How to Keep Foundations Dry

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Whatever type of foundation you lay for your home, and regardless of whether you're building a passive solar design or a traditional house, it is very important (especially for passive homes) to keep the foundations and their insulation as dry as possible. Moisture around the foundations is bad.

  • Moisture sucks heat out into the ground, reducing your home's energy efficiency and making it harder to heat.
  • Concrete attracts and absorbs water, so moisture in the ground can end up in your foundations, from where it can get into wall insulation, reducing efficiency.
  • Moisture in the foundations can travel into wall cavities, causing mildew and mold.
  • Moisture can also cause framing and sheathing to rot, which will lead to structural damage.

Although it is always best to build on an elevated site with great drainage, dry soil and a slope leading away from the building, it is not always possible to do so. For example, if the home is near the bottom of a rise, you may have to go quite some way up the slope to begin efforts to redirect drainage or prevent water running down to the building's foundations. Plants can help absorb water in this case, and earthen dams or alternative channels can be cut to divert any water around the building.

Downspout extenders and plastic tubing

How to add a downspout extender to keep foundations dryOne of the easiest (and cheapest) ways to reduce moisture in the ground around your home is to install a downspout extender or flexible plastic tubing. Either of these can be used to transport snowmelt and rainwater away from the building – ideally at least twenty feet away.

The water transported by these extensions can be used for irrigation or simply dumped. Be aware that the water will need to go somewhere, so don't dump it in a location from which it can run back to the house site!

Underground and French drains

How to install a French drain to keep foundations dryAn underground drainpipe can transport water away from the house in a similar fashion to a downspout extender. Underground pipes usually vent at surface level (they are "daylighted") at least twenty feet away from the home.

Once again, the water carried by an underground pipe can be used for irrigation, or even routed to an underground cistern which can be used to water gardens and lawns. Alternatively, the output can be directed to a drywell, which is a rock-filled hole that absorbs water and slowly allows it to percolate back into the subsoil.

French drains running alongside or under the foundations can also carry moisture away. A French drain is a rock-filled trench which has a porous pipe (or tile) running along the bottom. This pipe is pitched to allow water to run away from the house under the effect of gravity. The pipe is usually covered with a filter to keep sediment and other junk from clogging the pipe openings.

Perimeter drainage systems

You don't really need to consider a perimeter drainage system unless your site is in an area with high precipitation or the building is poorly situated (in a dip with poor drainage, for example). They work