Damp Proofing – The Lamest Sounding Important Moisture Defense

by Erik North on December 2, 2011

What is Damp Proofing

 

Unless you’re in construction or building science, damp proofing sounds like the lamest level of water prevention possible. Wait let me get this straight, it’s not water resistant or water retarding? It’s damp proof … so I’m OK in a slight humidity but screwed if there’s a moderate rain?

What is Damp Proofing?

Actually, damp proofing refers (in construction anyway) to applications coating the foundation exterior to retard the inward migration of moisture and damp proof courses for preventing rising damp. There is some debate over the merits of addressing ‘rising damp’ (another great phrase … it sounds like the worst horror film of 1982).

But in an energy conscious world, building envelopes are becoming tighter and more well insulated. With less energy transfer, there is less drying potential in the building envelope. As this happens, moisture needs to be closely checked in the attic, walls and yes the basement.

What is Damp Proofing – Moisture and Basements

Basements are constructed by digging an 8 foot hole into clay and mud, framing up concrete forms and pouring in a stone sponge. A concrete or rubble or brick foundation has thousands of pounds of water in it. The surrounding clay and soil has tens of thousands of pounds of water in it.

Even with no rain water, clay, high water tables, marsh soil, breaks, et. etc. a concrete foundation with all but the top 18 inches covered cannot dry outward.

What is Damp Proofing – Drying

Traditionally, foundations have been allowed to dry inward. Which made sense since it’d be impossible to dry out into clay. However, as I’ve scribbled about many many times, the foundation is the weakest part of the thermal envelope (next to possibly the windows). Increasingly, contractors are insulating the foundation walls to reduce heat loss.


 
A fun side benefit to insulating? You reduce the drying potential of the foundation wall. Makes sense, doesn’t it? More heat meant more drying. Less heat means less drying. And less drying means the moisture content of the foundation lingers longer and stays higher.

What is Damp Proofing – Moisture

Where can the moisture go? Not out … wet soil. Not in … insulated. Capillary action of the concrete pulls the moisture UP. And this is where damp proofing comes to the rescue.

To reiterate now that we have some context, damp proofing is a water retardant coating applied to foundation exterior or a damp proof course preventing rising damp. They minimize water movement into your concrete sponge.

What is Damp Proofing – Redux

The two forms of damp proofing are a damp proof course, which is a horizontal construction element to prevent capillary action from moving moisture up the foundation wall. The other is damp proofing materials, a barrier or application for slowing moisture movement.

What is Damp Proofing – Exterior Asphalt

The most common damp proofing coating approach is a layer of asphalt/bitumen applied to the foundation exterior during construction. Exterior asphalt coating effectively slows moisture movement but does have limitations.

The asphalt coating can dry out and will not stop moisture leaking through a larger foundation crack. However, asphalt damp proofing does a superb job of retarding moisture movement into the concrete.

What is Damp Proofing – The Damp Proof Course

The damp proof course is a horizontal stop on that moisture rising. Many materials have historically been used to stop moisture wicking up porous materials (imagine sticking a piece of cotton in water … the water creeps right up), like plastic, lead or copper sheets or slabs of slate.


 
In modern residential buildings, the most common capillary breaks is applications of Drylok during foundation curing or the humble sill seal.

Sill seals are strips of closed cell foam. They’re installed between the foundation wall top and the sill plate. The moisture may rise up the concrete wall but the sill sealer’s break stops it. The often leaky sill plate is also air sealed and insulated.

What is Damp Proofing – That Was Longer Than I Expected

OK, that was a longer post than I expected. Modern foundations deal with loads of moisture (OK, all foundations dealt with loads of moisture but it wasn’t as much a problem until we started insulating). Damp proofing helps reduce the amount of moisture moving inward and upward into your house. And less moisture means less problems.

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