"The guinea pig has long been a domesticated animal, from around 5000BC in fact. As a result, and much like dogs, their genes and health are a mess. A guinea pig is very far removed from any wild animal, domesticated for such a long period of time they are very vulnerable to temperatures outside of a very strict range - 65-75 degrees F which is 18-24 degrees C. Not even the UK, where I live and where is considered a temperate climate, can keep within these magic numbers for a single day and night.
"The hair of a guinea pig is no thicker than the hair on a human head. And as we all know from being made to wear hats in the autumn and winter as small children, that hair doesn't keep you very warm! On top of that, guinea pigs lack the ability to sweat, meaning that when they heat up they have no physical way of cooling themselves down again. The death rate of pigs kept outside in summer, or even left unattended in a run is sadly high.
"Many people seem to think guinea pigs are equivalent to rabbits, "designed" to live outside, happier when close to grass and so on. Now, I can't speak for rabbits, but with guinea pigs being such a social animal, housing them outside is very unfair. They love to welcome you into their herd, they love to have soft comfy things to lie on, they love to have lots of space to run around in and do their "zoomies", and they love to hear you doing things about the house. The idea that they are outdoor animals, as you can no doubt guess from their desired temperature scale, is just silly. This only came about because of this link with rabbits, which can still be seen by the number of people that keep rabbits and guinea pigs together - terribly dangerous!"
http://community.livejournal.com/guinea_pigs/tag/care:+indoors+vs+outdoors
I quoted this to you not to "get on at you" or start a debate but to give you the comfortable-temperature range and to then explain why it is so high.