How To Do Insulation For Basement

DEAR TIM: I'm going to be remodeling my basement and want it to be toasty warm. How would you do insulation for a basement remodel like this? Would you use basement foam insulation or just the old standby fiberglass?

What's the worst thing that can happen if you do the job wrong, and how do you avoid problems down the road? Is the insulation for basement wall in new construction any different than a remodel job? Kimberly C., East Hanover, NJ

DEAR KIMBERLY: Your questions are ones that could generate probably four or five correct answers. Any number of insulation experts would probably tell you their method is the best. The good news is that I know of at least two methods that work well.

The rigid foam insulation is placed between the stud wall and the foundation wall. The butt seams of the insulation must be taped. Photo Credit: Tim Carter

I find it interesting that you have thought ahead as to what is an unintended consequence of doing the job incorrectly. This is absolutely a concern because once you finish the walls, you hide the work and can't see problems until they get so bad they work their way through the walls.

I feel the biggest problem you can face in a basement remodel in a climate where the basement walls can be cool or cold is mold or mildew. Most basements, especially ones in houses built before the 1960's, have an issue with high or excessive water vapor. Water leakage into finished basements is also a huge issue. More on that in a moment.

This water vapor comes from two primary sources, water that's working its way through the foundation walls or the concrete floor. Prior to the 1960's, it was not a common practice to install a vapor barrier under the concrete floor, and not all houses had the minimum damp-proofing sprayed on the outside of the foundation walls before they were backfilled. Water vapor readily works its way through solid concrete.

The water vapor that concentrates and builds up inside a basement will readily condense on cool or cold masonry foundation walls. It may never get so bad that you see the beads of water. In most instances, it's just an invisible fog on the walls much like the mirrors in a bathroom fog up after you shower. You see the fog on a mirror because it obscures reflections. A water vapor fog on a concrete wall is absolutely invisible.

This water on the walls is the needed fuel for mold and mildew growth. When you insulate a basement in order to finish it, you need to account for this water issue.

There are two ways to insulate a basement that do a superb job. The first one works well if you're on a tight budget. The key is to create an air space between the foundation wall and the backside of the stud wall that you'll construct for your wiring and fiberglass insulation.

The air space can be as little as one-half inch and still be effective. This air space allows for minimum air circulation should water vapor get to the cold wall. You just make sure the fiberglass batts in the wall don't contact the foundation wall.

The rigid foam insulation is placed between the stud wall and the foundation wall. Photo Credit: Tim Carter

What's more, it's mandatory that you install a vapor barrier on the warm side of the stud walls before they're covered with drywall or paneling. This vapor barrier can't guarantee that water vapor will not find it's way to the foundation walls, but it surely helps funnel the water vapor to rise higher into your home eventually exhausting through great roof ventilation.

Perhaps the most effective way to install basement insulation is to use both rigid foam insulation in conjunction with stud walls that also contain fiberglass batts.

The rigid closed-cell foam is installed in large sheets, usually no less than one and one-half-inches thick, over the entire surface of the foundation wall. You want the foam to cover the foundation wall from the top of the floor slab all the way to the very top of the foundation wall.

The closed cell foam insulation is an exceptional vapor barrier, and when the seams at all butt joints and the corners are taped, it will be very hard for water vapor to get to the cold walls. The stud walls are built tightly against the foam pressing it to the foundation. Fill the stud cavities with un-faced fiberglass batts for a toasty warm basement.

The foam insulation method works best if the foundation walls are smooth. If the basement is an old stone foundation, spray foam insulation companies can spray expanding foam directly on the rough masonry. It'll be expensive, but it can be done and it will do a fantastic job.

If there are any issues whatsoever with leakage into the basement, this MUST be solved before you begin to remodel. The best method I have seen over the years is to control this water on the outside of the foundation before it comes into the house.

You can capture and divert subsurface water in a yard by using a linear french drain. I explain these in great detail at my AsktheBuilder.com website. Just type Linear French Drain in the search engine once there.