Why insulate your foundation wall?
8-12″ of concrete, brick, stone, and masonry carries the thermal resistance equivalent to R1 (R Value), or a single pane of glass. So yes, structurally your foundation wall is great, but it provides little to no thermal resistance. Another reason to properly insulate your foundation wall is to slow moisture movement (concrete reacts like a sponge)…slow, not block – but we will cover that in another post.
summer heat radiating into the basement through the foundation wall
In many cases, depending on the size of your home and the amount of exposed foundation (above grade) you have, you can save upwards of 5-8% or more on your energy bills…plus you will lower the moisture content in your basement, keep it cooler in the summer and warmer in the winter. The photo on the right is a great indicator of the transfer of energy through foundation wall materials. Energy travels from hot to cold, so in the summertime outdoor heat will warm the foundation wall and radiate into your structure. In the wintertime heat generated within your basement (boiler, furnace, washer, dryer, light bulbs, & even the earth) will radiate through foundation wall and outside, which means you’re wasting heat and money.
What is the best way to insulate my foundation wall?
uninsulated poured concrete wall
There are many insulating systems available and different approaches, and most work quite well. Before committing to just one option or material, make sure the moisture content within your basement is under control and that your foundation wall is structurally intact. We won’t get into proper conditions within this post, but if you are questioning these two factors call and Energy Auditor for their opinion, better to be safe then sorry!
Option 1 – solid foundation wall (poured concrete, cinder block, some brick surfaces…)
Install 1-2″ of Rigid Board Insulation (i.e. blueboard/pinkboard).
insulated foundation wall with 2" rigid board
Although it offers higher R Value/inch, I tend to stay away from polyisocyanurate (foil faced) as moisture does not permeate as easily as it can through polystyrene (blue & pink board). Only use polyisocyanurate if your basement is completely dry.
When insulating a foundation wall you want to insulate about 2′ below grade to reach the frost line, for most homes this is a 4′X8′ sheet of insulation hung lengthwise from the top of the foundation wall. We recommend to NOT start higher than the foundation wall (i.e. to the top of the joists) as it makes air sealing & insulating the sill plate/ perimeter joist difficult (i.e. closed cell spray foam).
You will need 2 people for this job, or some pieces of wood to sit the board on if doing it alone. Apply some dollops of construction adhesive to the back of the board and then either use concrete nails with washers or concrete screws with washers (about 4/5 per sheet). I like concrete nails best as it is fast and easy.
If this sounds pretty simple, that’s because it really is. If your a handy person and depending on size of your home, this can easily be a day job with a good helper. In some states it is mandatory to apply a certified thermal paint (15 min. fire barrier) to the exposed rigid board (contact your local/state fire marshal).
Another Solution: 1.5" thick Dow Wallmate foundation wall insulation - if you would like to finish your basement walls
the product edges are dovetailed so it can be strapped to foundation wall with concrete screws = no thermal break while maximizing floorspace as no wall studs are needed
