Several people have asked me recently whether floor insulation is worth it. It’s tricky.
First, let’s look at heat loss from the house during winter and compare your ceiling to the floor. Assuming your house is at 18 degrees, the ceiling will be around 20 and the floor around 16. The ceiling space will average 12 degrees while the underfloor will be around 14.
If the floor space is well ventilated it may be less, if you have heating ducts it will be more.
So the temperature difference across the ceiling is 8 degrees, across the floor it is 2 degrees. So insulating the ceiling is much more useful.
Also, it is much easier (and cheaper) to add ceiling insulation than floor insulation.
Now look at summer. Roof insulation works well to keep your house cool, but floor insulation works against you. The sub floor is normally cooler than the house so the floor cools the house. Insulation reduces this cooling effect.
There is another side to the problem, that of comfort. The floor is the coldest place in the room and your feet may be cold so you turn up the heater. Alternatively you keep your feet warmer.
You can wear thick socks. You can stop the floor ‘sucking’ heat from your feet by putting down a floor rug. You can place a piece of underfelt under the rug. You can even buy a ‘foot warmer’ that uses only around 20 watts.
So, I would concentrate on the ceiling. Fill the gaps between the batts, make sure the insulation is even and add another layer of batts. Only then is it worth looking at the floor and even then it is only economic if you can get cheap insulation and install it yourself.