I think I see the problem. I am not on the Choice team and a representative should answer you directly but what I think is happening is the way the sweat test is introduced is confusing.
The section starts off saying “sweat repellent” this suggests that the measurement is of the degree that sweat is prevented from entering the mattress. It then goes on to describe testing how well moisture escapes through the mattress, ie the permeability, which is the opposite.
I think the test is in fact of the latter property, how well moisture escapes and so how it allows the mattress to pass moisture away from you and to dry out. If this is so it is misleading to talk of how it repels sweat in the introduction.
Look at it this way, if you use an impervious mattress cover in hot conditions there is nowhere for sweat to go, you are going to end up lying in sweat collecting on top of the cover and soaked into your bedding. You will be more comfortable if the sweat passes through the mattress while you are on it and then escapes from it while you are not. So more permeability is correct.
Hmm. I agree that the article’s description of “permeability” is confusing. It appears to say that “permeability” measures the sweat barrier (repelling water intake).
From the article:
“If you’re a particularly sweaty sleeper or live in a hotter climate, you’re probably better off looking for a mattress with a higher sweat repellent score in our test (this assesses a mattresses’ permeability and water intake).”
Thanks for the question. It’s as @syncretic describes, it’s about letting moisture ‘pass through’ the mattress. Here are some more details on this part of the test:
Sweat repellent (human) The test subject lies on the mattress without moving for two hours in an environment with an ambient climate of 23°C. We measure the relative humidity between the body and the mattress beneath the waist of the subject.
Sweat repellent (technical) We perform this test in a climate chamber at 37°C for 10 hours. We place a measuring vessel under the mattress so that moisture can escape through it. The vessel is weighed before and after testing to determine permeability and water intake.
Thanks for the feedback on this point causing some confusion, I’ll pass it on to the team to see if we can perform an update to clarify the issue.