Solid walls typically allow twice as much heat to escape as uninsulated cavity walls - but they can also be insulated, either internally or externally. Very thick solid walls will have a better u-value than a typical brick wall, however they act as a massive heat sink (thermal mass - great if it’s hot). For a wall to have the same u-value as a typical 240mm brick wall with 100mm insulation, it would have to be 2 metres thick!
 
It is the received wisdom that internal wall insulation is cheaper to install than external, although in practice once everything has been taken into account the costs are broadly similar.
 
The budget costs that we use in our calculator (£100/m2) include for all material costs and labour of installing the insulation and completely finishing the wall.
 
Generally speaking, external insulation is the better option, as it can cover the whole wall and keeps the thermal mass inside the house, but there are pros and cons of both.
 
In many cases a mixture of the two will be the best solution - lots of older houses have an attractive brick frontage which wouldn't be suitable for external insulation, however the rear is generally less impressive (and less likely to be protected by conservation) and external insulation could be just the thing. The other walls can then be insulated internally, one room at a time if need be.
 
The alternative to solid wall insulation is to fix battens or framework to the wall, and then fill or cover with insulation; however there are no obvious benefits to the added complexity, apart from perhaps in listed buildings where the original wall surface must be protected.
 

External wall insulation

Rigid insulation boards are fixed directly to the outside of your walls using pins, and then rendered and if required painted, although pigments can be introduced to the render.
 
[Interesting fact – the colours of many rendered houses historically are down not to choice, but to the render additive available at that particular time of the season. Pig’s blood, rape, mustard, and cornflower were all used to improve adhesion and elasticity.]

Advantages

There are many very good reasons for choosing to have your walls externally insulated:
- can be applied with only minimal disruption
- does not reduce your floor area
- keeps the thermal mass inside, thus ensuring a more stable internal temperature and avoiding over-heating in the summer
- because it covers all of your walls, external insulation eliminates cold bridging (points which penetrate the insulation) and areas inside where condensation is likely to form. 
 
If you need to carry out external refurbishment works, it makes sense to install the insulation at the same time, particularly if you have rendered walls with damaged render or brick walls that need re-pointing – insulating at the same time may not cost you much more than the repairs. The new external rendered layer will renew the appearance of your walls and protect your brickwork, and improve weatherproofing, soundproofing, and draught-proofing.

Considerations

Of course external insulation comes with its own considerations:
- likely that you will require planning permission, since it can alter the appearance of your house drastically (unless of course it’s already rendered)
- you will need enough space alongside the wall for access during installation, ideally 600mm, or 900mm if it is a passageway
- your walls will need to be in good structural condition
- all rainwater goods will need to be moved
- detailing to roof eaves, window ledges, and door and window frames will all need to be thought out
...however, none of this is insurmountable!
 

Internal Wall Insulation

Rigid insulation boards are fixed directly to the inside of the external walls, and then plastered or papered and painted. This of course is very disruptive, although can be done room by room.
 

Advantages

- Installing internal insulation gives the option of splitting the costs and doing your house room by room as required.
- You can also insulate a wall whenever you’re doing something else to it anyway, such as fitting a new kitchen or bathroom or just generally re-decorating, cutting costs and disruption. 

Considerations

- Radiators, pipework, skirting boards, door frames, and other fittings will all need moving.
- Window sills will probably need extending.
- Walls will need redecorating, incurring further costs.
 
It is important to return insulation into windows and along internal walls, as well as insulating the walls between floors and ceilings so as to avoid thermal bridging and potential damp problems. The room area will be reduced by the thickness of insulation and finish on each insulated wall, and it is also not ideal to then hang heavy items from the wall.
 
 
 
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