Frost blankets and hessian sheets: How do they work?

Anyone who lives in the UK knows that the British winter is no friend to a construction schedule. When the temperature drops, the clock starts ticking on your concrete. If you’ve ever seen a fresh concrete pour ruined by a sudden overnight frost, turning a solid slab into a crumbly mess, you’ll know exactly why site protection is a necessity.

So the question arises, how do you protect your construction site and fresh concrete pours in the harsh winter conditions? Two of the most effective methods of protection are frost blankets and hessian sheets.

In this blog, we’ll discuss everything that you need to know about these materials, looking at what they are, how they work, and how to use them to ensure your project stands the test of time and survives the frosty temperatures. 

What is a frost blanket?

Frost blankets are essentially a high-performance thermal shield designed specifically for the construction industry. Unlike a standard tarp or a thin plastic sheet, a frost blanket is designed to provide high thermal resistance against extreme heat or cold. 

In the trade, you might also hear them called "curing blankets" or "thermal blankets." Their primary purpose is to insulate freshly poured concrete, mortar, or screed, protecting it from freezing temperatures that can compromise its structural integrity.

What are frost blankets made of?

To understand why they are so effective, we need to look at how they are made and what materials are used. Most professional-grade blankets are made from closed-cell expanded polyethene foam. There are two main parts to these blankets, which are the foam core and outer layer.

The foam core is made up of a closed-cell structure that contains millions of tiny trapped air bubbles. Because air is a poor conductor of heat, these bubbles create a strong barrier that prevents heat from escaping.

The outer layer is cased in a heavy-duty, UV-stabilised polyethene film (often silver or white). This layer makes the blanket waterproof, tear-resistant, and reusable across multiple construction jobs.

By combining these materials, you get a product that is lightweight enough to handle easily on-site but tough enough to survive the rigours of a busy construction environment.

How do Frost blankets work?

Now that we know what they are and what they are made of, we can look into how they actually work. Simply put, frost blankets achieve three main objectives, which are heat retention, exothermic management, and moisture control. Let’s break down these three objectives to gain a clear understanding of how frost blankets work. 

Heat retention

By trapping the heat through hydration, concrete cures through a chemical reaction called hydration. This reaction is exothermic, meaning it releases heat as cement reacts with water. In sub-zero temperatures, that heat is quickly sucked out by the cold air. A frost blanket acts like a thermos flask, trapping that heat and keeping the concrete within its sweet spot (usually above 5°C). This allows the curing process to continue even when the air around it is freezing. 

Exothermic management 

When water freezes, it expands by roughly 9%. If the water inside your fresh concrete freezes before it has reached sufficient strength, that expansion will literally rip the internal structure of the material apart. This leads to issues such as the surface flaking off, reduced strength and dusting, which is when the surface remains soft and produces constant dust. The frost blanket prevents this by ensuring the temperature of the material never hits the freezing point.

Moisture control

Many people assume frost blankets are only for warmth and temperature protection, but they also play a vital role in the setting of concrete by controlling moisture. In cold, dry winter air, moisture can evaporate from the surface of fresh concrete very quickly. This leads to desiccation, which is where the top layer doesn't have enough water left to complete the chemical hydration process. This results in a dusty-looking finish. A frost blanket acts as a vapour barrier, locking that moisture in, which prevents this dusty finish.

Alongside keeping the required moisture in, frost blankets also help with dealing with excessive rain. Rain can wash out the cement paste from the surface of a fresh pour, weakening the mix ratio. Because frost blankets feature a waterproof polyethylene skin, they keep the external rainwater out while locking internal moisture in.

By maintaining a consistent moisture level across the entire surface, the blanket makes sure the concrete gains strength at a steady rate. This reduces internal stresses and the likelihood of the slab curling or warping at the edges.

How to use a frost blanket for construction projects

Knowing how to use a frost blanket correctly is the difference between a job well done and a costly insurance claim. The first thing to note is that timing is very important. Don’t wait for the frost to arrive before you start rolling out the frost blankets. Monitor the weather closely, and if the forecast predicts temperatures of 5°C or lower, you should have your blankets ready to go.

Here is a quick look at the actual process of applying frost blankets when the time comes. 

