Basement Waterproofing

Everybody has a different idea about "how to waterpoof a basement" so that the basement remains dry and problem free. Unfortunately the various waterproofing alternatives pour scorn on the merits of their respective competition and add to the confusion. So here we try to offer our thoughts on waterproofing in general and waterproofing Polarwall formwork specifically.

All these waterproofing methods can (and should) work - but all can (and might) fail.

There are four main types of waterproofing methods to consider:

1. External Perimeter Drain

2. External Membrane

3. Waterproof Concrete

4. Internal Cavity Drain

External Perimeter Drain - This is a land drain which runs around the perimeter of the building just below the level of the concrete raft. The purpose of the drain is to remove water from the external face of the building. The land drain should be covered with a free draining material such as gravel and protected with a geo-textile membrane. The drain should be designed so that it can be cleaned out and de-silted when necessary. We believe that where a perimeter land drain can be used it should be used. However, such a drain is not a waterproofing solution, but only ever a part of the overall waterproofing design.

External Membrane - there are several types of external membrane on the market, but all essentially do the same thing and create a waterproof barrier on the outside of the wall. The most common form of external membrane is a bituminous "stick-on" plastic sheet. The main advantages of this system is that it is relatively inexpensive and appears to be quite simple, but there are several disadvantages. Firstly, when things get wet on site (or even when there is high humidity) the membrane can lose adhesion , and it is often fixed incorrectly by the contractor (corners can be a weak area). These membranes are also frequently damaged during the backfilling operation. Another type of external membrane is painted (or sprayed) onto the external surface and this has less room for builder error than the stick-on membrane, but again, it can be damaged in the backfill if not properly protected. We would generally recommend the use of a paint on membrane, which should then be covered by a cavity drainage board which provides a degree of protection from the backfill.

Waterproof Concrete - A chemical additive is added to the concrete to make it impervious to any passage of water. This is great in theory but it often fails due to site conditions, batching plant problems or the inexperience of the people placing the concrete. If someone is experienced in the use of waterproof concrete additives then everything should go fine, however in the hands of an inexperienced operative it could prove to be a waste of money. With any ICF system, the formwork stays in place and covers the concrete face, which means that no-one can be certain as to how well the concrete has been placed and whether there are any voids or honeycombing. Some specifiers believe that waterproofing additives are a foolproof solution to the waterproofing conundrum, removing all possible site error from the solution. Unfortunately this is definitely not the case. Waterproofing additive product warranties are invalidated by poor workmanship and are usually limited to the amount actually spent on the additive.

Internal Cavity Drain - The thinking behind this solution is that the methodologies described above do not work and that some water will still come through any basement wall - so this water should be controlled and taken to a drain, or a sump from where it can be pumped away. Many people do not like this solution as they think it is admitting defeat in the waterproofing battle, but in reality, it is the method recommended and preferred by NHBC, Premier Guarantee etc. In this solution any moisture coming through the walls is drained down inside the drainage membrane to a perimeter drain at the foot of the wall. From here it is either directed to a sump for pumping away or to a soakaway. In a pumped system the main danger is failure of the electric pump so it is normal practice to have two pumps in the sump chamber, and possibly a warning system to alert of failure of one of the pumps.

The design considerations of any new build basement should factor in soil type, conditions (including contaminants), the water table position, and the intended use of the structure. The extent of the risks leads to the choice of the waterproofing materials and it's design. A belt and braces approach needs to be adapted to keep your basement dry.