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I am looking at purchasing an older home that is brick on concrete blocks on plaster (from the outside in). I recently heard from someone else in the neighborhood that those homes were not constructed with insulation in the exterior walls and have inadequate insulation in the attic. I will be talking with the home inspector about this during the upcoming inspection but am curious what kind of task it would be to take this on myself and what kind of cost I would be looking at.

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The nature of task will depend on the exact type of wall. It would be best to wait for the inspection report before deciding anything.

If you have a cavity (for example) then it might be best to hire a professional. If you don't have a cavity then you need to add some insulation to the inside walls. You can get plasterboard with integral insulation which you can attach directly to the existing wall, but if you need more insulation you'll have to use thicker battens to allow for this. This will reduce the internal dimensions of your rooms. This is a job you can do yourself - but it is a big job.

Attic insulation is something you can do yourself too. The inspection should recommend what level of insulation you need to install.

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Thanks @ChrisF. I did more research and learned that there is 1" of furring between the concrete and the plaster - not sure if that is enough of a cavity. I'll look forward to the inspection for more info. –  justkt Mar 11 '11 at 16:22
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If it turns out that you cannot insulate a cavity, it is possible to insulate either the inside or the outside of the walls.

If you insulate the inside then:

  1. your walls will be "cold", this means that the materials in them will have to survive in whatever conditions you have outside, with much less additional heating from inside the house.
  2. you will lose internal space
  3. you will have to redecorate, move skirting boards, etc.
  4. your windows will be recessed more on the inside.
  5. this kind of insulation will leave cold bridges for example between the floors and where the joists meet the walls, etc.

If you insulate the outside then:

  1. the walls will run "warm" so the materials will be kept near the temperature of the house
  2. the walls remain part of the thermal mass of the heated space which may help you maintain a more consistent temperature inside the house, instead of it getting hot quickly when the heating is on and cool quickly when it is off.
  3. you will have to move any external fixings (drain pipes etc) away from the wall by the amount of the new insulation layer.
  4. your windows will be recessed more on the outside.
  5. the external appearance of your house will be altered (although there are many finishes available, including brick veneers).
  6. good at eliminating most cold bridges as you can cover the whole surface of the house in a continuous layer (apart from penetration by pipes, windows, etc).

In addition to thinking about how much insulation you will need and where to put it, you should also consider moisture control. Adding additional insulation anywhere will move the dew point (the temperature at which the water in the air in your house will condense), this could lead to condensation forming within the wall (interstitial condensation) and may require a vapour barrier, more/less insulation, additional ventilation, or for you to ensure that the wall can "breath" so that it can dry out when the conditions alter.

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