The Purpose of a Chemical Damp Proof Course
When a property's original physical damp proof course (DPC) has failed — or was never installed — a chemical DPC is the most common retrofit solution for rising damp. The goal is to introduce a water-repellent barrier into the masonry that mimics what a physical DPC does: stopping groundwater from wicking upward through the wall.
Two products dominate this market: damp proofing creams and chemical injection fluids. Both work on the same principle — introducing a silane or siloxane compound into the wall via drilled holes — but they differ meaningfully in application method, performance, and cost.
How Chemical Injection (Fluid) DPC Works
Traditional chemical injection uses a low-viscosity fluid, injected under low pressure through a series of holes drilled at regular intervals along the base of the wall (typically at 120mm intervals, 75mm above floor level). The fluid spreads through the mortar bed joints and masonry, where it cures to form a water-repellent zone.
- Injection method: Gravity-fed tubes or low-pressure injection equipment
- Penetration: Liquid disperses quickly and widely through porous masonry
- Application speed: Fast — holes can be drilled and fluid introduced in a matter of hours
- Skill required: Moderate; over-drilling or incorrect hole spacing reduces effectiveness
- Typical use: Widely used by professional damp-proofing contractors
How Damp Proofing Cream Works
Damp proofing creams (also called DPC creams or silicone cream injections) are a thicker, gel-like formulation of the same active silane/siloxane chemistry. Instead of flowing freely, the cream is injected or packed into drilled holes and diffuses outward slowly as it cures, forming the same water-repellent barrier.
- Injection method: Cartridge gun or by hand-packing into drilled holes
- Penetration: Slower, more controlled diffusion — less risk of waste or bleed-through
- Application speed: Slightly slower per hole, but no specialist equipment required
- Skill required: Lower — suitable for experienced DIYers
- Typical use: Popular for DIY applications and smaller jobs
Head-to-Head Comparison
| Feature | Injection Fluid | DPC Cream |
|---|---|---|
| Active ingredient | Silane / siloxane | Silane / siloxane |
| Viscosity | Thin liquid | Thick gel / cream |
| Equipment needed | Drill + injection tubes/pump | Drill + cartridge gun |
| DIY suitability | Moderate | High |
| Risk of product waste | Higher (can over-flow) | Lower (stays in hole) |
| Coverage per litre | Varies widely | Typically stated per cartridge |
| Effectiveness | Proven long-term track record | Comparable when applied correctly |
| Typical cost (DIY) | Lower per litre | Higher per treatment |
Which Should You Choose?
For DIY homeowners, damp proofing cream is generally the better choice. It's easier to apply correctly without specialist equipment, less messy, and the pre-measured cartridge format reduces the risk of under- or over-application. Products from reputable manufacturers come with clear instructions for hole spacing and depth.
For professional contractors, injection fluid remains common — particularly on larger or more complex jobs — because it can be applied faster in volume and is well-established in the industry.
Important Caveats
Neither product is a complete solution on its own. After chemical DPC treatment, affected plaster must be removed and replaced with a salt-resistant renovation plaster. Old damp plaster retains hygroscopic salts that will draw moisture from the air even after the rising damp itself has been stopped, causing the wall to appear damp long after treatment.
Also, confirm the diagnosis before treating — a chemical DPC won't fix penetrating damp or condensation, which are far more common causes of wall dampness than genuine rising damp.