Shade the glass from the outside, or you are already too late
An internal blind stops the light after the heat is in the room. External shading stops it at the glass. What the regulations assume about west windows.
By David White
A window is a hole in the wall that lets heat through in both directions. In January that is a cost. In July it is the single biggest reason one bedroom in a house is unusable and the one next to it is fine.
The instinct is to reach for a blind. It is the wrong end of the problem.
Once it is through the glass, it is yours
Sunlight arrives as shortwave radiation, and glass is nearly transparent to it. It lands on your floor, your walls, your furniture and your blind, and those surfaces absorb it and warm up.
Warm surfaces radiate too, but at a much longer wavelength, and glass is close to opaque to that. The energy comes in through the window easily and cannot leave the same way. This is the greenhouse effect operating in your back bedroom, and it is why a conservatory is unbearable in August.
An internal blind is inside that trap. It intercepts the light, which is real and worth something, but it does so after the energy has entered the room. A pale one reflects a useful fraction back out through the glass. The rest heats the blind, and the blind heats the room.
External shading is on the right side of the glass. What it absorbs, it gives back to the outside air.
The regulations already assume this
Approved Document O is the overheating part of the English building regulations. It is free on gov.uk and it is the only current, UK specific, freely available document with a hard shading requirement in it.
Two things in it are worth knowing even if you are not building anything.
The first is that shaded glazing in a high risk location has to reach a g-value of 0.4 or better. That is a demanding number and internal blinds do not get you there on their own.
The second is more interesting. In a high risk location, the document allows south facing glazing up to fifteen per cent of the floor area of the room, and west facing glazing only eleven. West is treated as the harder problem.
That surprises people, because south gets more sun over a day. West gets it later, when the outside air is already at its warmest and the fabric of the building has been absorbing heat since morning. A south window has its peak at lunchtime, when there is still evening ahead for the house to shed it. A west window delivers its peak straight into the evening you were hoping to sleep at the end of.
If you can only shade one window this summer, shade the west one.
What actually works, in order
External shutters. Louvred, hinged or rolling. Fully in front of the glass, adjustable, and they close at night in winter as well. The most effective and the most expensive.
External blinds and awnings. Same principle, less robust, usually cheaper. An awning over a south window is well suited to a high summer sun and much less use on a west one, where the sun is low and coming in almost horizontally.
A tree, a hedge or a climber. Deciduous, so it shades in August and drops its leaves before you want the light back. It is the slowest to arrive and the cheapest to run, and it works on the wall as well as the window.
Solar control film or glass. It reduces the g-value permanently, which means it is also reducing it in February. On a window that is a problem every summer and gets little useful winter sun, that trade can be worth making.
Internal blinds and curtains, pale and closed early. The weakest option on this list and still much better than nothing. Close them before the sun reaches the window rather than after, because once the room has warmed you are managing the heat rather than preventing it.
Then check whether you still need a machine
Shading changes the arithmetic rather than just the feeling. Solar gain is usually the largest single input to a room in summer, and taking most of it away moves the answer.
The room cooling calculator will give you the heat load for your own room, with and without shading. Run it both ways before you buy anything with a compressor in it. On a good number of British rooms the honest answer is a shutter and an open window at ten at night, and no machine at all.
The coefficients behind that calculator are provisional and the page says so. The ordering it gives you is sound; treat the watts as a band rather than a specification.
Questions
- Is an external shutter really better than a good internal blind?
- Yes, and the reason is where the energy ends up rather than how well either one blocks light. Sunlight that has already passed through the glass is inside the building. A pale internal blind reflects some of it back out, but it absorbs the rest, warms up, and then heats the room by convection and by radiating at a wavelength the glass will not let back out. An external shutter stops the same energy before it reaches the glass, and whatever it absorbs is warming the outside air.
- What is a g-value?
- The fraction of the sun's energy that gets through the glazing, counting both what passes straight through and what the glass absorbs and re-emits inwards. A g-value of 0.4 means forty per cent gets in. Approved Document O requires shaded glazing in a high risk location to reach 0.4 or lower, which is the one hard shading number in free UK guidance.
- Do I need planning permission for external shutters?
- This site cannot tell you, and neither can any site. Shutters are not covered by Class G, which is the air conditioning rule the planning tool here checks. Alterations to the outside of a building can be affected by a conservation area, a listed building designation, an Article 4 direction, or a leasehold or tenancy agreement. Ask your local planning authority before ordering anything.
- Does shading make winter worse?
- Not if the shutter moves. A fixed brise soleil is sized to block a high summer sun and let a low winter one under it, and a shutter you can open does the same job by hand. Closed on a winter night, a shutter adds a still layer of air outside the glass and slows heat loss, which is the job it was doing in northern Europe long before anybody was worried about overheating.