Concrete is not waterproof. It is riddled with capillary pores. If you backfill soil directly against the bare external wall of your basement, the concrete will soak up ground moisture like a sponge.
The most classic, proven and value-for-money solution for sealing those pores is bituminous coatings. These are thick liquids based on bitumen, brushed on with a large masonry brush or roller, creating a seamless, black, waterproof film.
In the store you will find the black tins divided into two large families. The choice depends on the wall condition:
Environmentally friendly, low odour and washable with water (while wet). Their huge advantage is that they can be applied to slightly damp concrete. If the contractor has just stripped the formwork and the wall still retains moisture, emulsion is the only solution.
These have a much stronger smell (like diesel/white spirit) and dry faster. They offer slightly better adhesion and penetration, but require the wall to be completely dry. If applied to a damp wall, they will bubble and peel.
Note: If you plan to bond insulation boards (EPS/XPS) onto the bitumen, use STRICTLY water-based emulsion. The solvent "melts" polystyrene!
Application is easy and fast, but must not be rushed:
The wall must be clean of dirt, form oil and loose material. Any tie-wire holes (from the formwork) must be plugged with repair mortar.
The first coat acts as a primer. We dilute the bituminous material with water (or solvent, depending on type) by 20%. This makes it more fluid, allowing deeper penetration into the concrete pores to "anchor" properly.
We apply the second coat undiluted in a thick layer. Once dry (the next day), we apply the third coat cross-hatched relative to the previous one.
You have applied 3 perfect coats. The wall is jet-black and waterproof. Then the excavator (JCB) arrives and starts dumping soil and stones into the trench to bury the basement (backfilling).
If soil falls directly onto the bituminous coating, the waterproofing is destroyed in 5 seconds! The sharp stones dropping with force will scrape, tear and puncture the black film.
Before backfilling, the bituminous waterproofing MUST be mechanically protected. Typically, extruded polystyrene boards (XPS) are bonded on top for thermal insulation, and over those the drainage membrane ("egg-crate") is installed to absorb stone impacts.
Our 10×10 house has a 3-metre-deep basement. The concrete has dried and we are about to bury it in soil.
The labourer grabs a brush, hastily slaps on one coat of diluted tar, leaving gaps ("clouds") where grey concrete shows through. The same day, the bulldozer dumps soil and stones. The stones scrape the tar. Come winter, the basement wall shows damp at exactly the points where the stones tore the material.
We clean the wall. We apply one diluted primer coat of bituminous emulsion plus 2 thick cross-hatched coats. We wait 2 days for it to cure into a tough rubber-like film. We bond pink insulation boards (XPS) and install the "egg-crate" membrane on top. The JCB drops the soil. The stones hit the plastic membrane, leaving our bituminous coating 100% intact. The basement is perfectly dry!
Final Verdict: Bituminous coatings are the most reliable, economical and fast solution for most basements with normal soil moisture. They are excellent at stopping damp, provided they are applied at the right thickness (3 coats) and are fanatically protected before backfilling.
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