Insulating your solid walls could cut your heating costs considerably, because solid walls let through twice as much heat as cavity walls do. The good news is they can be insulated.
One of the fastest ways to insulate an existing wall from the inside is to install plasterboard laminated insulation, such as Celotex PL4000 65mm, mechanically fixed to the inside face of the wall. This will achieve a U-value of 0.30 W/m2K.
Around door and window reveals install 27mm Knauf Thermal Laminate Plus to reduce the thermal bridging at this junction with the least reduction in size of the window frame.
Where internal and party walls run perpendicular to the external walls, the insulation should be taken back to a minimum of 600mm in order to reduce the effect of thermal bridging that occurs at these junctions. This is also the case in floor voids, both on the ground floor (where it is a timber joist construction) and all intermediate floors. The floor voids should be completely filled a minimum of 600mm back from the external wall.
Ensure the existing surface is even by applying a base coat to even it up. The insulation boards can be installed using a dot and dab system or, in extreme cases, battening out may be required. Ensure the walls to be retrofitted have any possible damp problems investigated and rectified.
When walls are retrofitted with insulation, it is just as important for the insulation in the loft to be up to date, otherwise condensation may form on cold ceiling areas.
At internal and external corners, plasterboard or insulation will need to be removed to ensure both the plasterboard and insulation layers meet each other.
Due to the fact that you are required to insulate the floor voids at a minimum of 600mm back from the external wall, it could be the perfect opportunity to insulate all of the floor voids.
By using plasterboard laminated insulation you can achieve a lower U-value with less intrusion on the existing footprint of the room in comparison to flexible insulation solutions.
Where electrical sockets are required to be brought forward, it is essential that the new boxes installed are completely sealed for air tightness. The bottom and top junctions of the new insulation system should also be sealed for air tightness.
Ensure any current ventilation is reinstated through the new wall insulation.