall cost to a building envelope as well as most of a home’s physical security vulnerabilities.
Architecturally, a window tells much about a building’s character. A poorly-constructed window will give the building it is housed in an overall cheap and artificial feel. An well-designed window will lend an air of value to an otherwise poorly constructed structure. The arrangement of window voids and openings, spacing, and repetition (rhythm in architectural terminology) combine with the overall window size and style to make a house appear either warm and inviting or cold and distant. Window selection is one of the most critical decisions that a builder will make on a structure.
Window Design Features
While researching wood aluminum-clad windows for my main house, I began noticing that none of them really struck me as well-designed windows. Even the more expensive brands were often constructed only somewhat better, while charging thousands of dollars more than economy units. Sometimes the issues could be traced to being intrinsically non-conducive to mass production but more often it was poor design itself that I felt was the root cause.
Looking back, most of the issues seem to group into the following general complaint categories:
- Poor hardware quality – die-cast pewter, zinc or even plastic levers and cranks, protruding locking lug tabs
- Structurally integral insulated glass units (IGU’s) – the wooden sash is not strong enough to support the IGU alone in most cases so it must be structurally glued to sash to form a integral unit. Integral IGUs complicate repair after breakage and sometimes require the replacing the entire sash. Bonding the IGU to the sash can also introduce stresses into the IGU and reduce it’s lifespan.
- Multi-piece aluminum facade – typical facades are made from 4 pieces. While some units are joined with tenons, some are not. This provides a path for water to seep in between the facade and the sash.
- Weak facade attachment to the sash – while upper end aluminum-clad are made from extruded aluminum (rather than roll-formed), most are attached to the sash via a protruding polymer strip that seats in a kerf cut into the sash. This method also usually still facilities moisture capture, contributing to sash rot.
- No IGU pocket drains – over time IGU-to-sash seals will inevitably leak, letting at least a little water into the IGU pocket on the sash. Without a way to drain, the water will accumulate rotting the sash and potentially degrading the seals on the IGU.
- Use of PVC or TPE seals rather than EPDM – though more expensive, EPDM is a longer lasting material
- Thin wood on crank covers for casement windows – will eventually break…though some brands were more hefty
- Window pockets can be difficult to clean – Nooks and crannies in the hinge area catch dirt, crank covers create a pocket that collects dust, spiders, etc.
This is probably not a complete list. In designing the windows, my goal was to eliminate or mitigate as many of these drawbacks as possible while building a strong, low-maintenance, high performance window for a reasonable cost. The window had to be repairable, look reasonably neat when completed, and most importantly, could be built at home with a basic selection of tools. While, I wasn’t able to eliminate every issue, I feel the result is a superior product in more ways than one. In the sum-total, it is possible to have quality home-built IGU wood windows with an aluminum-clad sash that offer most of the features of a commercially-produced unit for less than the cost of a vinyl window.
Compared with the classic single pane units of yesteryear which were relatively easy to build at home, one of the major obstacles to modern home-built dual pane windows is the increased weight due to the second pane of glass on the IGU. Because a conventionally-built sash generally cannot support this increased weight, the IGU is structurally glued to the sash in commercial units. This issue can be resolved, though, by making the facade a structural component. Here the facade supports and distributes the vertical forces and moments while the facade combined with the wooden sash resists torsion and bending forces. That eliminates the need for the IGU to be structural.
True to being an engineer, I designed the windows in a CAD program. It helped immensely while designing the multi-point lock. I caught most of the design errors in CAD; though, one error slipped thru on the locking rod and I didn’t discover the issue until assembling the lock. Fortunately, I was able to simply re-machine the correct features without scrapping the rod.
In designing the windows, one of my goals was to design the units for easy building and assembly. Doweled joints for the frame allowed quick and efficient joints with a minimum of precision. Only stock thicknesses of wood are used, eliminating any need to plane the wood to the correct thickness. The main metal pieces were either stock or modified COTS parts or were cut from sheet stock by a plasma cutter vendor. All holes were cut by the plasma cutter at the same time as the main shapes. Even if the holes needed adjusting later on, cutting them at the same time as the part provided accurate positional references which helped everything go together smoothly.
