So your interested in cooling your house down when it’s hot, and want to know, will opening the loft hatch cool the house?
If you paid attention during science class you will remember that heat rises. Opening the loft hatch with allow the hot area to rise and escape through ventilation points in your loft.
So the answer is yes, opening the loft hatch will cool the house considerably.
To improve the air flow even better it would be advisable to open windows down stairs on the cooler side of the house ie not in the sun, at the same time as the loft hatch is open. This will enable greater air flow, circulating cold air in through the windows and drawing hot air out through the loft.
The best time to do this is early in the morning when the air around the house is at its coolest. Getting the cooler air into your house for an hour or two, and then close the curtains and windows will allow the cool air to stay in the house.
I hope this information will help to cool your house by opening the loft hatch. While we have you, you might find this post interesting about the 3 Benefits of Air recirculation button in car.