With single glazing the Thermal Resistance, ‘R’ Value is 0.16.
Although they aren’t considered in energy calculations (see this link: Energy Rating) curtains can significantly decrease the heat loss from a window.
They also improve the feeling of comfort.
With different materials its not possible to give a definitive value for the insulation value of curtains however there are some indicative values in this post.
An effective curtain can increase the ‘R’ value of the window to between 0.3 – 0.5.
Even with a double glazed window curtains will typically further reduce the heat loss. (‘R’ value increased from around 0.33 to around 0.6).
To be fully effective curtains must:
- Provide a ‘seal’ around the window to stop air movement at:
- Top – Pelmet
- Bottom – Overlap window sill by at least 300mm, or down to the floor
- Sides – Overlap edges of window by at least 300mm.
- Use heavy close weave material, preferably lined.