Insulating Your Foundation

Greg, I am building a new home and although we didn’t hire you as our builder I have become very frustrated trying to get direct answers to my questions from the General Contractor in charge of building our home. My husband and I think that the foundation walls should be insulated but our GC says it isn’t necessary. What do you think?

Ginny and Bob

Ginny, I love this question. Each situation at each new house can be different and it doesn’t matter what I think because I am not an expert. However, it is my blog column so I can go on as long as I want. What I am going to do here is to pass along information gained from my “experts” who are building scientists (yes, there is such a thing), Joe Lstibureck from Building Science Corporation and William Rose of Building Research Council who understand way more about this than I ever will.

The short answer is yes, you should insulate the foundation walls of a new home. Now I will explain as best I can the reason why and the methods to do this.

The main reason why you should insulate not only the walls of the foundation but also the under slab area is because of the your home’s worst enemy – Mr. Water, who is also known as Mr. Moisture and Mr. Condensation whom, by the way, love to bring along their two nephews, Mikey Mold and Marty Mildew but I will digress from talking about these guys right now as most people are aware of the problems they can cause. Water in whatever form it takes (moisture, condensation, or a flood) is a problem and the best way to deal with a problem is to lessen the chances of it ever happening. Can you keep all forms of water out of your house forever? Of course not but you can take precautions to lessen the damage it will cause to your new home (or any home for that matter).

Let’s begin by the order in which a new foundation is installed. After properly excavating the site and leaving at least 4 inches of undisturbed soil in the areas where the footing will be placed (the form guys hate when I do this because now they have to go back to their truck and get a shovel and actually have to hand dig those four inches of soil before they can set the height of the footing) and then form for and place the concrete in the footings. Now here’s a critical step – on top of the new footings you should place a footing barrier. This is a product that resists the capillary wicking and upward migration of moisture from the footings into the foundation walls. This is known as “rising damp”. Why is this critical? Because, no matter how much waterproofing you apply to the exterior side of the foundation walls, this rising damp will allow a lot of moisture into your basement. How does this happen, Greg? Well, Ginny, this happens because concrete is porous. It has millions of tiny little air pockets in it that gives moisture a freeway to the inside of your house. By setting the footing on bare ground and then installing concrete walls on top of it, without this barrier, it allows moisture to wick right up the walls and then transfer itself to the air in your basement. From there it will do it’s best to get to the rest of your house. And that’s a story for another time.

Now the walls of the foundation. It is always best to insulate on the exterior side whether it is a concrete wall or a wood wall above grade. Why, because that is where the elements of hot and cold start. If done properly, this will do more for your house, your comfort, and your utility bills than any other component of your house. And, because through magic called gravity, whenever it rains the water eventually winds up on the ground. (yes, I know, some of it goes into the air but most of it goes into the ground, and sometimes into your house but we don’t want to talk about that now either.

So, below the ground is your foundation wall that your builder has had enough foresight to use a WATERproofing product instead of a DAMProofing product that the building code allows for and now it’s time to install an insulating product that will also help to remove from the wall all of that rain that falls out of the sky once in a while here on Cape Cod. One of the ways that I like to do that is with a drainage board that also has some insulating value to it. One of the products I use is Warm-N-Dri Board manufactured by Tremco. It is placed against the concrete walls that have already been waterproofed, insulates up to a value of R-10, and allows water to pass through it on its way to wherever water goes when it gets past the bottom of your footing. Hopefully somewhere besides the other side of your foundation wall because that would mean it was in your basement. Not good!

So, there is the long drawn out answer to your question Ginny. Good luck in convincing your GC to comply with what you now know is the correct way to install a new foundation. Maybe if you bring him coffee and donuts? GC’s like donuts almost as much as cops do.

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