There is no space between bricks so we would like to add R20 to the exterior of the house also providing a vapour barrier. A previous owner had taking down the lath and plaster and installed two by twos with a bit of fiberglass insulation but no effective vapour barrier.
Insulating an old brick house from the interior or exterior and adding a vapour barrier are two different things, so let’s address them separately. And since you didn’t write ‘vapor barrier’ I suspect you are writing from Canada? There’s no place in the great white north where you would want to put a vapour barrier on the outside of a building, so hold that thought. Not having an interior vapour barrier can be rectified, but creating an air barrier after the fact is more tricky. The first step is to understand what each one does, and please keep in mind that most people (builders included) don’t realize that they perform two entirely different roles, so have a read here first –
First let’s cover that existing situation – If I understand, you probably have a total of 5.5” inches of insulation by adding 2x2’s to old 2x4s (which are probably a true 4 inches rather than 3.5inches) and your concern is that there was no 6 mil poly vapour barrier added? I'm just confused by your mention that there wasn't an 'effective' vapour barrier. I'm wondering, as you will come to understand after reading the above, if you don't actually mean effective 'air barrier'. Let me know, but I'll proceed with the vapour barrier solution
When you remove air leakage from the equation (as explained in that article above) stopping vapour diffusion becomes a bit more manageable. One option may be to apply vapour barrier paint / vapour retarder primer as we did on a demo house a few years ago.
About insulation now – you could insulate the outside of the bricks, but rather than applying something that will hold moisture, you’re best to install something like Rockwool mineral wool insulation that will let moisture move right through it, which will allow your walls to dry to the exterior. We can try to help a bit more, but can you let us know what climate zone / city you are in? That’s a determining factor for how much insulation you should add.
Another page that may help you is our Building Science Made Easy Video. Once you know what a wall is supposed to do, the options will make more sense.
Fitting extra insulation to an older home when renovating can be a great idea, but when insulating exterior walls from the outside take care to do it right. Ec…
Vapor retarder latex based primers can be twice as effective as the requirements laid out in many building codes regarding vapor barriers requirements.… Vapor …
Weather barriers (or more accurately water-resistive barriers) keep bulk water away from vulnerable building components and allow walls to dry at the same time…
Basement mold and mildew can easily be avoided with better designed walls. Common building practices trap moisture in wall assemblies preventing them from dryi…
When it comes to choosing a building envelope there are many variables to consider: what climate zone you're building in, performance, durability, health, comf…
Can a house be too airtight? No it cannot. The difference between air barriers and vapour barriers is often confused. Also confused, is which one is more impor…
Approximately 100 times more water vapour is carried into our walls by air leakage than is carried by vapour diffusion. That makes air barriers 100 times more …
Insulating an old brick house from the interior or exterior and adding a vapour barrier are two different things, so let’s address them separately. And since you didn’t write ‘vapor barrier’ I suspect you are writing from Canada? There’s no place in the great white north where you would want to put a vapour barrier on the outside of a building, so hold that thought. Not having an interior vapour barrier can be rectified, but creating an air barrier after the fact is more tricky. The first step is to understand what each one does, and please keep in mind that most people (builders included) don’t realize that they perform two entirely different roles, so have a read here first –
The difference between air barriers and vapour barriers
First let’s cover that existing situation – If I understand, you probably have a total of 5.5” inches of insulation by adding 2x2’s to old 2x4s (which are probably a true 4 inches rather than 3.5inches) and your concern is that there was no 6 mil poly vapour barrier added? I'm just confused by your mention that there wasn't an 'effective' vapour barrier. I'm wondering, as you will come to understand after reading the above, if you don't actually mean effective 'air barrier'. Let me know, but I'll proceed with the vapour barrier solution
When you remove air leakage from the equation (as explained in that article above) stopping vapour diffusion becomes a bit more manageable. One option may be to apply vapour barrier paint / vapour retarder primer as we did on a demo house a few years ago.
About insulation now – you could insulate the outside of the bricks, but rather than applying something that will hold moisture, you’re best to install something like Rockwool mineral wool insulation that will let moisture move right through it, which will allow your walls to dry to the exterior. We can try to help a bit more, but can you let us know what climate zone / city you are in? That’s a determining factor for how much insulation you should add.
Another page that may help you is our Building Science Made Easy Video. Once you know what a wall is supposed to do, the options will make more sense.