Yes, ideally domestic cats should be kept indoors during the heatwave in the UK if practical and appropriate depending on other factors. This is because all the experts say that the best way to keep the inside of your home cool is to open the windows at night to let in cool air to reduce the interior temperature of your home and then when the air temperature warms up during the day to close everything up, draw your curtains and rely on the cooled interior to suppress the growing heat during the hottest part of the day. Clearly, I am referring to homes without aircon which is the norm in most of Europe as opposed to the USA (but see Jon’s comment below please). Also, this is about protecting cats from sunburn not heat per se. Although cats do not tolerate heat as well as humans which may surprise people.
And the experts also say that cats should be protected from sunburn during a heatwave such as currently being experienced in the UK with temperatures rising towards a projected, possible record of around 40°C. Cats can get sunburnt ear flaps. Separately, it is advised to use sunscreen on a cat’s ears if they are allowed outside. It should be sunblock for pets and not human sunscreen as the latter can contain ingredients which are toxic to pets (see article below).
A lot of cats in the hottest areas of the USA such as Texas are kept inside airconditioned homes full-time anyway for reasons of cat and wild animal welfare. The beneficial spin-off is protection from sunburn.
RELATED: Do Pets Need Sunscreen?
My personal opinion is that cats do find shady spots naturally and therefore perhaps arguably the fear of a domestic cat living in a home acquiring sunburned ears is possibly misplaced. I think you will find stray cats and feral cats, particularly white ones, getting sunburned ears because they live outside. They are constantly in the elements and it’s a foregone conclusion that they will receive a lot of UVB light during hot weather.
But it is my distinct impression that domestic cats living in good homes, particularly if they are indoor/outdoor cats, will be sleeping during a large part of the day because they’ve been active at night and they’ll find shade if and when they venture out if it is hot. So, I think this problem takes care of itself to a large extent provided sunblock is applied.
I’m not saying that responsible cat owners should ignore the possibility of their cat becoming sunburned but at the same time I don’t think the problem should be over-egged.
However, to return to the question in the title, in a strict sense, the answer is YES if your cat companion will tolerate it. Domestic cats who are habitually allowed to go outside do not tolerate well being confined to the home by having their cat flap locked shut. They become confused because cats are creatures of habit just like humans. If they’ve done something for years, they expect it to continue.
My cat has literally forced his way through a cat flap that was shut on one occasion. He simply head-butted it until it broke. He made his point. It is the kind of behaviour that might cause injury and it certainly is the sort of behaviour which destroys cat flaps.
Below are some more articles on sunburn.