Routing Sash for Insulated Glass Units

Wood repairs for sashes, frames and sills.
johnleeke
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Routing Sash for Insulated Glass Units

Postby johnleeke » March 5th, 2012, 4:05 pm

Bob Yapp has sent the following letter to the National Park Service about the newly revised Preservation Brief #3.
We intend to revise this letter into a "position paper" on the issue of routing for IGUs, and included a statement on it in the final Standards.

Please let us know your views by clicking on "post reply" above.

**********the letter:

March 5, 2012


National Park Service


A large group of preservation artisans and social policy people across the country were excited to see that Preservation Brief #3 Improving Energy Efficiency in Historic Buildings was recently updated. There's lots of good, solid information in the brief. That is to say, many were excited until we read the strings about historic windows. While we're grateful for all the cautionary comments about not replacing windows and using storm windows, it is disconcerting to see the same document give advice that is contradictory and inaccurate. The following constitutes a gathering of opinions from many in our national organization the Window Preservation Standards Collaborative (WPSC) and others:

1) Preservation Brief #3 says, "Where the sashes are sound, but improved thermal performance without the use of a storm window is desired, some windows may be retrofitted with insulated glass. If the existing sash is of sufficient thickness, it may be routed to accept insulated, clear low-e glass without extensive loss of historic material or historic character."

We contend that insulated glass retrofits into existing wood sashes is a bad idea regardless of sash thickness. This practice, over the last 30 years, has proven disastrous to the structural integrity of wooden sashes. Residential windows, 1-3/8" thick, cannot accommodate a typical 1/2" or 5/8" thick IG unit. This is no less true for commercial wood sashes, which are typically 1-3/4" thick.

The problem begins when the sashes are deeply routed to accept the IG unit. This step causes permanent and irreversible damage to the mortise and tenon joints which hold the sash together. These efforts to increase the depth of the glazing bed, penetrate into the most vulnerable areas of the mortise and tenon joints, weakening them permanently. The added weight of the IG unit then puts stress on these structural joints causing the potential for complete failure of the entire window sash.

Through observation in the field, many of us have witnessed IG retrofitted wood sashes, done over the last 30 years, failing as described. An additional issue is that in almost all the cases, traditional putty glazing is not used in favor of wood glazing strips. These wood strips allow moisture to penetrate the stiles and rails of the sashes accounting for more deterioration of the original sashes.

In a typical 1-3/4" commercial sash, the deepening of the glazing bed for an IG unit and wood glazing strips puts the depth of these beds at 1" to 1-1/4". This is well over half the thickness of the sash and structurally catastrophic. This practice conflicts with thousands of years of basic woodworking knowledge.

The national Window Preservation Standards Collaborative is a group of over 100 of the top window restoration and weatherization professionals in the country as well as architects, city staff, consultants and SHPO personnel. We are currently in the editing stage for the first national Window Preservation and Weatherization Standards. To my knowledge, not one of the WPSC artisans was ever consulted before this revised brief was re-issued. We will be recommending that this practice never be done. We will also be publishing our own objective testing data which show that weather stripped sashes with a weather stripped storm, exceed the 2012 International Energy Code for air infiltration. As you eloquently point out in the brief, storms are really the answer.

My goal in writing this letter is to ask you to change this section of PB #3 to reflect these facts. To date, no one has ever done a study on the sustainability of these retrofits over the last 30 years. We'll be looking at the possibility of gathering existing data and future testing but for now, there isn't any basis to even recommend it. So many old house and building owners depend on the Preservation Briefs, as do I. Many historic tax credit property developers leap on any vagueness in what's allowable. Recommending this practice is akin to allowing the sandblasting of brick if you want your brick to be really clean.

Steve Stier
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Re: Routing Sash for Insulated Glass Units

Postby Steve Stier » March 5th, 2012, 5:03 pm

I think Bob's letter is good, & we all should make our individual feelings known to NPS. Even better that you will rework this into a position paper to send on behalf of WPSC.

