Guinea pigs are susceptible to heat stroke if left out in the sun. This is why its important not to place your guinea pigs hutch in a conservatory or sunroom during summer time. Also a green house should never be used to house your guinea pigs hutch - summer or winter.
If the hutch is in a shed please make sure that there is lots of ventilation by keeping the windows open all day and possibly the night in hot conditions. Make sure the guinea pigs spend lots of time out of their hutch in the warm weather and are placed in a run on the grass which they will mow very well for you. Better still you could buy your pets a double rabbit hutch with an exercise run underneath, this way they can decide where they want to be during hot spells.