Understanding rooflight performance

Understanding the performance of a rooflight can be difficult, especially for those of us who don’t have a degree in physics or read British Standards in our spare time.

What does ±2000 Pa serviceability mean? Is it relevant to my project? Do I even need a performance tested rooflight?

Without getting into the boring long-winded codes of the British and European Standards, windows, doors and rooflights should be tested for “weather tightness”. This has three main components to it, air permeability, water-tightness and wind resistance.

Air permeability – how draughty is your rooflight?

Image of a draughty shack

Have you ever been in an old house where the door doesn’t fit quite right and you can feel the cold air rushing in around the edges?

Effectively, air permeability is a measure of how draughty the rooflight is. This is tested by putting a full size rooflight on to a sealed chamber and turning on a big vacuum followed by a big fan. The vacuum tests to see how the rooflight performs when the air is being sucked into the rooflight, simulating high pressure outside and low pressure inside (positive).

The fan blows the air out to see how it performs when there is a high pressure inside and low pressure outside (negative). This is where the ± comes from when you see a value of say ±900 Pa air permeability. The “900 Pa” is the pressure it has been tested at, the higher the pressure the unit has been tested at the better.

You will also see a “Class” attributed to the performance of a rooflights air permeability, Class 4 being the best and Class 1 being the worst. This is based on the pressure the rooflight is tested at and the amount of air that leaks out over the area of the rooflight or the length of the opening joint. The Vision AGI Slide Over Fixed Rooflight we tested in April 2019 achieved Class 4. As you can see from the graph below, it passed with flying colours.

Graph of test results

Water-tightness – pretty much what it says on the tin.

Every homeowner’s nightmare is to wake up after a night of rain and find water running down from the ceiling.

Man holding a bucket under a leaking roof

Testing for water-tightness is the best way to make sure that doesn’t ever happen.

The test for this is a bit more straight-forward than air permeability. First, with the rooflight still fitted to the sealed chamber, you soak the outside of rooflight with water for 15 minutes. Multiple nozzles and controlled water flow are used to ensure the product is well covered.

After the 15 minutes are up, and the unit is soaked on the outside, the vacuum is then turned on (with the water hoses still on) increasing set pressures to try and draw the water in, simulating high winds or large pressure differences.

At the point when water penetrates the product, the test stops, and the product is classified to the previous set pressure it reached and didn’t fail at.

Many windows, doors and rooflights are tested up to Class 8A or Class 9A, 450 Pa and 600 Pa respectively. Above Class 9A, the final performance is not shown as class 10A, 11A or 12A etc. Instead it is designated using an E followed by the maximum pressure (in Pascals) achieved.

For example, the Vision AGI Sliding Rooflights we tested in April 2019 were tested to 1050 Pa (Class E1050) because we wanted to make sure that they were the best on the market. They still didn’t fail at this pressure, but it was the highest pressure that the test chamber could safely reach!

Wind resistance – nobody wants their rooflight to fly off or collapse in high winds, which is where wind resistance testing comes in.

House being swept away in the wind

Wind resistance is split into two main tests, serviceability and safety.

The serviceability test simulates changes in high winds and measures the deflection of any internal framework or glass. The rooflight is subjected to three high positive pressures (vacuum) and three high negative pressures (fan).
Depending on the pressure applied and amount of deflection caused during the tests, the rooflight is categorised into classes A, B and C, where A is the worst and C is the best. The maximum test pressure is grouped into 5 classes, where Class 1 is the lowest performance and Class 5 is the best.

The Vision AGI Sliding Rooflights tested in April 2019 achieved Class C5 so can withstand ±2000 Pa of pressure, which is the equivalent to 127mph winds. (Bearing in mind that a 74mph wind is classed as hurricane force – this is quite impressive!)

The safety test is a higher-pressure test where the defections aren’t considered. The Vision AGI Sliding Rooflights were tested to ±3000 Pa which means it was subjected to a negative pressure of 3000 Pa for 4-10 seconds then a positive pressure of 3000 Pa for 4-10 seconds. This simulates gusts of wind at around 156 mph!

So, is any of this important or necessary?

Well I hope after reading the above you think that it is. Draughty rooflights will not only be bad for the energy bill but the risk of condensation increases dramatically. Leaking rooflights are a nightmare for everyone involved. One of the main reasons Vision AGI do such rigorous testing is to protect not just the customer but also ourselves. We want to be sure your rooflight doesn’t leak.

Our testing gives you and us the confidence that we won’t have any leaks to fix.
Safety has got to be most important. When you put something made of glass and metal above your head you want to know that it is going to stay there.

At Vision AGI we believe that these tests are paramount and should always be done for all rooflights in the marketplace. Unfortunately, this isn’t always the case and some products on the market, even from large companies, are still untested.

If you want to be sure that your rooflight is safe and free from drafts and leaks – we recommend that you always ask the question “what is the weather-tightness performance of these rooflights?” and don’t settle for “we’ve been doing this for years”.

If you want to have a look at our rooflights range, please go to https://www.visionagi.co.uk/the-rooflight-collection/

If you would like to see our Test Certificates, please go to https://www.visionagi.co.uk/the-rooflight-collection/sliding-rooflights/ you will find them when you click the ‘Downloads’ Tab.

Or if you want to talk to someone and get some further advice please call 01223 79 22 44

And finally – if you are thinking about buying a rooflight from anyone else, always ask to see their test certificates!