Understanding Mold: They Don't Build 'Em Like They Used To

By: Mark Clement , Contributing Writer
In: Old Houses, Old House Construction, Old House Musings

Question: I don’t get it. I’m from New England and we’re surrounded at every turn with old homes that have stood winter and summer for what feels like ever. Yet I hear the horror stories of new homes being consumed by mold and see all these mold-resistant products out there. Mold’s not new but new houses seem to be full of all these new products like particle board walls that are cheaper and less durable than plain-old wood 1×8s. A few years back I saw a house being built in Maine. It had the old fashioned boards and I said, now that house will be here for a good long time. What’s the deal?

Answer: Wow, great question. The answer is both simple and complex. Let’s start with simple and focus on the mold. I should also point out that I’m generalizing here and that different situations may have different details. So, generally speaking:

First, a mold fact: Mold needs three things to grow–a food source (usually wood or paper-backed drywall), proper temperature (warm), and moisture. Take one of these three things away, the mold can no longer grow.

So, with that in mind, the reason newer homes are more subject to mold problems is because they have something old homes don’t, namely insulation. Insulation’s job is to slow or stop air movement through the wall. The better insulated a house is, the less air moves through it–which is good because if you’ve paid to heat that air you want to keep it inside in February.

Now add this to the equation: All buildings leak water–even old ones. Even brick and stucco buildings. But the difference between an old home and a new home is that the old home isn’t packed with insulation and–with all that air moving around in its bones–is able to dry out. More tightly insulated homes don’t fair as well because there is less air moving through the walls. Less air movement traps the water. And because the air that is there is warm, mold grows. Think about wearing a ski jacket in summer. No moisture moving through bad boy and pretty soon you’re soaked, right? Same idea.

As for the types of products used building a modern home–you mention “particle board” and solid sawn 1×8 sheathing above for example–mold grows on both of them. They both constitute a food source. Note: the particle board you’re referring to is OSB, oriented strand board. The flakes or “particles” its made from come from fast-growing, young trees and trash wood. 1×8s come from much older trees which, considering the house you mention in Maine may well have been felled and milled by the builder from his own land and produced for free-ish. Sheeting a house with 1-by today would be monumentally expensive. Put another way,  carpenters long ago would have used plywood and nail guns if they existed.

The secret to making a new home or addition mold free–and thermally efficient–is to give the water that inevitably gets in a way back out. Flashings, building paper, trim details–these all combine to weather- board a house such that it sheds water efficiently and evacuates moisture that penetrates the building envelope. As one builder told me once: think like a raindrop.

There is, though, at least one element the builders of old got right that our world of super-tight budgets sometimes prohibits: overhangs. Windows were recessed into the building envelope not nailed on the outside as they are today. Eaves were, generally, larger. You can probably walk down your street and see a modern house with an “eave” that’s a 2×6 laid flat and wrapped with aluminum. Wide eaves, porches, even overhangs at the doors protect the home, the materials its built from and the people in it.