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Understand Thermal Blinds and Curtains

Thermal Curtains and Curtain Lining – Understanding Thermal Properties


This document is a work in progress and will be updated as time goes on.

Thermal curtains are designed to stop heat energy entering a space and the heat / cold escaping from a space.

So if you are using them in your home they complement your windows to create a further barrier to the outside.



Why Use thermal Curtains

Thermal curtains add a level of insulation to the space and work to keep your space;

  • Warm and comfortable in the winter
  • Cooler in the summer


Understanding Solar Gain









Understanding Thermal Acronyms and information

When talking about thermal properties there are a variety of words used and these combine to provide the thermal information of the fabrics used.

U-value is a measure of thermal transmittance which is the ability of a material to transfer heat. All elements of a building have U-values for example walls, windows and other insulation materials such as plasterboard. The lower the U-value, the lower the heat loss through that particular material. Therefore a material with a low U-value is a good insulator. The U-value of glazing is always improved by installing blinds.

Example U Values of Windows

G Value
EN 14501Clear Single Glazing5.80.85
EN 14502Clear Double Glazing2.90.76
EN 14503Double Glazing with low emissivity1.20.59
EN 14504Solar Control double glazing1.10.32
EN 13363-1Triple Glazing20.65
EN 13363-1Double clear glazing with low emissivity1.60.7



Understand Gtot

Total solar energy transmittance (gtot) relates to the performance of solar protection devices such as blinds or curtains and is the percentage of solar energy incident on the fabric of a blind or curtains that is transmitted to the interior of the building, including both the transmittance of the solar protection device itself and of the window glazing.


The lower the g-value or gtot, the lower the heat gain. The value of gtot is between 0 and 1, where 0 equates to no radiation being transmitted into the room and 1 means all radiation (100%) is transmitted. So a gtot of 0.25 (25% heat gain, 75% heat rejection) reduces heat gain three times more effectively than a gtot of 0.75 (25% heat rejection). External shading helps to significantly reduce gtot values and has a much more significant impact on gtot than internal shading.


Although the levels of Reflectance (Rs), Transmittance (Ts) and Absorptance (As) can vary depending on the type of material, its colour, thickness, etc., their total will always add up to 100% of Solar Irradiance. Reflectance (Rs) + Transmittance (Ts) + Absorptance (As) = 100% of Solar Irradiance


What is the goal of thermal curtains

The goal when using thermal curtains is to reduce the solar transmittance into the space and as such reduce the amount of solar energy passing into the building and inturn keeping the room cooler in the summer.

The opposite is also the case when trying to keep heat inside a room. We want to stop transmittance of energy passing back out of the window. So again putting a fabric between the room and the window will reduce heat loss through the window.




What to do with your blinds and curtains to improve


To maximise the thermal benefits within your space your should do the following



  • Close the blinds at night on the east-south elevations to protect from early morning heat gains
  • Open the blinds at night on the west and north-west elevations to assist night time cooling


  • Close the blinds after the sun goes down to retain heat
  • On Sunny days open the blinds during the daytime to maximise heat gain from the winter sun and close blinds at night.
  • Winter Blinds in unoccupied rooms should always be closed.

Blackout Thermal Curtains

Heat Loss through windows

  • Conduction – direct loss of heat through the window to the outside
  • Convection - where the warm air in the room hits the cold surface of the glass and cools the air inside the room
  • Radiation and Re-radiation - this is where the cold surface of the glass absorbs the heat from inside the room
  • Air leakage – heat lost through cracks in the frame or from around poorly fitted glazing


Improvements in Blinds when using a blind

Clear Single GlazingDouble Clear GlazingDouble Glazing with Low emissivity
Glass AloneU=5.8 W/m2KU=2.9 W/m2KU=1.1 W/m2K
High Air Permeability42.41
Average Air Permeability3.52.21
Low Air Permeability3.22.11
Percentage Improvement
Glass Alone5.82.91.1
High Air Permeability31%17%9%
Average Air Permeability40%24%9%
Low Air Permeability45%28%9%



Credits and

BBSA website for some of this information