I did this to some windows in my 1860's farm house about 12 years ago, and some of the IGU's are failing. The sash were not designed to hold the weight and tend to fail under more than twice their design capacity, even without the routing. The windows look terrible cause the metal between the glass always shows. It's just a bad idea, and unnecessary on top of that!
Steve Stier

oculus
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Re: Routing Sash for Insulated Glass Units

Postby oculus » March 5th, 2012, 5:08 pm

I agree with the stance that Bob has proposed. I have always thought that re-routing was a disaster for historic sash. Too bad this discussion did come up a couple of years back for the Oregon State Hospital project. They eventually decided to re-rout and put in an interior shatter-proof storm panel. Who know what is growing between the panel and the historic sash by now.

Here is my old blog post about the project. It was a real heart-breaker.
http://oculuswindow.blogspot.com/search ... 20Building
Amy Harrington McAuley
Oculus Fine Carpentry, Inc.
http://oculuswindow.blogspot.com/
oculuswindow@gmail.com

"Opportunity is missed by most people because it is dressed in overalls and looks like work"-T.Edison

Bob Yapp
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Re: Routing Sash for Insulated Glass Units

Postby Bob Yapp » March 8th, 2012, 11:56 am

I put the letter up on Forum-L at the National Trust. Neal Vogel made the following thoughtful comments:

Bob-

I can appreciate your position on this and agree that the statement is misleading at face value and problematic for most residential windows.
I should also disclose that I interned in the Technical Preservation Services office in the mid 80s.

Much of the staff's collective experience stems from review and (sometimes painful) appeals of tax credit applications; developers and their legal counsel lobbying for approval of their specific projects (and I'm guessing calls from their Reps and Senators happening behind the scenes, albeit much of my distrust is fueled by Stewart and Colbert).

In any case, I experienced a sharp, knowledgeable staff with very high principles attempting to balance pure, authentic and idealogical approaches to preservation with cost-data, spreadsheets, legal interpretations and yes, sometimes twisted definitions of the Standards for adaptive re-use projects.

I suspect this specific clause/viewpoint in PB #3 came from large urban tax-act projects where developers were required to save the original windows on high-rise and relatively large urban buildings while still attempting to achieve Class A rentable tenant space in competition with new or modern office space.

In many of these cases, the windows are enormous and are in fact 2¼" thick sashes. Moreover, with the introduction of central-air/climate control, many of these large, once operable windows are/were converted to fixed windows. In the case of the Board of Trade and Wall Street Banks, perhaps the operable windows were converted to fixed windows to reduce anxiety over jumpers!

I digress but obviously exterior storms are impractical on such buildings as well.

I know one can debate the logic of converting operable windows to fixed windows in a technical brief on energy but sustainability has not reached the point of inspiring the general public to return to the days before air-conditioning and it certainly won't work in an urban office space under these conditions. (Have you ever opened an alley-side window in Chicago in July? It brings a whole new meaning to getting teary-eyed about historic buildings).

One example of an IGU retrofit is the Rookery project in Chicago completed in the late 80s. I believe the heavy commercial sashes are in fact 2 ¼' there and they were retrofitted with thermal-panes. I also believe some operable sashes were converted to fixed windows. It may be worthwhile to revisit the building to evaluate performance today. However, I recall that it was a tax act project, and producing competitive Class A office space was the developer's intent. We were not involved but I won't be surprised if others on Forum will weigh in on this.

If thermal-panes are a compromise in commercial sashes that prevents replacement with the clad $*^& produced today would you support this scenario?

Best regards-

Neal Vogel
Restoric, LLC

Bob Yapp
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Re: Routing Sash for Insulated Glass Units

Postby Bob Yapp » March 8th, 2012, 1:24 pm

Neal Vogel's post about retrofitting IG into historic sashes makes some really good observations.

I think most of us are aware of how difficult it is for the NPS to deal with historic tax credit developers. Neal asks, "If thermal-panes are a compromise in commercial sashes that prevents replacement with the clad $*^& produced today would you support this scenario?"

If it were the only and best compromise/alternative I would say yes, but there are less damaging and cost effective methods which get the developer what they want. Yes, some commercial sashes are 2-1/4" thick but the deepening of the glazing bed or an IG panel is still over half the thickness of the sash which cuts into the most important structural part of the structural joint, the tenon the shoulder area.

