No windows
. Sorry there are two double glazed windows one at the end of the extension and one on the side wall along with a back door.
Walls sound hollow alright.
Okay, good info there.
The following will give you an idea of what
may be going on with respect to your heat loss. I emphasise "may" because in the absence of an onsite inspection by experienced eyes, the following is still all theory.
1. I assume the extension is jutting out off the back wall. This means that 5 of the 6 sides of the extension (3 external walls, floor and roof) are heat loss elements and the total area of these elements in relation to the internal floor area is significantly larger to that of the original house. This means that, all things being equal, the heat loss rate will be much larger than the rest of the house. Therefore, to compensate of this increased heat loss rate, either more heat needs to be supplied to this space and/or the heat loss elements need to be optimised to reduce the losses in the first place.
2. The heat is likely being lost by a) conduction through the roof/wall (lack of insulation or insulation badly installed) and/or b) convection through roof/wall (aka draughts, air leakage). Depending on which it is will result in different approaches.
- If it's a) then you need to tackle the insulation of the walls and roof with the aim of significantly decreasing the u-value (thermal transmittance) paying special attention to the increasing risk of interstitial condensation and mould so external insulation and a warm deck flat roof detail would be advised.
- If it's b) then you need to first understand the nature of air leakage in buildings such as the extension (for example, internally drylined walls typically are devoid of an air tight barrier), how and where to rectify etc and also be cognisant of the need for appropriate ventilation to avoid indoor air quality issues and especially condensation and mould.
- or it could be a combination of a) & b)!
It is not unusual to spend a lot of money (and even receive gov grants) on a measure, for instance insulating walls & roofs, only to find out after the works are done that the problem still persists or the result is not as good as you thought it would be. This is because the cause of the heat loss was not well understood initially and the wrong measure was taken.
I'm not sure if this helps you somewhat but I want to show that questions such as yours are not always about just adding more insulation especially where a targeted approach is desired.