Bifold window & door design features.
What do they look like, how do they work and what are the differences between Bifold window & door versions?
A bifold door functions in exactly the same way as a bifold window, but uses glazed door panels in place of casements. Once folded to the sides, the panels can “sit or stack” either to the inner side or outer side of the main frame.
The main objective of both versions is to leave the maximum amount of clear space once the unit is open – in this case it, up to 90% of the aperture remains unobstructed when the windows or doors are fully opened. When closed, they look just like a regular patio door or casement window, it is the opening function that sets them apart from other designs.
Both of these versions can really make a room feel spacious because of the wide open space they create, due mainly to the absence of centre frames or mullion posts that you get with other window designs.
Bifold Window & Door Opening Function
Bifold window outer-frame Construction.
Bifold Window Used Indoors
Typically, Bifolding designs feature an outer frame in which the door or window panels are housed. The panels themselves are attached to guide tracks at the head & the foot of the frames, one of which will be taking the weight of the panels.
Either the upper or lower track can be designated as the loadbearing section. Where the upper track takes the weight the installation is referred to as “top hung”, and where the lower track is the loadbearing one, the installation is referred to as “bottom hung”.
Top hung bifold doors or windows require a securely fixed overhead frame to ensure the load is supported properly. If you are unsure of the type of lintel that is being used in the head of the aperture, then opt for bottom hung doors or windows to be on the safe side – the top track is then just for guidance and non-loadbearing,
There are 2 types of frames; the outer frame and the sub-frame. Sub-frames are the window or door panels themselves and can both be made from either timber, uPVC or Aluminium.
- Timber Frames / sub-frames: There are broadly 3 options for timber frames, hardwood, hardwood veneer or engineered wood.
Hardwood is self-explanatory and is an excellent choice as it is a hardwearing, and naturally insulating, material that many favour due to the “natural” look and feel.
Hardwood veneers are a combination of softwood frames with a thin layer of hardwood bonded to the surface. It’s not that common to see them used as external patio doors or windows.
Engineered wood is virtually the perfect wood to use for this type slim framed window or door. Because it is composed of alternating layers of wood, it has no “grain” to speak of. This lamination of cross grained timber strands is extremely resistant to warping, shrinking or other “similar flaws” that wood is usually subject to.
- Aluminium Frames / sub-frames: With aluminium you get 2 options, all aluminium or aluminium clad (sometimes called aluminium composite). The full aluminium frame profiles are chambered to make them lighter and boost strength, but also within the chambering is what is known as the thermal break.
Typical Bifold Window Design
Thermal breaks for aluminium profiles used in bifold windows and doors are very important. As aluminium is a good conductor of heat or cold, there has to be a physical barrier built into frame profiles to stop any transfer.
Aluminium clad bifold windows and doors don’t need the thermal break as the sub-frame itself is usually engineered timber (various softwoods or even European Oak) and an aluminium skin is then bonded to the external surface. The aluminium cladding is only on the outside and so cannot transfer heat or cold in or out of the property.
- UPVC Frames / sub-frames: Much like aluminium uPVC frames are chambered to make them lighter and stronger. Unlike aluminium, uPVC is a bad conductor and therefore uPVC bifold doors or windows don’t require the thermal break.
UPVC is not as strong as aluminium, and so the outer frames and sub-frames can include galvanised steel internal reinforcing where necessary to create greater structural integrity. This is very useful for larger bifold door installations, where the larger doors can get quite heavy.
What are the differences between Bifold windows & doors?
Apart from the obvious difference in size and purpose, the function and method of operation for a bifold door or a bifold window is exactly the same.
The differences are only minor, one of them being the need for a threshold for a bifold door. Whilst both designs have a bottom track, the height (or lack of it) of a bifold door threshold can impact upon how comfortable the unit is to live with on a day-to-day basis.
A high threshold can be somewhat of a safety hazard as well an inconvenience, someone could trip over it and suffer an injury, especially concerning for children, the elderly & infirm or a wheelchair user.
Panoramic UPVC Bifold Doors
Fortunately, there are low or rebated options which keep the threshold at a very low and easily accommodated height or they can be fitted to be flush with the floor level. Bear in mind that you will need to ensure that the version you are looking at has been fully tested against wind & water penetration.
You may also need to think carefully about using flush thresholds if your garden area is subject to flooding or has a lot of standing water that could come into the house.
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