Routing Sash for Insulated Glass Units

Wood repairs for sashes, frames and sills.
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Joined: April 13th, 2011, 7:34 pm
Full Name: John Leeke
Location: Portland
Organization: Historic HomeWorks
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Location: Portland, Maine

Re: Routing Sash for Insulated Glass Units

Postby johnleeke » March 9th, 2012, 3:27 pm

Steve writes:
>>Are the standards going to bend because of pressure or perceived pressures? <<

No. We are writing the Standards to stand right where we want them with no bending necessary.

>>Most of his points were mere justifications to smooth the pressures.<<

It's important to understand that Neil is giving us his insight about the office that publishes the Preservation Briefs so that we can better understand and influence the people who work there. He also provides us with his experience in dealing with the people who decide to have sash routed and IGUs installed, about which we need to know as much as possible so we can be effective at influencing those decisions.

My own feeling about this is that I should rout out a sash and install an IGU, and then live with the result so that I can speak from personal experience when I say whether it is good or bad thing to do. Otherwise, it's just my opinion.

By the way, the old authentic Thermopanes were good. I sent a high-fly ball through a brand new one in 1961 and learned first hand about welded-glass edges as I helped my dad pick up the pieces and install a new one in that picture window. I've got a couple of these old thermopanes in my current house here in Portland that have lasted half-a-century and still perform well with absolutely no fogging. (they are 3/8" thick, and would fit nicely into many old thick sash with no routing needed)

But, the old authentic Thermopanes are not to be confused with the contemporary double-pane, insulating glass units (IGU) that are sealed at the edges with metal strips and goop. These seals have always had a high failure rate and the window industry knows it, but keeps this a secret from their customers.

Also, Eli Cohn, a window specialist in NJ and WPSC Stakeholder, worked for several years as an Andersen warranty Service Provider, I'm sure he knows all about the thermopanes, meet Eli here: ... p=338#p338
Standards Co-Founder
Standards Editor

Bob Yapp
Posts: 59
Joined: May 9th, 2011, 8:39 am

Re: Routing Sash for Insulated Glass Units

Postby Bob Yapp » March 9th, 2012, 3:53 pm

John Leeke says, "My own feeling about this is that every window specialist should rout out a sash and install an IGU, and then live with the result so that they can speak from personal experience when they decide and say whether it is good or bad thing to do. Otherwise, it's just opinion or hearsay."

John, my brother, this is not hearsay or just an opinion. This is 30 years of observation and basic woodworking knowledge. We don't have to do one to know it's a bad idea. I have many objective studies about IG failures and I have seen hundreds of failed, routed out IG sashes. Now, should we as a group, look for examples and do a future study on the ruinous practice, of course. Here are some other responses I recieved:

Once again, Bob Yapp gets to the heart of the problem. I cannot get any definitive answer to questions about low-e glass and our experience (windows installed without application) was that they looked like darkened car windows. That is certainly not the appropriate look for historic houses.

We encourage everyone to restore their windows and install storms but so many have bought into the "save 20% on your fuel bills" with new windows nonsense. One long time window salesman admitted that you would never make back the outlay for the new windows in fuel savings and that by just inserting sash kits (which most of the replacements are) you are not tackling the real issue - air infiltration around the edges.

I would be very interested in seeing the report. Every piece of information we can give to homeowners to get them to retain historic windows helps.

Lynn L. Petersons
Operations Manager
The Homeland Association, Inc.

Good points.

Another factor in retrofitting insulated glass into an existing wood sash is increased weight. After this intervention, the existing pulley weight system cannot be used, so some kind of spring/friction track system is used instead to hold the window in place. In my experience, because these systems typically employ a lot of plastic pieces, they break and deform and eventually no longer hold the sash in place. I know of many issues where, in less than 10 years, these retrofitted sashes were replaced with new windows after the track system repeatedly failed.


Hear, hear!!

I am not a fan of IG and I am happy to see someone stand up against this practice.

Obviously exterior storm windows are the best option, but there are cases on prominent facades, where other options might be explored. I would be interested to see if laminated glass with traditional window putty is an option that reduces heat/cold loss while still keeping the windows structurally intact and maintains the historic appearance of the window and surround.

Thanks for forwarding this around our office,

Michael Radeke
Restoration Project Manager
Kentucky Heritage Council

Bob, Thanks for doing this. Chris.

Christopher C. Skelly
Director of Local Government Programs
Massachusetts Historical Commission

I forwarded your letter Bob to Tommy Kleckner, IN Landmarks as well. He has a classic case across the street from his office of a building in which IG was replaced in the lites of historic windows. I believe they are not even 5 years old and are failing. I asked him to take some pics for us to use as examples, but I imagaine we all could come up with some!!!

Historic Madison, Inc. Director of Programs
RLD Glass Art and Restoration/Ol'House Experts
Historic Preservation Trades/Educator
Madison, IN

Hi Bob

Thanks for doing this. We are in the process of revising our guidelines and look forward to the data from the collaborative especially about storms and energy efficiency.


Mike Cowhig, Community Planner
Planning and Community Development
City of Greensboro

Bob Yapp
Posts: 59
Joined: May 9th, 2011, 8:39 am

Re: Routing Sash for Insulated Glass Units

Postby Bob Yapp » March 13th, 2012, 5:28 pm

A reply on the National Trust's Forum-L Tuesday, March 13, 2012:

"It is enjoyable to lurk as Bob and Neal work though these issues.thanks to you both for your viewpoints.

I must comment, however, on Bob's point on what may be a larger issue related to fixed v. operable windows. Stan Cox's fascinating study Losing Our Cool (The New Press, 2010) presents what at best can be seen as a pretty short event horizon for the continued use of air conditioning as we have grown accustomed to it. If he's right, and the numbers certainly support that theory, then the replacement of operable windows with fixed, of any design, has grave long-term implications for energy policy and actually moves structures toward a less and less energy-efficient standpoint overall. I know that I was incredibly frustrated by the silly requirements of TARP-funded upgrades to historic structures intended to improve energy efficiency by mandated fixed windows.

I should state that a large part of my practice is with public and private utilities. many of whom are busily fending off demands that hydroelectric projects be jettisoned in favor of "green" sustainable energy generation with a higher groovy factor that an updated 19th century dam at the same time that they face huge increases in overall power demand (Think computer farms and electric cars). With the demand for electricity, however generated, we are either going to have to build massive new infrastructure or face serious shortages of power.

I expect that 20-30 years down the road any building with fixed windows above the storefronts is going to a dinosaur. Opening your windows, historic or replaced, will again become a necessary option for many property owners. I expect the window vendors will next figure out a way to make replacement of your double-paned low-e fixed glazing the "Green" thing to do."

George Kramer
Ashland, Oregon

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