In Canada, hot summers alternate between cold winters, with frost on the ground and melting snow. In addition to its decorative role, exterior parging can create a barrier against the inclement weather of our northern climate, storms, and the sun. It's up to you to choose the right parging!

The Purpose of Parging

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The part of the foundation that protrudes from the ground, meaning the base of the exterior wall of your home, is exposed to open air and the sun. This part of the foundation walls must be covered with a coating known as parging. Applied to mask the form lines and the imperfections of the surface, whether poured concrete or in concrete blocks joined together, parging functions as a decorative feature. Note that its application is not required by the Construction Code.

If foundation walls are insulated from the outside, applying foundation parging reinforced with a wire mesh or another rigid finishing material is mandatory to protect the insulation from bad weather and the sun.

What are the different types of parging?

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Cement parging

Simple and affordable, cement parging is applied to the trowel and smoothed over with a sponge to give a sleek finish or sprayed to get different textures. There are several mixtures of cement parging: Type N or S masonry cement.

When it comes to cement parging, retailers offer new products such as Ultra-bond parging mix that adheres to concrete foundations, brick and wire mesh. You can also find cement parging in a dry mix specifically designed to conceal irregularities such as joints, colour variations, and deformities, as well as conceal tiny cracks (microcracks). One bag covers roughly 130 sq. ft. and all you need to add is water. You can also purchase a ready-made mixture. 

Type N masonry cement is manufactured with Portland cement, lime and sand. It's used to lay brick, natural stones, and cinder blocks. It has the benefit of being sold already coloured.

To wrap this up, let it be said that these products are both very adherent and easy to use, which makes their installation that much easier.  

Do not change the brand or manufacturer during your project, otherwise, you risk seeing colour variations in your exterior foundation. Unless you wish to use it for artistic expression, the decorative role of parging remains to mask the foundations in a uniform way.

Acrylic parging

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Colourful and with greater elasticity than cement parging, acrylic parging increases adhesion, flexibility and tightness. Yellow, red or blue, this product offers a large palette of colours and textures while being ideally used in places exposed to bad weather and de-icing salt. On the other hand, these benefits do come at a cost. Acrylic parging is more expensive than cement parging.

Stratification is done in two layers:

• The base layer is a liquid mixture of acrylic polymer and Portland cement to which a fibreglass mesh is added so that this layer is shock-resistant to help avoid cracking.

• The top coat is composed of polymers which contain colours. As soon as the coating is dry, it will retain its clean appearance for many years.

Instead of the first layer, you can opt for lightweight concrete panels consisting of a concrete core, both sides of which are covered with a fibreglass mesh. These panels are highly resistant to moisture, they do not rot or disintegrate, and they do not swell when exposed to water. In addition, they are shock-resistant and extremely durable. The second layer remains indispensable.

Crushed aggregate 

Eco-friendly, colourful and textured, crushed aggregate parging combines the advantages of acrylic parging with those of cement parging. Lightweight, supple, resistant and durable, this type of parging can resemble granite and is applied with a trowel just like cement.

What type of parging is best for straw bale foundations?

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Cement and Lime Parging

Cement and lime parging uses raw materials (lime, cement and mortar sand) that are readily available on the market and don’t require much handling. The application can be done in two layers, the first of which must be striated to apply the second. This type of parging resists erosion and protects against rainfall, in addition to establishing a thermal mass.

Mixing cement with lime allows for offsetting any inconveniences that may occur when applying cement to lime by hand with a trowel, which can result in surface warping, fissures, and lack of adherence. In fact, mixing cement and lime gives a much smoother material.

Albeit its numerous advantages, cement and lime parging aren’t very resistant to water vapour, which can lead to a water build-up within the walls. To remedy the situation, it's best to use latex glue.

Clay Parging

Clay parging uses inexpensive raw materials, as well as being easily repaired (simply wet the parging and add water). Unlike cement, clay isn't corrosive, adheres very well on the surface of a bale of straw and withstands water vapour. The hydrophilic property of the clay allows this material to capture excess moisture. As for cement, this type of parging can form a thermal mass.

On the other hand, clay parging requires a lot of work since it must be extracted about 1 year prior for it to be dry when used, on top of the four layers required (1 layer of argyle, 2 layers of parging, and 1 layer of lime finish coating). Note that the finishing layer is a must to ensure that the parging is adequately protected against rainfall and snow. Furthermore, it’s not especially resistant to erosion, and can’t be applied near ground level due to its absorption capacities and the risks of capillarity.