Sudden deterioration of concrete block foundations explained

Why does the concrete block foundation of a 50 or 60 year old building, which was in perfect condition a few years ago, suddenly appear to be weakening, deformed or showing significant damage? Surprisingly, the source of such a problem may be a homeowner’s idea to reduce their energy consumption by insulating the foundation. Interior renovations, particularly in the area of insulation, are often the cause of damages that may not be noticed until years later. Frédérick Beaucage, civil engineer at CEP Forensic and Regional Manager of the Ottawa office, answers a few questions about concrete block foundations, which are quite common in older buildings and cottages. He explains why sometimes it’s best to just “leave old foundations alone”.

  1. First, can you tell us why some foundations are built with concrete blocks while others are built with poured concrete, and what are the major differences between the two?

One of the major differences is the installation method of the foundation walls, which is reflected in the cost. For instance, when it comes to cottages, concrete blocks are easier to transport to a confined area than having a concrete truck mixer come on site. In addition, there is no formwork to set up with concrete blocks.

For the majority of handymen, it is easier to lay a concrete block foundation than it is to build sound formworks for poured concrete. Moreover, when you consider older foundation walls, there was also a cost issue, as labour was cheaper than today, while poured concrete was more expensive.

The other major difference between the two types of walls, concerns the structural resistance of the foundation wall in terms of lateral thrust (meaning the inward pressure of the soil against the foundation). Concrete blocks, with their many joints, are more sensitive to inward movement than cast-in-place concrete, which can be seen as one big block. In terms of vertical strength, there is no real difference between both foundations, they both do a very good job of supporting the building.

  1. What types of damages associated with concrete block foundations do you encounter most often in your work?

If you eliminate the damages related to soil movement that can be observed on both types of foundations, we often find inwardly displaced blocks with concrete block walls. This type of damage can be caused by ground pressure, ground freezing, hydrostatic pressure or a combination of all three. It is also important to note that this type of damage is only present when there is a basement or crawl space deep enough for there to be a pressure difference between the exterior and interior. What I mean is, when there is a high enough ground level on the outside, in comparison to the basement floor, there is a pressure difference from the ground that pushes the wall inward. From the inside, there are no restrictions to the soil pressure, since there is only an empty space for people to walk. Over time, the joints between the blocks break and the blocks shift slowly inward. 

  1. Can you explain why doing interior renovations, such as insulating concrete block foundation walls, may not be a good idea?

Foundation walls of this type, especially older ones, need to “breathe”. There needs to be a place where the moisture can escape. Moisture from the outside will pass through the blocks due to the porous nature of the block material. From the inside, the blocks use the interior heat to dry and evaporate the moisture contained within them. This process will increase the humidity level in the basement, but that’s a whole other topic!

So, insulating walls with urethane, extruded or expanded polystyrene, or fibreglass insulation with a vapour barrier, limits the amount of heat reaching the wall and traps moisture in the blocks. In addition, insulation also limits the heat loss that can be directed through the ground to the outside. The escaping heat was preventing or limiting the freezing of the soil around the foundation since construction. If there is no frost outside, there is usually no frost pressure, and the blocks can therefore survive for a very long time.

  1. Are damages caused by adding insulation worse in cold, dry winters or in mild, rainy winters?

A cold, dry winter is often less harsh on the foundation, but this depends on the type of soil along the foundation. If you have frost-susceptible soil and poor drainage, most of the problems observed are related to soil expansion occurring when the heat loss from the interior has been eliminated. With gravel or sand backfill and a good foundation drain, wall insulation will usually not cause problems with a cold continuous winter and a good snow cover.

However, in a mild winter, and with winter rains, the water can stay on the surface longer because of the frozen ground and the reduced percolation. And, if there is a rapid drop in temperature after the rain, the water will freeze and push on the foundation walls. 

  1. In conclusion, is there a way to insulate a concrete block foundation without fear of causing its long-term degradation?

It is possible, but it is expensive! First, you need to limit the amount of water on the outside along the foundation walls. Good drainage is therefore necessary, as well as the waterproofing of the wall from the outside that will limit the amount of water that can penetrate the blocks.

It is also necessary to have non-frost-susceptible soil along the foundation (the backfill) and a good foundation drain. Afterwards, it is possible to insulate from the inside, without the risk of long-term damages, since no more water can freeze during the winter. If there is no water in the soil, there is no expansion on the outer side of the foundation walls, so no lateral thrust on the blocks.

Therefore, to save the planet and your wallet, you will have to invest at the start to save on heating costs now and on new foundations further down the road. This is something to think about!

For any additional questions on this matter or to discuss a claim with one of our experts, contact our Civil & Structural Engineering team.

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