While not my favorite, commercial grade extruded aluminum, operable exterior storms work well on the high rise properties Neal describes. These can be installed on the exterior while working from the interior of the building with no need to have lifts, cranes etc. I have yet to see a commercial window that didn't have some sort of storm bed on the exterior. Properly weather sealed interior storms are another great solution for high rises where fixed sashes are so seemingly important. The cost to weather strip an original 2-1/4" sash, keep the sash weights and add a storm is usually less than the cost of retrofitting an IG unit that will fail in 6 to 15 years. Another alternative is 1/4" or 3/8" laminated glass in the sashes and/or storms.

Neal's point is that many of these building developers don't want operable windows or may have "jumper" issues. I agree but I really think developers arguments on these points are misguided and a diversion from the primary issue. My objection to retro-fitting insulated glass into these sashes is that putting and IG panel into a sash doesn't eliminate the primary energy issue with old and historic windows and can permanently ruin the sash for future use. As rightly stated in Brief #3 the primary energy issue is air infiltration.

For the NPS, they have always described "best practice" and recommending an alternative method that is costly, damaging to the sashes and uses a glass panel (IG) that is not sustainable assures the sashes won't be around in 15 to 30 years and will be needlessly replaced in the end. That doesn't seem to be a compromise or reasonable alternative.

Bob Yapp
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Re: Routing Sash for Insulated Glass Units

Postby Bob Yapp » March 8th, 2012, 8:20 pm

Another email from Neal Vogel:

"Thanks Bob, you made excellent points but you know I abhor aluminum even more than IGUs (which ain't easy). I do in fact recommend lami for most commercial installation and just installed 55 pieces in a big steel and cast iron fixed lobby window last month. To date, Restoric has never retrofitted any sashes for IGUs and strictly promotes storms but I even crossed a Frank Lloyd Wright house recently that have very, very early IGUs (still trying to date them) many where the seals are still intact so blanket statements about failure should be qualified or we come off sounding like the replacement companies marketing every old window as worthless. Convenience and the perception on energy efficiency and comfort play into developers decisions (since window cleaning and removing a storm for window cleaning annually or bi-annually is also a consideration for operating costs-how much expense does this add to clean a storm and primary window---the bean counters contemplate this stuff and we need to counter the economic reality."

Best regards-

Neal

johnleeke
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Re: Routing Sash for Insulated Glass Units

Postby johnleeke » March 8th, 2012, 8:47 pm

Neal writes:
>>...blanket statements about failure should be qualified or we come off sounding like the replacement companies marketing every old window as worthless.<<

I agree, which is why I keep "lobbying" for thorough documentation of a few cases of IGU failures and routed sash failures. This is one way to qualify the statement. Since it is so common, then we should be able to come up with a few cases that we can document.
John
Standards Co-Founder
Standards Editor

http://www.HistoricHomeWorks.com

Bob Yapp
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Re: Routing Sash for Insulated Glass Units

Postby Bob Yapp » March 9th, 2012, 1:23 pm

As I suggested as well, we need everyone to let us know where they've found failure and success with retrofitting IG into historic sashes. There were no blanket statements made in the letter to NPS or in my resposes but, Neal makes good points about economic issues we must always have at the forefront. IG failures are well documented.

sschoberg
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Re: Routing Sash for Insulated Glass Units

Postby sschoberg » March 9th, 2012, 2:36 pm

yadda yadda yadda! He told you where the pressure was coming from! Most of his points were mere justifications to smooth the pressures. His only good pont was that most of his examples related to 2 1/4" sashes and those may very well be able to take routing out for IGU's. But the briefs dont say that so the wording will alway be interpreted to mean all sashes.

Another point----Thermopane I think was a trademark for Anderson's initial welded glass technoloy. To date I have not seen any Thermopane glass that is foggy. (or fail)

Are the standards going to bend because of pressure or perceived pressures? If so the disposable window and glass companies will win again.

Bob Yapp
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Re: Routing Sash for Insulated Glass Units

Postby Bob Yapp » March 9th, 2012, 3:03 pm

Of course they won't Steve, I'm just being polite.


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