What is the best exterior insulation to put over existing double brick?
We are in the process of doing a major rennovation + second story addtition to a double brick 1940's bunglow. The inside will be be well insulated to code with an HRV, etc., but we also are looking to add 1" of rigid insulation (our builder has suggested 1" GPS) to the full exterior and finally clad it with Hardie Board. My question is around the best approach to the wall construction and application of the insulation & siding over the existing brick. I've done a fair bit of research and had a couple of people suggest that it might be a good idea to wrap the entire house (including the existing brick) with a house wrap used as an air barrier and installing the insulation on top of that.
This however, does not seem to be a common practice in the industry from what I can tell. So I'm wondering if adding housewrap directly over the brick is right thing to do, or is there a more appropriate approach to this scenario? The reason we are adding the insulation to the exterior it so help combat any possible freeze/thaw issues as well the continous insulation benefit. My main concern is around allowing the existing brick to breath. What would be the most appropriate approach for our situation here? Thanks!
Bricks will absorb moisture from the interior of a home, so it is best to have vapor permeable materials on the outside. Foam insulation will act as a vapor barrier - less so with just one inch since the permeability of foam goes down the thicker it is - but we would point you towards Mineral wool insulation to allow moisture to pass through it. Here is a page that can give you an idea how moisture affects brick; its about painting but the principles and importance of keeping it breathable are the same –
Is Painting Brick a Good Idea & Which Paint to Use?
A brick wall in good condition and without cracked mortar will not leak much air, but it really depends on the condition. Adding an air barrier is not a bad idea, but again I would suggest using something as vapor permeable as possible. House wraps are a bit like coats – there are waterproof raincoats that won’t allow water and moisture to pass through, and there are windbreakers that will stop air and repel a bit of moisture, but they will also let moisture escape. See this page here –
Choosing a house wrap, which is best?