Read questions and answers for architects on restoring historic wood windows.
Note: This Q&A is transcribed from Brent Hull’s webinar, Repairing Historic Wooden Windows: What Architects Need to Know. The webinar was hosted by the Traditional Building Conference and is available on-demand. Watch now.
Q. Why isn’t oak on the list of the best types of woods to use?
A. Oak isn’t on my list because we don’t end up using it often on historic buildings where they had originally made the window or door in oak. White versus red oak is very important. White oak is an excellent performing exterior wood. Red oak is not.
Q. How can you find out which woods would be appropriate for the region you are working in?
A. In general, if you are west of the Rockies, you are looking at Western woods. Fir and redwood are the best long-performing exterior woods. If you’re in the South from Texas over to Florida and up into the Carolinas, you’re in the longleaf cypress area. This continues up into Tennessee and Kentucky. Above this, on the Eastern side from Minnesota over, you’re in the Eastern white pine area. If you’re working on a building from the 1870s—almost anywhere— the wood would be white pine. This is because they weren’t harvesting wood from the South yet for millwork. So it’s a two-part thing: It’s the region you’re in and what building you’re working on. There might be state and county foresters that you could speak with or the federal forest products labs. There are also university state forest labs that publish research as well. You might also reach out to your state historic preservation officer.
Q. Are old growth woods more color stable from fading? What are the most color stable woods that will not bleach out, fade or darken? Which are the worst?
A. I’m assuming you’re talking about a stain-grade wood that’s fading out on project. The only wood that doesn’t lighten in the sun is cherry, which darkens over time. Walnut will lighten over time. It’s probably the worst because it will tend to yellow over time. Cherry is probably the best because it darkens.
You need to control the UV and pay attention to the finish. Certain finishes cause the wood to bleach and yellow more than other finishes like oils. Most stain-grade woods are hardwoods. Longleaf pine can even darken with sun. Oak will also bleach. There’s a book by Bruce Hoadley on wood called Understanding Wood that might be a useful resource.
Q. How does dipping the sash affect the wood long term?
A. It depends on the system you’re using. There are many systems available for getting the paint off wood, like steam boxes. You can dip the wood. If the wood isn’t left in very long and used with a dipper who is mindful of waiting until the solvent starts to break through the paint, it’s fine. If you leave it in the solvent too long, it will cause problems.
Dipping doors is a bad idea because of the glues and the panels. If softwoods are left in too long, they might get a little fuzzy. Once it dries, though, you can usually sand those things off. It’s really a matter of using it properly otherwise it gets soft and can lose its integrity.
Q. Have you worked with any of the specialty woods like an acetylated wood?
A. The technology with wood preservatives is improving. Accoya is a trade name of wood that has basically been pickled. It’s been heated in an oven which changes the molecular structure of the wood. I once worked on a project with Accoya gates. It’s probably been 10 years. The gates were painted and have withstood extreme exposure from the sun and they’re still rot-free. The quality of that product has a great future. The problem with this particular wood is that it’s very expensive—over $10 per foot last time I priced it.
Q. Have you used any of the vacuum insulated glass for replacing glass in historic window restoration?
A. The quick answer is no. We’ve tried on two projects to get vacuum insulated glass specified. One time was with the energy code. They haven’t been tested enough to know that it’s better. I’m a big believer in the vacuum glass and what it can do. Aesthetically you have the problem with the button, but then you don’t have the problem with the muntin bars. I think we’ll see more of it in the future once the energy code people catch up.
Q. How do you specify repairs to weather check on the sills, particularly in the courthouse project with the green paint on the sill you had discussed?
A. My thoughts are that the sills are a pretty typical hard abuse area. The sills are laid flat in the sun. One side is exposed and the other side isn’t. The competing nature of the two sides of wood can cause it to check. When it’s flat, water will lay on it longer. My experience with old growth wood is that we’re replacing that only about 20% of the time. The integrity of the wood is still decent even if there’s damage. Sometimes we use a product like Abatron to fill in any checks, but there usually isn’t much rot. Certainly, there are times where we have to cut out the wood of the sills and replace it, but it’s not as often as you’d think, largely because of the integrity of the wood.
Q. What about patching small rotted areas rather than replacing the entire wood component?
A. Yes, I think that’s money well spent. The only problem is that sometimes for checks, for instance, you can repair them with an Abatron because there hasn’t been much rot there. Rot is a different thing. It’s a living, growing fungus. In my experience, it’s been challenging to get all of it. So if you have a joint, to peel off that rot and try to harden it, and then put Abatron over it—it’s a Band-Aid. The rot will continue to move through that wood, away from the repair. For some areas it works great and for others it’s just a Band-Aid.
Q. Do you have any further thoughts on weatherstripping, resetting stops or making fascias more airtight?
A. I’ve been amazed at our ability to make a window weathertight at the adjustable stops at the end of the window by adding weatherstripping. Weatherstripping is a historical product that really can make a huge difference in the performance of the window.
If you gave me a double hung window without weatherstripping, where when the wind hit and the glass would rattle inside its pane, I could take that window, strip it, reset the stops and add weatherstripping and that window would perform 10x better than the original one. The great news is that you can restore old windows that haven’t been touched in a hundred years and you can restore them with very little work. You can have a great performing window very effectively and cheaply compared to buying something new.
Q. Contemporary windows can be quite large. What advice do you have for the maximum window size when the frame needs to be larger to perform and operate properly with the weight of the window glass?
A. We worked on the state capitol in Texas. The total height of the window was 12’ tall and those sashes were about 6’ tall. The stiles and check rails were not 2.5”! The bottom rail on the sash was about 6”. The side styles on were about 4”. So those numbers absolutely need to change when the window sizes change. This was obviously a monumental building, but the scale still works and looks right when you’re looking at it from the street. I like a bottom rail to be at least 3-4”, especially with a pitched sill because they tend to last longer.
Q. IBC is no longer allowing the mounting of plywood as an effective device for hurricane protection. How can we protect restored window sashes once they’ve been replaced when they’re threatened by a weather event?
A. I didn’t know they weren’t allowing plywood. We use a lot of bulletproof plywood for courthouses when there are some safety issues. You can buy bulletproof plywood in 4 x 8 sheets. Nothing will blow through that, not even a bullet.