Feeling the pressure of innovation?
As the game changing GL250 loft access door turns 15 years old, the competition is still playing catch-up.
“If it ain’t broke, don’t fix it” as the saying goes, but just because something works it doesn’t mean it can’t be made better. So it was that as UK housebuilders began to face ever increasing restrictions on air tightness since the introduction of sample plot testing of domestic dwellings with the 2006 update to Approved Document L, there was always going to be room for improvement with the robustness of certain details and the products that fit into them. As methods of testing improved and a wider spread of plots were assessed, patterns began to emerge; details like pipe and joist penetrations kept causing issues, and key products within the outer thermal envelope of the building kept leading to failures … first and foremost of these was the loft access door.
During an air tightness test, a fan is affixed to the front door opening and air is sucked out of the building at a pressure difference of 50 Pascals (Pa). This creates a consistent negative pressure inside far beyond the natural air interchange you would see day to day which would only ever be between 1 and 10 Pa depending on how windy it was outside. This extreme negative pressure serves to pull all the external doors and windows shut tight onto their seals, instead focusing the influx of air being drawn into the property to the weak points in the building fabric. A drop-down loft access door is a direct path through into the cold, uninsulated roof space and typically consist of a rectangular door panel that is hinged at one end with a catch at the other. This arrangement functionally works, however the advent of air tightness testing exposed a fundamental weakness in its design; as the negative pressure was increased, doors began to bow downwards centrally along their length being pulled off their air seals by the negative pressure and thus opening a direct air leakage path into the void above. A positive pressure would never have exposed this issue as it would have pushed the door upward, tightly onto its seals, but when fitted horizontally in a ceiling already fighting against gravity, the air tightness of a drop-down door becomes increasingly susceptible to failure under negative pressures.
As a foremost manufacturer of loft access doors for the UK construction industry, Manthorpe Building Products was approached by several of the leading housebuilders with this very problem back in 2008 who were seeking ways to help them pass the pressure tests that were only going to become more stringent in time. We could have ignored the problem and kept with the same old traditional design, the door still worked perfectly well as an access door after all, but we didn’t, we saw the opportunity for improvement and as designers welcomed the chance to pursue that most elusive of mistresses, innovation. This was an opportunity to not only resolve a direct issue for the housebuilders in terms of air leakage, but a chance to design a loft door for the modern home; sleek and contemporary in appearance but well insulated and most importantly airtight under negative pressures.
It was apparent from the off that there was a number of fundamental design criteria to meet, any new loft door design needed to achieve the following:
- Not bow along the long rectangular side of the door panel under negative pressure.
- Have no air leakage penetrations through the door panel (for the catch etc.) that are inboard of the air seal.
- Ensure the seal is continuous and unbroken between both the door to frame and frame to ceiling.
- Ensure that the insulation coverage is maximised to limit cold bridges.
- Increase the accessible opening through the ceiling in use.
- Improve ease of installation by making the door removeable.
As we had carte blanche to approach the project without constraint it allowed us to explore avenues of design that had never been considered within the limitations of a traditionally manufactured drop-down loft door, and what ultimately emerged out of over 12 months of design and development was something truly revolutionary for the industry.
So what did ‘game-changing’ look like?
It all started with the hinge; we knew that a traditional barrel hinge design located in the corners of the accessible opening wasn’t good enough. So, by pulling the hinge point outward into the edge of the frame, this not only created a bigger accessible opening but created space for the air seal to run unbroken inboard of the hinge geometry, removing a primary air leakage path on older door designs. This new style of hinge was also easy to install and remove allowing the door to be quickly detached during installation of the frame, not only allowing the frame to fitted by a single person, but also allowing the door to be taken out in use to further increase the accessible opening if needed and prevent damage to the insulation when moving large objects through the opening.
While the new position of the hinge offered technical improvements, the aesthetics of these outboard hinge slots needed some work, and it was through the development of this visual aspect that the true innovation was achieved. In order to hide the hinges, the idea of a sliding cover was conceived, a panel that could move back to hide the hinges when not in use but move out the way to allow them to operate. It worked in principle, so what else could be do with this otherwise ‘sliding vanity panel’, and there it was … the answer. Our biggest issue was that we didn’t have enough catch points, having them at one end opposite to the hinge was insufficient, we needed more down the long sides of the door to hold it up tight to its air seals to stop the bow and limit air leakage. We also didn’t want to have multiple individual catches, the door should still be operated from a single point, a solution should not come to the detriment of the usability of the door. But what if we had one big catch? One big catch that can move to unlock multiple fixing points in one go, whilst also acting as a vanity cover for a clever hinge design. So it was that the sliding catch surround was created, an innovative solution to securing a loft door panel in place and preventing it from sagging under negative pressure. Rather than one, two or three catch points, the design instead has twelve separate anchor locations around the full perimeter of the door which robustly holds the door panel tight up against the frame when locked, no amount of negative pressure testing will get this to bow.
This clever sliding design allowed us to push all hinge, catch and fixing geometry fully outboard of the air seal, meaning no secondary air leakage paths. Moving all this away from the opening also meant that we could maximise the amount of space within the door panel that is insulation, keeping the uninsulated gap to the edge of the structural opening as small as possible and making the insulation a consistent, uninterrupted block, no notches and cut outs here creating unwanted thermal bridges. Couple this with a wide range of insulation materials and thicknesses available, and we have an option for any thermal requirement.
Along with the removeable door, the ease of installation was further improved by having the frame screw directly up into the ceiling, this is not only easier than juggling fixings sideways into the timber opening, but it also creates a much more consistent pressure on the frame to ceiling seal, improving air tightness yet further. With the frame fitted, the door is quick to replace with simple snap fit hinge blockers there to ensure no unwanted removal.
The end results …
So after a year of design and development, several patented principles and a sizeable investment in new tooling, the new GL250 was ready to be put through its paces. An independent 3rd party test by the BRE to BS EN 13141-1 would show us how the new design stacked up against the rest of the market.
The Building Regulations Part L ‘reasonable limit’ for the design air permeability of buildings was a rate of less than 10m³/h.m² at 50Pa, with the requirements for a “Well Sealed Ceiling” as detailed in BS 5250 was an air leakage rate below 1m³/h at a pressure differential of 2 Pa. The Manthorpe GL250 achieved rates of 0.19m³/h.m² at 50Pa against the Part L requirement and 0.05m³/h at 2Pa for the BS 5250 rate, easily eclipsing the requirements of the standards when tested at both positive AND negative pressures, improving on the next nearest competition by over 300 times the leakage rate at -50Pa.
What these results led to was the GL250 becoming the no. 1 specified loft access door by UK housebuilders due to its unparallelled improvement in air tightness, coupled with its comprehensive thermal coverage, its speed of installation and it’s sleek, contemporary styling which helps it to blend seamlessly with the finished aesthetic of a modern home.
15 years after it was first launched, the GL250 is still by far the market leader and is tried and trusted by housebuilders to help them pass the air tightness test time and again, which has never been more important now every new home is being tested. We not only have that testimony, but we also have the testing data to back it up which is freely available on our website for all to see. So while the competition continues to play catch up with tame evolutions of the same traditional design, you can rely on Manthorpe being as far ahead of the curve now, as we were back then with the innovative GL250 loft access door.