Close tolerances were eliminated as much as possible. For example, because the wooden part of the sash is no longer handling vertical or moment loads at the joints, I was able to make the length of the mortises slightly more than the tenon height which enabled the milling to be much less precise than a standard mortise and tenon design. Also, because of the wide clearances required around the IGU, the need for highly accurate wood parts is surprisingly low. While the multi-point lock did have some tighter tolerance cutouts, in general the accuracy required for building these windows seems less than it did for a run-of-the mill casement window that I once built years ago.
Summary of main window design features
- Exterior structural facade (cladding) from 3/16″ aluminum sheeting. The facade holds the window together and absorbs the shear and bending moments at the sash joints.
- Sash joints are standard construction (Mortise with floating tenons). The tenons are slightly smaller than the mortise pockets to allow for non-precision in the setup. This is possible because the shear forces and bending moments in the sash are taken up by the facade.
- Stock wood thicknesses are used on both the fame and sash
- EPDM material for seals and IGU pads (20-year working life)
- Dual sash seals
- Drain tubes along the bottom rail of the sash drain any water that accumulates in the pocket below the IGU.
- Dual pane insulated glass units (IGU) with low-E on outer pane
- Anodized exterior facade provides long lasting color with minimal to no maintenance
- IGU’s have only interior grid lites. This allows window glass to be cleaned efficiently but still provides the “bite-size” viewpoint chunks. The interior grid colored a dark bronze. (Note: choosing a light grid color will cause the grid to appear slightly green when viewed from the outside due to the low-E coating.)
- Multi-point lock from stainless steel with a custom heavy-duty gear drive
- Hardware accepts standard Euro 7mm square-drive handles
- The exterior facade is attached to the wood frame with screws. This allows the exterior facade to be removed when necessary for maintenance or in order to replace the IGU.
Building the Windows
The first step was to mill the lumber. I employed a chop saw to cut each piece to approximate length, rip it to width on a table saw, and then trim to exact length using the chop saw again. Once that was complete, I inspected each piece for approximate straightness (I rejected a few of the longer pieces which were badly warped). Each group of pieces were bundled with blue painter’s tape. Colored sticky tabs provided identification of which window type and the location (eg. upper window, RHS sash piece). After that, I began to add the milled features. Since a lot of pieces had the same or similar features, I was able to put together a setup and then run the entire lumber stack thru in assembly line fashion which helped save a lot of time. I put together half-a-dozen jigs or router patterns over the course of the build which also helped to save a significant amount of time.
My window profile-cutting bit was only intended for single pane windows. Adapting it to fit a 1/2″-thick IGU was a little tricky. The IGU is significantly thicker than a single pane which can mostly be accommodated for by adjusting the specialty window bit or using a rabbiting bit. However, for the interior sash joints I wasn’t able to to gain enough cutting depth with the stock bit, so I used a slotting cutter holder with the window bit pinned to it up-side-down. This reversed the cutting edge so I had to temporarily modify the router to turn in the opposite direction. Fortunately, router motors are universal motors, so reversing the motor direction was as simple as swapping the brushes.
Each aluminum facade composed of four straight pieces of 3/16″ sheet stock, welded together to form a single part. While it made a lot more work, cutting each of the pieces out used stock material efficiently and theoretically saved some costs. (I paid a plasma cutting shop to cut out the pieces.) The first order of business was to countersink the holes for the mounting screws. Next, the outer edges of the facade were rounded with a wood cutting bit on the router table. While not an ideal bit and the aluminum behaved a bit gummy, the wood cutting bit did work alright. Once complete, I carefully laid the pieces out on a flat table and joined the pieces together with a TIG welder. The welds were ground flat and the entire surface of the aluminum was lightly sanded and the rounded edges were blended in. Finally, I had them hard-coat anodized.
