Illustration showing my foundation wall assembly. Sometimes it’s easier to see what’s going on with a diagram, so I’ll do a few of these throughout the project to show things like wall, floor, and roof designs.
In the past, a lot of concrete foundation walls had no waterproofing or insulation, and many basements were cool and musty. Some homeowners remediated dank basements by waterproofing and insulating the interior side of the foundation walls. That works! But in new construction, water barrier and thermal barrier layers are usually put on the exterior side of the wall. This keeps the concrete warm and dry, preventing mold or frost damage. Of course, you can still add additional insulation to the interior side of the wall. I’ll do that, but it’ll come at a later stage in construction so I haven’t included it on this diagram.
This should be a pretty solid foundation assembly: it uses a high-performance waterproofing membrane, a big foundation drain and far more below-grade insulation than most homes in Anchorage. Is there anything missing? …maybe. Some higher-end builders also add a drainage mat to the exterior side of the exterior insulation. Drainage mats protect the insulation and create an air space between the foundation assembly and backfilled soil, so that any water that reaches the assembly falls down to the footing drain. I didn’t include a drainage mat for a few reasons: they’re difficult to find here, they’re expensive, and my rocky soil already has great drainage. Also, I’m using dense, nearly-waterproof 25psi XPS insulation that shouldn’t really need protection from the backfill. If I were building a finished basement in a wet part of the Anchorage bowl I would have considered a drainage mat, but at my site it just felt like overkill.