I've taken an interest in building new old window sashes. That is, traditional wooden double-hung windows, as you'd find in any home in America before about 1940. This little essay contains some of my notes on things I have learned about the process.

Why make traditional sashes?

My interest formed when I started in on dealing with rotting sashes and drafty, leaking windows in our 1927 Seattle house. I received some bids from general contractors, but they all proposed to remove the old windows and replace them with new modern windows from Anderson or Marvin.

Those companies make good products, but I didn't like that plan for a few reasons:

  1. Longevity: Modern windows generally rely on double-pane insulated glass. This technology uses two panes of glass, with an air seal separating them. Eventually, that seal will fail, even microscopically. When it does - not if, when - it will let in moisture, which will fog the window and make it practically opaque.

    Most window manufacturers cite lifetimes around 20 or 30 years. But even when poorly maintained, traditional windows will keep working for over 100 years. They're just a more robust, and in some ways simpler, design.

  2. Aesthetics: In the early 20th century, windows were usually made locally by one of a few shops. That means that the windows ended up having a local character. Here in Seattle, that meant (for example) ovolo profiles with a narrow quirk, frequent use of leaded panels, and generally an undivided bottom lite. This forms a design language that is characteristic of your city.

    When you buy a modern window from a big national manufacturer, they don't have any interest in matching the local style. You get a little bit of choice, but mostly in things like wood species and hardware color. If you want divided lites, it's going to be with "simulated divided lites" (essentially sticks pasted onto the surface of the glass to fake the look of real muntins).

    It's hard to describe, but these modern windows always, always look out of place on an old house, and I think it's because they swim upstream against that local design language. They just don't look right, like a strange and plasticky mask.

  3. Window size: Modern replacement windows are generally installed as "inserts". This means they remove the old window sashes from the wood framing around them, then install a new window frame inside the old frame. This means that you end up with a new window which is a few inches smaller in both height and width than your old window.

    They do this for a few reasons, most prominently that new windows are manufactured as one integrated piece. In the 1920s and earlier, windows were often at least partially crafted on site by the carpenters: they'd install pulleys and weights into a jamb, and that counterbalancing system holds the window sashes up. But to make installation faster and less skillful, window manufacturers have developed new suspensions which require no on-site work. Nail the window up, and it's good to go. But this is incompatible with old openings.

    But if you reproduce the old wooden sash, you can reuse the old jamb, so you keep the old dimensions, of course.

  4. Cost: This surprised me! High quality windows cost a lot. We're talking $2,000 to $4,000 per window for modern windows which include wood and at least attempt to blend in with an old house's aesthetics.

    But an old-style window just isn't made of that much wood, and single-pane glass is pretty cheap. The raw materials, even getting best-of-the-best wood, end up costing under $200 per window. If you have the tools and the skill, reproduction windows can be cost competitive, even including cost of your labor.

What about energy efficiency?

The most serious argument for modern-style windows in an old house is energy efficiency. There are three sorts of heat loss we're contending with:

  1. Conduction: Glass will just conduct heat directly between the air in the interior and exterior of the home. The greater the temperature difference, the more this matters.
  2. Radiation: Light which passes through the window carries energy with it. For infrared light, this is a major source of heat loss.
  3. Air movement: Since windows have moving parts to open, they can't be perfectly caulked and sealed. This means they have gaps which let air through - and that air carries energy with it, letting heat out.

It's really hard to generalize about the relative importance of these factors. Effects are different for heating versus cooling, whether it's windy or still, and whether in the shade or not.

There is no question that double-pane insulated glass conducts dramatically less heat than single-pane glass. Many (not all!) modern windows are also equipped with "low-E" coatings which make them nearly opaque to infrared light, which cuts down on radiation loss.

Air movement is a bit trickier; it will at first depend largely on the competence of the installers. If they are diligent, and they carefully seal all gaps and flash the window completely, it can be pretty tight. In addition, sliding parts of the window that let it open and close are sealed with weatherstripping at the factory. All modern manufacturers I'm aware of use various plastics and foams for this, which probably last about 10 or 20 years until they wear out.

Traditional windows have tools to deal with each of these issues.

The most important tool is a storm window. This is basically a second, lightweight window which you can attach to the outside of the house's window frame. It ends up forming a "double pane" but without any fancy seals; it's intentionally got some air gaps to ventilate moisture. Storms work very well to reduce conduction because they create an insulating boundary of air. They'll also help reduce air movement losses in an old window.

For radiation, there are two approaches. The simplest is window coverings: curtains, blinds, shutters, whatever - if you can't see through the glass, you're probably not going to be losing much to radiation. A more fancy solution is to use low-E coating glass on a modern reproduction of a traditional window. A good glass supplier should be able to provide single-strength glass like this.

Finally, good traditional windows use interlocking metal weatherstripping. I'll write more about that at some point, but suffice to say that interlocking metal weatherstripping dramatically reduces air movement, improves the mechanical function of the window by giving it "tracks" that keep it moving straight, and lasts pretty much forever.

So, I think there are plenty of tools in the arsenal for traditional windows, especially in a mild climate like Seattle's.

Traditional sashes are a good challenge

The last reason that I've taken to learning about making traditional double-hungs is that they're pretty challenging. I've been doing fine woodworking (mostly furniture) for about a decade now, which gives me a good foundation for taking them on. But they're very technical and demanding.

If the sash is out of square at all, it'll bind when slid in the opening, so you need to be real careful when preparing wood to use in the frame. Everything needs to be jointed and planed carefully, and it's critical that you select only the highest quality pieces of wood that won't twist and warp at all, which takes an experienced eye (and some degree of good fortune).

All joints need to be dead tight. One sashmaker I talked to said his tolerance is usually measured in thousandths of an inch. Any gaps are a serious problem because windows are exposed to constant weather. A gap in a joint quickly becomes a spot where water will seep in, rot the wood, and break the window apart.

The window parts have a fairly large number of grooves, angles, and rabbets to accomodate all the pieces that make up the window. The interior face is generally given a molding profile, which needs to be accomodated in tenon shoulders. You need a place for the glass, of course, but also for the ropes, and a notch to let the sash slip over the pulley sheaves that are in the jamb. If you take on interlocking metal weatherstripping as well, that adds more complexity, especially at the meeting rails of the window. In short - there's a lot going on.

I find all of this thrilling. Inside all that complexity, you can also see that there's a lot of room to get very, very good at making windows, especially if you can establish and streamline a process, efficiently moving from rough stock to milled parts, on to cut joints, to stuck profiles, all the way to assembly.

Anyway, I'm learning a lot about it, and I hope to write more. I've made a few sashes and I intend to write about what I've learned since it can be hard to find much material on the internet. Beware, I don't write as a seasoned expert for decades, just as a nut who is wading deep into this stuff, and interested in, perhaps, making things just a little clearer for anyone following me.