What's the best way to insulate my foundation - By Emily Mottram

It’s building season! One of the most common questions I get from people getting ready to build, or people who are looking to improve their energy efficiency is: Do I really need to insulate my basement? The answer to that question is yes! You really do need to insulate your basement. When you think about heat loss, you think “heat rises” everyone knows that. What you don’t think about is, if heat rises, then cold air is coming from somewhere – and guess where that is…your basement!
 

Existing Homes:

If your existing home has a dirt floor in the basement, prior to installing any insulation on the basement walls you should cover the basement floor. There are many ways to do this with 6 mil poly, epdm, or a full basement system with poured concrete floors and drainage.

It is very rare that I would ever encourage someone to insulate the basement ceiling instead of the basement walls. This is the wrong place to insulate. Let me repeat that, this is not where your insulation goes. If you have any of the following in your basement, please do not insulate your basement ceiling: Plumbing, washer/dryer, or your heating system. 

If your basement is granite or rubble stone, the best basement insulation is spray foam. It’s one of the only insulations that can get in full contact with all the uneven surfaces of your basement walls. You need to insulate everything above grade, and in most places in Maine – to four feet below grade to get below the frost line. All spray foam in Maine needs to be covered with a 15 minute thermal barrier per the Maine State Fire Marshal, this application should be done by a professional and you should make sure your quote includes the thermal barrier (paint). I often say this type of insulation looks like melting snow! It is great to brighten a basement. 

See video online from our friends at Dixfield Foam Insulation.

Concrete basement wall, you can install rigid insulation glued and screwed directly to the foundation walls. Homeowner do-it-yourself types can do this easily on their own. Make sure you buy Thermax Sheathing foil faced or white finish that complies with ASTM C 1289, Type 1 Class 2 as required by the Maine State Fire Marshal. Tape all seams with approved tape to be used with this type of sheathing. If you’d rather not buy this type of rigid insulation you’ll need to cover the insulation with a 15 minute thermal barrier like the spray foam. Also, ½” gypsum wall board and some OSB will meet this requirement.

New Construction:

On the Exterior: The best way to insulate a basement during new construction is to add 4” to the exterior of the building. If you choose to do this, you need to get creative with flashing details that lap under the siding/skirt board and out over the 4-inch projection of the insulation. Exterior insulation also needs to be covered with a protective coating to prevent its breakdown from UV rays and wear and tear on the foundation from mowing and maintenance. For this reason, many people opt to insulate on the inside of their foundation. We like the coatings by Styro Industries where you can buy FP Ultra Lite Panels that already have the coating on the panels, but there are many companies that make coatings to cover your foundation insulation from concrete or stucco parging to rock veneer.

ICF basement blocks. If you are planning to finish your basement, this may be the right way to go. The foam needs to be covered on the interior for fire protection and exterior for UV and maintenance protection. With only two inches on the exterior you can skin the outside of your exterior walls with rigid insulation and eliminate the flashing details at the foundation. You can hang sheetrock directly over the insulation on the inside and have a very comfortable finished basement space.

Interior insulation. There are a number of ways to insulate the interior of the foundation, so instead of telling you how to do it I’ll just make a few notes.

1.      Always consider a layer of rigid insulation between the stud wall and the foundation wall as a thermal break.

2.      Do not leave an air space behind the stud wall, this defeats the purpose of the insulation

3.      Do not use cellulose anywhere that it can come in contact with the concrete. Cellulose insulation is wonderful, but it can hold a lot of moisture and it will pull moisture from the concrete.

4.      Always use moisture resistant or paperless drywall in basements because you never know when you might have a moisture problem

5.      And last, but never least – the current energy code requires R-15 continuous insulation (so rigid insulation) or R-19 cavity insulation (think stud walls with fiberglass batts)