The frost blanket application process

1. Wait for the concrete to set: You shouldn't throw a heavy blanket onto wet concrete immediately, as it will diminish the finish. Wait until the concrete has reached its initial set (it should be firm to the touch but still a bit damp)
2. Allow the blankets to overlap: Never lay blankets edge-to-edge. To prevent any cold air from seeping through the gaps, you should overlap the seams by at least 300mm.
3. Secure the edges: If the wind picks up, it can lift the blankets and cause some issues with the setting. Use timber, bricks, or sandbags to weight down the edges and the overlaps.
4. Double up if needed: In extreme cold conditions (below -5°C), consider using a double layer of blankets or combining them with hessian sheets.

What are hessian sheets?

While frost blankets are more commonly used, hessian sheets (or burlap) remain a staple on UK sites. Hessian is a 100% natural, biodegradable fabric woven from the skin of the jute plant. For construction projects, a specific grade of heavy-duty hessian is used to provide a breathable protective layer for concrete work.

Unlike synthetic plastic covers, hessian is open-weave. This means it allows for a degree of airflow while still providing a physical barrier against the elements.

How to use hessian sheets

To get the most out of its natural properties, you need to follow a few site-standard rules when working with hessian sheets. It’s not as simple as just throwing a sheet on a wall or a concrete project. Here is a quick guide on how to effectively use hessian sheets.

Pre-soaking

In many scenarios, particularly during the curing phase of concrete, you don’t want the hessian to be bone dry. Dry hessian can actually take the moisture away from your concrete, which is the opposite of what you want.

To avoid this, soak the hessian in a bucket of water before application. This allows it to act as a damp reservoir, slowly releasing moisture back onto the surface for a perfect, crack-free cure.

Making use of double layers

For frost protection, a single thin layer of hessian won't do much when the temperature hits -2°C. To create an effective thermal barrier, you can fold the hessian over itself to create multiple layers of trapped air.

If you’re working on a vertical surface (like a newly built brick wall), drape the Hessian over the top and secure it with scaffold ties or boards to ensure it doesn't blow away and leave the top courses exposed.

Combining with polythene

Hessian is excellent at holding moisture, but it’s not waterproof. If it gets soaked by heavy rain and then the temperature drops, you end up with a sheet that could be frozen solid draped over your bricks, which is a recipe for disaster.

One way to combat this is to place your hessian layer down first, then cover it with a layer of heavy-duty polythene. This keeps the Hessian damp (but not soaked by rain) and adds an extra layer of windproofing.

How do hessian sheets compare to frost blankets?

Hessian works slightly differently. On its own, a single layer of hessian provides limited thermal insulation compared to a foam-core blanket. However, hessian does do a great job when it comes to:

• Moisture retention: In summer, we soak hessian in water to keep concrete cool and damp. In winter, it acts as a breathable first layer.
• Cost-effectiveness: It is generally cheaper than thermal blankets, making it ideal for smaller projects or as an extra layer of protection.

Don’t let your profits freeze this winter

In the UK construction industry, we are at the mercy of the elements, but we aren't defenceless. As we’ve discussed, the science behind how frost blankets work is all about managing the delicate balance of heat and moisture during those first critical 48 hours of a pour or a build.

Choosing between frost blankets and hessian sheets usually comes down to the specific needs of your site. Go for frost blankets when you need maximum thermal insulation for structural concrete and high-speed reusability. Or stick with hessian for traditional concrete work, and projects where breathability and moisture retention are the priorities.

Whichever route you take, the goal remains the same: protecting the structural integrity of your work and making sure that you don't have to face the nightmare of a blown slab or crumbling mortar come spring. A small investment in the right protection today saves a massive headache tomorrow.

Protect your concrete pour with CCP

Don’t wait for the first frost warning to start scrambling for supplies. At Complete Construction Products, we pride ourselves on being the one-stop shop for contractors who refuse to let the weather dictate their schedule.

We stock everything you need to keep your site moving through the coldest months, including heavy-duty frost blankets for maximum heat retention and high-quality hessian sheet rolls for curing and masonry protection. 

Not sure which grade of protection your project requires? Our technical team is on hand to offer expert advice tailored to your specific site conditions.

Contact the CCP team today, or browse our full range of frost protection products to secure your winter essentials with next-day delivery options.