Foundation Wall Insulation | Rigid Board Insulation | Rook Energy Solutions

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. 

Depending on the size of your home, the amount of exposed foundation (above grade) you have, and the amount of heat production you have in the basement, you can save upwards of 5-10% or more on your energy bills; keeping your basement cooler in the summer and warmer in the winter. The photos to the right are 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 (top photo). 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 (bottom photo).


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.

Foundation Wall Insulation - Thermax rigid board insulation

Option 1 – solid foundation wall - see photo at right (poured concrete, cinder block, some brick surfaces…). Install 2″ of Rigid Board Insulation (i.e. Thermax White Finish rigid board). Thermax White Finish is the best option as is comes with the 15 minute fire barrier in tact (the white side), although it is more expensive, it meets code and is the best option for home owners not interested in finishing their basement. If you plan on finishing your basement, purchase Polystyrene (blue/pink/green board) as the 1/2" drywall or 3/4" pine board will act as the 15 minute fire barrier, therefore the less expensive rigid board will suffice. 

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). You can use Ramset charges with nails and washers, but the best option is to use a good hammer drill, coarse thread concrete screws, and PVC washers. If using the screws and washers, no adhesive is needed. A good corded drill is worth it in this instance.