Forums DIY Ground Source Heat Pumps Reply To: Ground Source Heat Pumps October 25, 2018 at 2:45 pm #447 nab 23 Posts If the listing of the building allows I would certainly look at some good insulation on the insides of the external walls. As you say it will make the window reveals even larger and there might be room features (eg coving) that would make it hard to achieve but I think it would make a big difference. Something like 100mm Kingspan or to be more environmentally friendly sheep wool.