Britain’s summers keep getting hotter, but for millions of renters, staying cool is not that simple. Many tenants cannot drill into a ceiling, rewire a light fitting, or fit permanent equipment without written permission. That leaves them stuck between two bad choices: sweat it out or spend hundreds of pounds on a bulky air conditioning unit that guzzles electricity. Many shared houses and flats also do not have anywhere sensible to fit a window unit in the first place, which rules out that option before it even gets considered. There is a quieter option that more renters are trying this year,…