Stop the sun before it reaches the glass and you stop most of the heat. Once sunlight has come through the window it has already become heat in the room, and an interior blind can only slow down what is left.
Stop the sun before it hits the glass
External shade is the single biggest lever you have. Sunlight that lands on a fabric hanging outside the window heats that fabric and the air around it, and most of that heat drifts away rather than ending up in your lounge room.
Outdoor roller blinds do this well on a deck, a balcony or a courtyard, and they double as a windbreak. A folding arm awning does the same job over a wider opening and folds away when you want the winter sun back in. Motorised roller shutters go further again, because the shutter closes over the window with an air space behind it.
Anything outside beats anything inside for heat, every time. That is the whole principle, and everything else on this page is a refinement of it.
West-facing windows are the worst of it
West-facing glass takes the low afternoon sun straight on. The sun is at a low angle, so eaves and verandah roofs give almost no protection, and it arrives at the hottest part of the day when the house has already warmed up.
That combination is why a west-facing room can sit several degrees warmer than the rest of the house by five in the afternoon. If you can only afford to shade some windows, shade these ones. North-facing glass is easier, because the sun is higher and an eave or an awning does a lot of the work.
What an inside blind can and cannot do
An interior blind cuts glare, cuts the direct radiant heat you feel on your skin, and stops furniture and flooring fading. What it cannot do is keep the energy out of the room, because the glass has already let it in.
Some fabrics do better than others. A sunscreen roller blind is rated by openness factor, usually between 1% and 10%, and a tighter weave at the 1% to 3% end reflects more of the sun while still letting you see out. Lighter fabric colours reflect more heat back out through the glass. Dark colours look sharper and give better clarity of view, but they absorb more and then radiate that warmth into the room.
Blockout fabrics with a reflective or white backing work harder than plain dark blockout, because the backing faces the glass and bounces heat outward. Curtains with a good lining help too, mainly by trapping a layer of still air against the window.
Double glazing, or a covering, or both
Double glazing and window coverings deal with different halves of the problem. Double glazing reduces heat conducted through the glass and it makes a real difference to winter warmth and to noise.
It does much less about direct sunlight streaming through the window, which is what makes a west-facing room unbearable in February. Glass with a low-e or tinted coating helps with that, but sunlight coming in at a low angle still carries a lot of energy.
If you are choosing where to spend, external shade on the worst windows will usually give you a bigger comfort change than reglazing them. In a new build or a major renovation, do both and let the glazing handle winter while the shade handles summer.
Motorised roller shutters
Motorised roller shutters are the heaviest tool in the box for a window that cooks. The shutter sits outside the glass, and when it is closed there is an insulating air gap between the shutter and the window.
They run off a motor with a remote or a wall switch, so you can close the western side of the house before you leave in the morning and come home to a room that is still liveable. They also cut noise and give you security, which is why they turn up on bedrooms facing a main road.
The trade-off is that a closed shutter is dark. Most people use them on bedrooms and on the one or two rooms that suffer worst, rather than across the whole house.
A practical order of work
If you are working through it on a budget and in stages, this order gets you the most comfort earliest.
- Shade the west-facing glass from outside first, with an outdoor blind, an awning or a shutter.
- Put a tight-weave sunscreen or a light-backed blockout on the remaining hot windows inside.
- Deal with east-facing bedrooms next, since morning heat carries into the day.
- Add lined curtains where you also want warmth in winter and less echo in the room.
- Leave south-facing windows until last, because they get very little direct sun.
Simple habits help as well. Close the blinds on the sunny side before the room heats up rather than after, and open the house up once the outside air has dropped below the inside temperature in the evening.
If one room in your house is always the hot one, book a free measure and quote. We will look at which way the glass faces and what is shading it now, and tell you honestly whether the fix belongs outside the window or inside it.
Common questions
Are outdoor blinds better than indoor blinds for heat?
Yes, by a clear margin. An outdoor blind stops the sun before it reaches the glass, so most of that heat never enters the room. An indoor blind is dealing with heat that is already inside, so it manages glare and radiant warmth rather than preventing the gain.
What colour blind keeps a room coolest?
Lighter colours reflect more heat back out through the glass, so a light or white-backed fabric runs cooler than a dark one. Dark fabrics give a clearer view out and less glare, but they absorb heat and release it into the room.
Do I still need blinds if I have double glazing?
Usually yes. Double glazing reduces heat conducted through the glass and helps a lot in winter, but it does not stop direct sunlight pouring through a west-facing window in summer. Shading and glazing solve different parts of the problem.