Exterior Foundation Insulation

If you are excavating to waterproof your foundation, it's also a good time to consider adding some insulation to your foundation. Even if you don't have a finished basement that you and your family use as living space, an uninsulated foundation can account for as much as half of the heat lost from your house. Adding insulation reduces your heating costs, makes your basement more comfortable and increases the value of your home.Graph of heat loss by depth from a fondation

The amount of heat that is lost from a foundation wall depends on the difference between the temperature inside and the temperature outside. The deeper you go into the ground, the warmer the ground will be in the winter, so the most heat is lost from the highest parts of the foundation wall.

Your foundation can be insulated from the inside, the outside or both. A combination of the two is often the best solution.

Reduce Condensation

First, by insulating the outside of the foundation, the foundation stays warmer. A warmer foundation means that there is a smaller difference between the temperature of the concrete and the air inside the house so there will be a lower risk of condensation when warm moist air meets a cold wall. This subsequently reduces the possibility of mold growing.

An Insulated Foundation Saves you Money

Insulating your basement exterior walls to R10 can reduce your heat lost through those walls by 55%. The payback period on the investment depends on where you live in Canada but you can expect an 8-10 year payback period. This period is reduced further when you consider that there is currently a Government of Ontario rebate program available to people who upgrade the insulation in their homes (including adding insulation to the foundation) so your capital expenditure is even lower. In addition, unless your basement is empty, insulating inside is much ore time consuming and disruptive. Finally, adding insulation when you’re already excavating your foundation to do waterproofing is a small incremental cost compared to doing the insulating on it own.

Illustration of temperature gradient with interior vs. exterior insulationThe drawback of exterior insulation is that it's difficult to deal with above-grade. Below grade, it is hidden and protected by the backfill. However, above grade, the insulation must be covered for aesthetic purposes as well as to protect it from physical damage and degradation from exposure to sun. You are also faced with an aesthetic issue since the insulation will extend out further than the exterior finish (brick, siding, stucco, etc.) of your walls leaving a large lip. In addition to the undesirable visual effect of this lip, it may also present a place for water to get behind whatever finish is applied to the insulation. It is for this reason that we recommend a combination of interior and exterior insulation. Exterior below grade and interior above grade.

Adding exterior insulation while your foundation is fully excavated adds only a small incremental cost to the job but reduces your future heating costs, increases the value of your home, makes your basement more enjoyable. If your basement is already finished, adding insulation on the exterior is much easier and cheaper than adding insulation inside. Even if your basement is already insulated from the inside, you can still benefit from adding exterior insulation.

Government Helps Pay for Insulating Your Foundation

Finally, if you're still not convinced that adding more insulation is a good idea, consider that the Ontario government has a rebate program in place which helps you recover some of your costs for insulation immediately. See our brief summary of this program in Ontario Home Energy Savings Program.

For more information on the long term cost savings from R10 insulation on a full basement see:

For information on heat-loss from basements



# Administrator 2011-09-11 22:16
Nick asked:


We are waterproofing our basement this weekend and decided to add go with the blueskin, exterior insulation as well as the platon membrane. I was wondering what order should I add the insulation? Should it be put on before the bluskin, or between the blueskin and platon? Thanks for your VERY usefull website!



Blueskin should be first against the foundation to give you the waterproofing. Then, for the order of insulation or platon, see comment #2 above. You can go either way, but there are pros and cons to either approach.
# Insulation Blower 2011-06-14 14:46
I'm a newbie on this stuff. I like your posts, but where can I find other sites too?
-1 # Siegfried 2011-06-07 08:26
I would suggest having a couple of contractors assess whether your whole foundation needs reparging. It may not be necessary and if you're adding blueskin and platon, you're not likely tho have waterproofing issues. Some people advocate filling cracks with injection type fillers which are supposed to solve the problem and shouldn't need parging over top. Talk to you home center sales people to see what products they offer.

Parging is a special mix. It's thinner so that it can be spread more easily. There is a bit of art to making parging look good, so start at the back of your house and work your way around to the front. Hopefully your technique will improve as you go. This might give you some parging insight: http://rodcroskery.wordpress.com/2009/08/25/stone-house-reno-v-parging-mix/
# Siegfried 2011-06-07 08:25
Putting the platon on last adds physical protection to the XPS particularly during back-filling and if the XPS is above grade it gives on-going protection from accidental or intentional damage (think kids here). Putting it last however, is a bit awkward because the platon attaches to the wall and then has to go out over the XPS and down the wall. Also, you want the insulation to go as high as possible because the greatest heat loss occurs at the areas above the ground. However, then you end up with less-than-attra ctive membrane and insulation visible. If possible you could do the exterior insulation below ground level and then do interior insulation to cover the upper portion of the wall.
+2 # Chris 2011-06-07 07:56
1. If we are insulating with XPS (2") do we still need the Platon membrane? I've found its not a large additional expense, so if it'll help what should the order be? (blueskin, platon, xps? or perhaps blueskin, xps, platon)?

2. Do you advocate the parging over the entire foundation, or just spots there cracks exist? How thick should the parging be, and is a special mix required?