High pressure salesmen, building regulations, FENSA, designs and styles, insulation, energy rating, PVCu vs. aluminum vs. galvanization - it's enough to make your head swim! All you want is new windows for your house - who knew you had to become an expert in glazing, building codes, historical design and heat conduction to pick the right ones!
Let's make the process a little simpler by defining a few terms that you'll hear in association with double glazing and hung windows.
Double glazing
Windows are hung with two layers of glass separated by a 'dead air space'. The space between the two windows is anywhere from 16mm to 28mm. Because air is such a poor conductor of heat, the air trapped between the two panes in double glazing serves as an additional layer of insulation to keep cold air on one side and hot on the other. The dead air space may also be filled with an inert gas such as argon to maximize the insulating property, sometimes with desiccant to prevent or minimize condensation between the panes. In any case, double glazing is far more insulating than single glazed windows (ones with only one pane of glass). Thus, double glazing will keep your rooms warmer in winter and cooler in summer.
Frame
The material into which the glass is set makes a difference in its cost and energy rating. Double glazing window frames are most often made of aluminum, PVCu (or uPVC) or wood. Each has its own advantages and disadvantages.
FENSA
Fenestration Self Assessment Scheme - an association of window manufacturers that sets standards for manufacture and quality.
U value
The u value is a measurement of heat transfer through glass. The lower the measurement, the better the rating. Part L of the Building Regulations specify a U value of 2 or less for PVCu and wooden framed windows and 2.2 for aluminum, though this may be modified for historical preservation.
Pilkington K
Pilkington K glass is glass that is coated on one surface with an energy conserving material.
Low E Glass
Low E stands for low-emission glass - glass with a microscopically thin coating of metal oxide. The coating is invisible to the eye, allows heat and light to pass through one way, but blocks heat flow in the other direction. Most often, it is installed to reduce heat loss from the interior of a room.
Sealed Units
The term sealed units refers to the actual 'glass sandwich' - two pieces of glass that are sealed to each other with a spacer bar between to maintain a specified distance. Often, the spacer bar is filled with a 'desiccant', to soak up any moisture between the two panes.
Sash Windows
Sash windows are windows that are hung within a sash and open by sliding up and down within the channel.
Casement Windows
Casement windows are hung on hinges of some sort and open in and out rather than up and down.