Although it sounds like a mix by DJ Tiesto, it's actually a description of the reflex hinge (sometimes called a "friction stay") fitted to most of today's top-hung and reversible windows. Low stack refers to the depth of the hinge arms, Sinidex is a trade name and the Eurogroove is a standardised 18mm wide rebate in the jamb of the window frame. If the terminology means nothing to you, perhaps window specification is a more complex business than you realised...
At architecture school, I developed a hit-and-miss knowledge of windows from reading journals and occasionally picking up brochures and adverts from the trade press. In my first project at my first job after graduation, composite timber/ aluminium double-glazed windows were specified, and that was the starting point for a journey which gradually taught me about window technology.
Having previously specified NorDan, Velfac and Rationel windows on projects, a few years ago I spent a while collecting information then tried to compare the performance of all the composite window systems that were available in Scotland. Recently I revisited the exercise for a new project, and some changes are obvious. We've moved on from double glazing with low-E glass, and are on the brink of triple glazing becoming the standard.
Trying to find the perfect window? Sadly, it's like punching smoke. But unlike the DIY’ers, self-builders and internet wiseacres – architects have the advantage that we specify again and again, so we have some idea of how to measure success and failure. Plus we have access to proper technical resources such as BRE digests and TRADA research. That's important, because the modern window is a high tech enterprise and in order to specify well, you need to know a bit about building science.
For example, the window frame sections consist of many-times-machined timber and complex extrusions, with drips, anti-capillary grooves and fixings for gaskets machined in. The angles of components, the gaps between them, and the geometry of the labyrinth which prevents rain being driven towards the interior, are carefully calculated.
During the 1950’s, the English Joinery Manufacturers Association developed the EJMA “Stormproof” window, where the casement is outward-opening and sits proud of and overlaps the frame. This standardised design with its “weather check” replaced the various styles of flush casement windows common until that point, and for the next 30 years or so house builders had a simple choice: timber frames of standard quality, eg, Magnet & Southerns “M4”, or timber frames of slightly better quality, eg, John Carr/ Boulton & Paul.
The stormproof window was adopted in Scotland, but Scotland is and always has been different to the south. The lower temperatures and much higher exposure to wind-driven rain make our climate much more like Norway than Surrey. We've fitted windows into rebated jambs and set them back further in the opening for hundreds of years, whereas the English Reg's still allow you to fit windows flush against the ingo of the outer leaf or brick skin.
When double glazing started to come in, partly driven by the Technical Standards and partly by replacement window firms such as Everest, the EJMA Stormproof window frame sections had to be beefed up in order to compete. The first Scandinavian windows I came across were in the timber clad, timber kit Moelven houses that Aberdeenshire Council built across the field from my grandfather's farmhouse in Kemnay.
A single-glazed window (as originally fitted to buildings in 1960’s) gave you an overall U-value (Uw) ~ 4.8 W/m2K, but a typical PVC-U double-glazed window from the 1980’s (4*12*4 with air space between glass) gave you Uw ~ 2.8 W/m2K. Almost twice as good. In a country where it feels like the last big innovation was the duplex sash and case window, that was big news. Of course, with traditional sash windows, effective sealing is always in conflict with the operation of the window. This means a traditional sash window is either airtight and hard to slide open, or easy to open but draughty.
Scotland and Scandinavia can be very windy, so their casement windows have traditionally opened outwards and they developed effective drainage profiles and sealants because wind and weather proofing is important. As I discovered during my research, Scandinavian expertise lies in top-hung reversible windows (using "H"-type or Spilka) gear, with double-glazing and noble-gas filled cavity to give Uw ~ 1.3-1.4 W/m²K, improving to 1.1-1.2 W/m²K with low-E glazing.
By contrast, windows from Austria and Germany are often inward-opening, but they do get extremely cold winters, so good U-values are judged more important and they pioneered thermally-broken frames and “warm edge” spacers to improve them. Austrian practice is to use tilt & turn rather than reversible gear, with an insulated laminated timber frame and insulated glazing bead, plus triple-glazing and noble-gas filled or vacuum cavity; even in the late 2000’s, they could achieve Uw ~ 0.65-0.7 W/m²K.
What’s state of the art now? 92 mm window frames, now with 48mm triple glazing (4*18*4*18*4), which provides Uw ~ 0.8 W/m²K. That’s the level set for new build Passivhaus buildings, along with EnerPHit renovations in the cold climate zone, ie. northern Scotland. To achieve that, windows and doors are fitted into reveals with all edges taped using flexible foil tape, and the reveals sealed with Compriband externally and silicone internally
If you think glazing units are like razor blades, where manufacturers keep adding more and more blades to the cutting head … you’re right. While researching this, I discovered that the Scottish company Enviro make Uw = 0.35 W/m²K quadruple glazing. As far as I’m aware they’re the first in the UK, no doubt others will follow. Meantime I’ll end with a series of cutaway images showing some of the different frame profiles available on the market today.
In some respects it’s surprising that they’re so varied, and that makes specification more difficult since you’re not able to compare Coxes with Mac Reds, but the main two types are Alu-clad timber windows where the aluminium acts only as an external finish (almost like a rain screen cladding) and Composite timber windows where both the timber and aluminium parts are structural.
Alu-clad timber windows:
Rationel Auraplus, Uw = 0.79 W/m²K and lifespan of 80+ years with 4*20*4*20*4 (made in Denmark)
NorDan NTech One, Uw = 0.8 W/m²K and lifespan of 60+ years with 4*12*4*12*4
Optiwin Resista Modern, Uw = 0.64 W/m²K
Allan Brothers Horizont/ Alu Clad, Uw = approx. 0.9 W/m²K
Broxwood Alu-clad Timber Tilt & Turn, Uw = 0.7 W/m²K (made by Arbo in Latvia)
Katzbeck Combina Passiv, Uw = 0.71 W/m²K (made in Austria)
Norrsken P41A
HON Quadrat Studio FB, Uw = 0.85 W/m²K (made in Czech Republic)
Viking SW14 Uw = up to 0.60 W/m²K (made in Estonia)
Composite timber/ aluminium windows:
Velfac 200 Energy, Uw = 0.82 W/m²K and lifespan of 60 years (with 48mm glazing) 4*18*4*18*3, Uw = 1.06
IdealCombi Futura+, Uw = 0.74 W/m²K (made in Denmark); 4*13*3*14*4, Uw = 1.04
Gaulhofer Fusionline 108, Uw = 0.65 W/m²K
Internorm HF310, Uw = 0.62 W/m²K
Silber Fenster Passive, Uw = 0.71 W/m²K
Neuffer Eco Idealu, Uw = 0.78 W/m²K
Green Building Store “Ecocontract Ultra” (made in UK); 4*18*4*18*4, Uw = up to 0.68 W/m²K
Footnote - As with my piece a couple of years ago about electrical accessories, this is written from my own personal experience, and neither I nor Urban Realm have links to any of the manufacturers. Copyright in all images rests with the respective manufacturers.
