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FAQ: The abbreviation FENSA stands for FENESTRATION - SELF ASSESSMENT. The uk dictionary definition of the word fenestral or fenestrate is " belonging to or like a window" The word fenestration is "the arrangement of windows in a building". FENSA is used by double glazing uk trade companies to certify that their replacement windows and doors installed comply with Building Regulations Document L (and more), actually checking av. 1% of all replacement windows installations. Did You Know?
Not a vacuum, but many, or even most possibly, will have a negative pressure, compared to our atmospheric pressure. This is not deliberate, but purely an accidental result of part of the manufacturing process. There are a number of different types of edge seal materials and processes, and some are more likely to produce a greater negative pressure, sealed within the unit, than others. A high negative pressure is not a good thing, and under certain circumstances a unit can implode, although a slight negative pressure will in itself will not normally prove to be a problem, except under certain circumstances when they can implode. See why did my sealed unit explode?
Here is one example: When it comes to larger sized sealed units there is an added reason why a negative pressure can be accidentally sealed in. Imagine a sealed unit for a Patio Door, probably 3ft to 4ft wide and around 6ft high, laid horizontally on a flat work bench. The lower pane is supported by the flat bench, the spacer is placed around the perimeter and the other pane of glass is laid on top prior to sealing up around the edge. The top pane will sag somewhat, and if completely sealed like this it will have quite a high negative pressure built in. Because manufacturers know this, they will either a) clamp a suction pad in the middle of the top pane and pull out the sag until the unit is completely sealed, or b) seal most of the perimeter of the unit, then lift it off of the bench, stand it vertically until the inside pressure has equalised with atmospheric pressure, and then seal the last little bit. For more information on this subject see All about 'misted up' sealed units Do Sealed Units have a vacuum? Why did my sealed unit explode?
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