I know this is probably pretty self explanatory but bear with me. The three main areas to consider when insulating in an old or an existing house are:
There are potential pitfalls for each and some in my opinion, may not be worth the hassle depending on the house, construction type, exposure and whether or not you’r prepared to embark on a major renovation of the property.
For example a house with solid ground floors will not be easy to insulate as you’ll have to dig up the floors, breach any damp proof membrane that may (or may not) be present and then relay them forming a good seal between the new damp proof membrane in the floor and the damp proof course in the walls (which will be difficult and quite expensive).
Also room in roof construction, where you have a room in the loft area will make it much more difficult to access the areas where you need to install the insulation. So unless you’re about to renovate this area anyway then it might be better to wait until you are.
Provided it's carried our correctly:
Unfortunately there’s no straight forward answer to this question as it’ll depend on the construction type of your house;
Cavity walls - normally they’re easy to insulate by getting an insulation company in (it’s not a DIY job), but sometimes it can be better not to do it!
Solid walls - more difficult to insulate and can be quite expensive, but can certainly be done. There are two types of solid wall insulation; internal insulation (cheaper option) and external insulation (more expensive but better performing).
Open loft area - relatively easy to insulate and fairly cheap to do. But there are some things you’ll need to be aware of.
Room in roof construction - This is where you have a habitable room within the roof area and limited access to the underside of the roof. This type is much more difficult to insulate without spending a lot of money and major upheaval, unless you’re about to renovate the area anyway.
Solid ground floor - very difficult to insulate, but not impossible (it will be expensive and disruptive).
Suspended timber ground floor - relatively easy to do but if you do it wrong you could cause as many problems as you fix.
Short answer is… No!
Traditionally if you wanted to insulate your underfloor or loft you just laid out some fibreglass wool (or equivalent) and that was it, job done! Old houses tend to be a lot more draughty than newer properties and even if you do the best job ever when insulating your old home in the traditional way and ensure there are no gaps, if there are draughts in the area (and there will be draughts, they are a Building Control requirement in lofts and subfloor areas especially when increasing insulation) then the insulation won’t do very much.
At this point it would be a good idea to understand the difference between controlled and uncontrolled ventilation. But I don’t want to get distracted at this stage, so if you’re not sure what they are follow the link and then come back to finish this page.
The most common issues I come across in old houses belonging to clients is condensation and mould. These are often directly linked to uncontrolled ventilation (draughts) and insulation, or the lack of insulation, in the property (albeit there are other factors in play as well such as relative humidity, air pressure and sometimes damp walls (have a look at my article on Condensation for more information).
But suffice to say, if you can insulate and make the property as airtight as possible (in addition to providing controlled ventilation), you’ll increase the surface temperature of the main external walls, floors and ceilings you’ll reduce the risk of condensation occurring and therefore reduce the risk of black mould forming and make it more comfortable and cheaper to heat effectively.
In conclusion, provided you use the correct method and the best insulation products when insulating an old house there’s no reason why you can’t insulate and draught proof your existing property and at the same time reduce your heating costs and make your home much more comfortable.