The hardware was a fun challenge. I was careful to design the lock to only require a minimum of machining and utilized self-locating features when ever possible (for example, the latch plate is sized so that it’s lateral position is set by the jamb). The only machining required was to machine mounting areas for the rack on the lock rod, trimming bushings to correct size and broaching the main drive gear.
Sourcing latch handles, hinges, and window stays was difficult. The latch handles were especially difficult as most of the US stock of Euro-drive (7mm) handles are cheap and ugly. I tried a California-based distributor of Italian handles. The handles were nice but I couldn’t justify the prices. Ultimately, I bought a few handle styles from a UK-based company, From the Anvil, and had them shipped for an expensive but affordable price. The handles were black-painted, but otherwise I was pleased with the quality of the castings and the styles.
Finding black mounting screws to match the anodized facade was another challenge. I tried buying screws and unsuccessfully aging them until they were black. I also tried dying the screws but it still wasn’t a good match to the anodize. In the end, I painted them with a black enamel spray paint and carefully let them dry in the sun for about a week prior to installing them on the windows.
Assembling the windows went fairly smoothly. I was careful to ensure that the frames and sashes were glued-up sparely. This was most important with the sashes because the truncated tenons allowed the joints to be far out of square. Once the sashes and frames were assembled, I test fit the complete assembly with the hardware, then proceeded to disassemble, sand, and apply the finish.
The finish consisted of two coats of a Tung-oil varnish mixture. It was let dry for several days and then buffed mediumly with a soft rag to bring it to a dull lustre. Looking back, it took a significant amount of time and probably wasn’t worth the effort to use an oil-finish on these windows. It was nice to learn first-hand what people mean when they talk about the labor of an oil-rubbed finish, though. 🙂
Once the finish was complete, the hardware was installed for the final time and the windows were ready for installation.
Installing the Windows in the 10×10 Shed
It didn’t take a lot of time to install the windows in the shed. The process was similar to old-style windows before the advent of the nail fin. I used shims to hold the windows in place, checked level and squareness, then pre-drilled and nailed the frames to the rough opening with 16d finishing nails. One or two spots where the frame bulged inward significantly needed screws but I was able to hide those spots behind the latch plate.
Update: as this post goes to print, it’s been a couple months after first starting this article and nearly four months since the windows have been installed. All-in-all, the windows seem to be functioning very well, though I would definitely make some changes were I to make these again. Most importantly would be re-design of the multi-point lock so that it is a one-piece assembly with the latch plate. While making the lock design slightly more complex, it would eliminate boring the holes in the sash rail for the locking rod, likely eliminate the (expensive) lock rod, and allow for easier maintenance and repair. The precision required to bore the lock rod holes is difficult to achieve especially repeatedly over the entire length of the sash rail. The other change would be to choose a tougher finish. The finish that I used hasn’t held up very well over the summer to UV or to water. I was expecting minimal once- or twice-a-year maintenance since it was an oil-based varnish but it looks like I’ll have to be careful about maintenance, at least until I get a new oil finish established. I’m sure there are other changes and design mods that should be made as well but those are the main two so far. Otherwise, I am very pleased with them.
I’m so thankful for the opportunity to build my own windows! Working with my hands to create something functional has been both satisfying and mentally challenging. I’ve also appreciated learning new skills as well as re-using ones previously learned. At times I’ve wondered whether it’s worth it; whether it’s worth starting from scratch. Certainly, for many things it’s not – especially when we consider how long we live and how temporary life is. Learning to know when to build and when to buy is a valuable life skill. For these windows, each one required about 45 hours to assemble and finish. Applying moderate design and manufacturing improvements, some of which I’ve mentioned above, should drop this number to around 20-30 hours. While that is still a significant time investment, it is possible to build one over the course of a few weekends.
Blessed is the man that trusts in the Lord, and whose hope the Lord is. For he shall be as a tree planted by the waters, and that spreads out her roots by the river, and shall not see when heat comes, but her leaf shall be green; and shall not be careful in the year of drought, neither shall cease from yielding fruit. - Jer. 17:7-8
