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U-Values, Sightlines and Weather ratings

Questions we get asked - and the one you should always want to know the answer to!

The high quality gasket on a Sunflex SF55 Bifold Door system

There are two questions we get asked on a regular basis...along with how much does a system cost?

What is your sight line?
Note: A sight line is the amount of frame that will be on view, i.e. the thickness of the door profile that you see when the doors are closed. 

Of course the slimmer the better so you are maximising the light into your room and enhancing your view. When we quote sight lines for bifold doors and sliding doors we will always say the best one we can achieve. Always get a section drawing of the door you are ordering detailing the sight line sizes as different opening configurations offer different sight lines.  All doors in one direction generally offer symmetrical sight lines. If they are opening from the centre or off centre then there is usually a larger sight line at the meeting section so your sight lines would not be symmetrical.

What is the U-Value?
Note:An overall U-Value is the thermal performance of the frame and the glass – measurement of heat loss – the lower the number the better, this tells you how good the doors will be at keeping heat in your room.

Building regulations for thermal efficiency states a maximum U-Value of 1.8 W/m²k for replacement doors. Some companies may claim a U-Value of 1.3 W/m²k but fail to mention that is the best their system can achieve but only with triple glazing or very expensive glass.  Also make sure you are not getting just the glass U-Value (centre pane) as this will be the lower figure and not give you the full picture. Always ask for the overall U-Value of the doors and glass you have been quoted - this is the figure IDSystems will always upon request.


While these two questions are important, there is a more important question that hardly ever gets asked.

How weather tight are the doors?  You may have been told the doors you are interested in offer an excellent overall U-Value with a slim sight line but this is only half the story if you do not know how weather tight these are.  A good U-Value does not mean the wind and the rain are kept out. Your doors need to be suitable for the middle of winter - not just for summer. There are two tests that door systems have.

Air Permeability Rating: This rating shows how air tight the doors are – do they keep out the wind? Air pressure is applied to the doors to see how much they withstand before leaking air. This pressure is measured in Pascals so the door could achieve anything from 50 Pascals which would be poor, up to 600 Pascals which is the best. This also rated as from Class 0 which is the lowest or Class 4 which is the best.

Water Tightness: This is tested the same way as air – water pressure is applied to the doors to see how much they withstand before leaking water. Again this goes from 50 Pascals up to 900 Pascals and  from Class 1 being the lowest up to Class 9 being the best.

How can you tell a doors weather rating? The only accurate way is to ask for written confirmation of the result - ideally this should be in the form of copy of the actual independent test results. Any company should be proud to confirm the figures they are quoting.

Is there any other way of telling? Not accurately, but a double rebated top and bottom track can be a good indication of a good seal, so the door panels can close tight against 2 points. Also check to see if the gasket between each panel carries on where there is a hinge which would be a weak spot if not - there should certainly be a gasket on both sides (inside and the out) of each door. You should also find out what the gasket is made of – is it made to look good and offer a colour choice but basically just made of foam? or is it made from long lasting rubber to offer a long lasting seal?

An unscientific test on folding doors is the ‘paper test’. Put a single sheet of paper between the door panels and then close them - how easy it is to pull that piece of paper out will offer an indication of how good the seal, and therefore the weather rating, is.

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