Suspended floors can show particularly dramatic heat loss images, and may be easier to improve than you think.
If you have a basement, it may be more efficient to properly insulate the ceiling as work is about to be done; alternatively leaky timber floors can be covered with a cork layer or carpet.
Around floors, also pay attention to skirting boards, pipes and cables: the gaps need filling. The cheapest method is to make papier mache with torn paper (newspaper is fine) and diluted PVA glue and press it into gaps. Alternatively use a commercial filler or an expanding foam. The foams are quite tricky to use but are ideal for large gaps.