What is Rising Damp – The Least Scary Sounding Threat to Your Home

by Erik North on December 7, 2011

What is Rising Damp?


Up front I’ll admit that the main reason I’m writing this article is that everytime I say ‘Rising Damp’ I crack up. It sounds like the worst horror movie of the 70s; maybe something John Carpenter knocked out before the studios committed dozens of dollars to make “Halloween.” What’s it going to do? Chase you into a spooky forest corner and get you slightly moist?

Actually, rising damp is a very real (and potentially Freddy Krueger-scary) problem for any building incorporating porous construction materials like brick and concrete. So that’d be all of them. Unless appropriate damp proofing and capillary breaks are used in the foundation (especially when insulating), rising damp means moisture moving into your house.

What is Rising Damp?

Rising damp is the colloquial term for the upward movement of moisture in porous building materials like brick and concrete.

Have you ever seen paper towel touching a liquid? The liquid, be it spilled coffee, water or your puppy’s little mess (don’t ask), will absorb into the paper towel and quickly begin traveling up it. Or how the wick on an oil lamp burns despite the top being 3-4 inches over the surface of the oil. These are both examples of capillary action, the driver of rising damp.

Why is this a big deal? I mean we’ve built houses for ages without worrying about the goofy sounding ‘rising damp.’

First, there’s evidence that builders going back to Roman times have known about this problem. Second, it is absolutely a problem for houses. Scientists calculate that lacking a break, capillary action could move moisture up 6 miles of concrete. How tall is your house? 25 feet?

Wood, once it has absorbed enough moisture, will begin to mold and given time it will rot. Over time any untreated wood sitting on wet brick or concrete will have loads of moisture pushed up into it by rising damp. And over time, they’ll rot out.

How to Stop Rising Damp

Stopping rising damp requires reducing the amount of moisture and introducing capillary breaks.

A capillary break is a hydrophobic material which stops capillary movement. One modern example we’ll get to is inserting a foam sill seal between the top of the foundation wall and the wood sills. Historic construction has used courses of a non-corroding metal like copper, tin or lead. The sheet of metal or rock would be built right into the construction.

This first pass at addressing rising damp was introducing horizontal waterproof courses. These were one course of a moisture impermeable material horizontally across the porous stuff. An example would be inserting a brass or lead sheet in the mortar layer between two brick courses or a layer of solid slate.

How to Stop Rising Damp – Modern Construction

A second stab at addressing moisture was mandating the use of pressure treated lumber in contact with hydrophilic building materials. The big plus of pressure treated lumber is that it won’t rot no matter how much moisture is in it. The big downside of pressure treated lumber is that it won’t rot no matter how much moisture is in it.

Pressure treated lumber is, like attic venting, a moisture band aid. Rather than solve the moisture problem, you’re using a material which won’t rot. It will still soak up moisture and communicate moisture up into the house. This is where the previously mentioned capillary breaks are used.

My feelings toward pressure-treated lumber are much the same as those toward Twinkies. Any “food” which can sit unrefrigerated for years cannot be good for you.

In modern construction, capillary breaks can be any material of sufficient thickness and water impermeability to stop capillary movement. This can include various metals, glass, foam or plastics. Drylok latex masonry paint is often used in this capacity, being applied to the interior and/or exterior for full waterproofing.

Rising damp, despite its thoroughly goofy name, is a powerful natural force moving moisture into your home. It may sound ridiculous but we have several hundred years experience with wood rotting out from under us to know that rising damp needs to be addressed.

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