temperature of chicken house

hi, i moved my 4 week old hybrid chicks out into their chicken house to give them more space, they have their electric hen still. i am worried about the temperature out there i have just been out and itis 15 oc, they are all under the electric hen at he moment although i have seen them up and about earlier today. they all seem to be eating and drinking, they have been outside for 1 week. when we coverted the shed we have left  ventilation strips in the eaves of the roof, should i block them up for now, do i need to insulated the shed it has been made out of 16mm thick tongue and groove panels. thanks for your help, i just dont want anything to happen to my chicks as i have got them this far. does anyone when the danger period is over for loosing chicks.     

thanks for your help, at least i can sit down and have a cup of tea now.