Why is upstairs so much hotter than downstairs in summer?

In winter the complaint is the opposite way round, and the answer is a different one — see the companion guide on an unevenly heated house. In summer the reason people get stuck is that they treat it as an airflow problem and open the landing window, which helps a little and then stops helping. Knowing which of the three mechanisms dominates in your house tells you whether to spend an evening on it or a weekend.

Step by step

1. Understand the roof as the main source.A roof in direct sun gets far hotter than the air around it — a dark slate or tile surface can reach a temperature you cannot hold your hand against. That heat conducts into the loft, warms everything in it, and then radiates downwards through the ceiling into the rooms below for hours after the sun has gone. This is why upstairs is often at its worst at nine in the evening rather than at three in the afternoon.
2. Understand the stack effect as the second.Warm air is less dense, so it collects at the highest point in the house. In a house with an open stairwell, heat generated anywhere downstairs — cooking, appliances, people, sunlit rooms — ends up on the landing and in the bedrooms. Real, but a fraction of the roof contribution in a sunny spell.
3. Understand the glazing as the third.Bedrooms are typically shut and unshaded all day while everyone is out. South and west-facing bedroom windows admit sunlight for hours into a sealed room with nothing removing the heat. By the time anyone opens the door the room has been charging up since morning.
4. Work out which one you have.If the upstairs is hottest in the late evening and the loft hatch feels warm to touch, it is the roof. If it is hottest in the late afternoon and one particular room is much worse than the others, it is glazing on that room. If the landing is hot but the bedrooms are only mildly warm, it is the stack effect.
5. Check the loft insulation before anything else.Loft insulation at the ceiling level slows heat coming down into the bedrooms just as it slows heat going up in winter. Insulation that is thin, patchy, compressed under boarding or missing over part of the ceiling is the single most common reason a house cooks upstairs. Look at it with a torch rather than assuming it is fine.
6. Do not block the loft ventilation while you are up there.A cold loft is designed to be ventilated at the eaves so moisture can escape. Stuffing insulation hard into the eaves blocks that path and buys you a condensation problem in winter. Insulation goes up to the eaves, not into them.
7. Shade the bedroom windows during the day, not the evening.The heat has to be stopped before it enters. Closing a curtain at bedtime on a room that has been in sun since ten in the morning achieves nothing, because the heat is already in the walls, the floor and the furniture.
8. Deal with the stack effect last.Closing doors to unused sunny rooms and keeping the stairwell clear of heat sources helps, and it costs nothing. But if the loft is uninsulated no amount of door management will fix the bedrooms.

Tips

• The evening timing is the giveaway. A house whose upstairs peaks hours after the outdoor temperature has peaked is telling you it is storing heat in the roof and the fabric, not receiving it from the air.
• A loft that is boarded for storage usually has the insulation squashed flat underneath the boards, which loses most of its value. Raising the boards on battens above the insulation restores it and helps in both seasons.
• Roof colour matters but is rarely worth changing on its own. Insulation depth at ceiling level does far more per pound.
• If the top floor is a loft conversion rather than a normal bedroom under a cold loft, the problem is different again — the insulation is at the rafters and there is no buffer space. That case has its own guide.

Common mistakes

• Treating it purely as hot air rising — It sends people to open windows and buy fans when the dominant source is a hot roof radiating downwards. Air movement does not remove radiant heat from a warm ceiling.
• Opening upstairs windows in the middle of a hot day — While it is hotter outside than in, an open window is importing heat. The whole benefit of opening up comes after the outdoor temperature drops below the indoor one.
• Assuming loft insulation is only a winter measure — It works in both directions. A house with poor loft insulation is expensive to heat and unbearable upstairs in summer, for the same reason.

Questions people ask