Buying a house with double glazing – misted units, ventilation, condensation

Buying a house with double glazing

When buying a house with double glazing you might think great, no draughts, lower heat loss, etc.  However, double glazed units can fail and require replacement, particularly older units.

Double glazed windows have now been in use for several decades.  Early double glazing was often in either a timber frame or a metal frame, sometimes with insulation within the frame members.  Some of the older units had narrow sealed units which were not as effective against heat loss as more modern units.  Today, the vast majority of double glazed windows, whether replacement windows or used in new housing, have PVCu frames.

Double glazing is virtually essential for a modern dwelling to enable it to meet the requirements of Building Regulations in respect of heat loss.   The sealed air gap (filled with air or another insulating gas) between the two panes of glass reduces heat loss from the dwelling and prevents condensation on the glass itself.

When buying a house with double glazing it is important to check for the following:


Failed double glazed units

The sealed double glazed unit can fail either due to a defect in manufacture, damage or deterioration over time.  This can be identified by misting/fogging, ie, condensation occurring between the two panes of glass which form the double glazed unit.  The amount of misting can vary from day to day depending on weather conditions.   Once a sealed double glazed unit has failed it cannot be repaired.  It may be possible to replace the unit only, or it may be necessary to replace the whole window.  If only one or a small number of units have failed then other units of the same age may fail in the not too distant future.

If you are buying a house with double glazing and note one or more failed double glazed units, ask the vendor when the windows were installed and check whether there is a warranty to cover failure.  If there is a valid warranty, check whether this can be transferred into your name.

If the whole window requires replacement then contact a FENSA contractor to obtain a quotation and replace to ensure that the work is carried out in accordance with Building Regulations.


Insufficient ventilation

The purpose of double glazing is to reduce heat loss, reduce the risk of condensation on the glazing and help to maintain a comfortable temperature within the home.  However, as homes have become better insulated they sometimes suffer condensation due to a lack of ventilation.  It is important to provide adequate ventilation to a dwelling to allow excess moisture to escape and reduce the risk of dampness, condensation and mould.  Condensation isn’t simply water running down a window, it can also occur within furnishings resulting in dampness, a musty smell and possibly mould spores within the fabric.

Ideally, windows should have a combination of large opening casements and smaller top hung opening vent lights.  Windows should have locks so that they can be locked in an open position without reducing security, enabling windows to be left slightly open at night to prevent a build-up of moisture.  If there is mould on window frames and around window openings then this is a sign of inadequate ventilation.

If you are buying a house with double glazing, check whether trickle vents are provided.  Trickle vents allow background ventilation and can help to avoid a build-up of moisture and the risk of condensation.



  1. Check the configuration of opening windows.
  2. Check whether windows can be locked in an open position.
  3. Check whether there are trickle vents (usually found in the head of the window frame).
  4. Check the working order of window handles.
  5. Check whether there is any misting or fogging within the double glazed units.
  6. Check that any replacement windows have been installed by a FENSA contractor (if replaced since April 2002).
  7. Check whether there is a warranty for any replacement double glazed windows or doors and whether this can be transferred into your name

See also buying a refurbished house. 

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