Plastic vs. Metal Insulation Fixings: Which One Should You Choose?
Both plastic and metal insulation fixings can hold insulation boards securely. But they perform very differently when it comes to heat loss, fire safety, and load capacity. This guide explains the key differences so you can choose the right fixing for your project.
What Are Insulation Fixings?
Insulation fixings are mechanical fasteners. They hold insulation boards firmly against a wall, ceiling, or other surface.
Without fixings, insulation boards can slide, bulge, or fall away from the substrate. This is a serious problem in external wall insulation (EWI) systems where boards must stay flat and secure for decades.
Most insulation fixings have two parts. There is a plug or anchor that goes into the substrate, and a pin or nail that goes through the insulation board and locks into the plug. A large plastic disc or cap sits over the board to spread the load and prevent the fixing from pulling through.
Fixings are available in two main materials: plastic and metal. Each has strengths and weaknesses that make it better or worse suited to different jobs.
Plastic Insulation Fixings Explained
Plastic insulation fixings are made from nylon or polypropylene. They are lightweight. They do not rust. They are the most commonly used fixing type in UK external wall insulation systems.
The most common design is the plastic hammer fixing. This has a nylon plug and a plastic or nylon pin that is hammered in to expand the plug inside the substrate. Some types have a screw-in pin for extra grip.
The large plastic disc on the surface of the insulation board is an important part of the fixing. It spreads the load across a wider area. This stops the fixing from pulling through the insulation board under wind load or the weight of the render.
What Are Plastic Fixings Good For?
- Fixing EPS, XPS, and mineral wool boards to brick, block, and concrete substrates
- External wall insulation systems on buildings up to 18 metres
- Projects where minimising thermal bridging is a priority
- Coastal or damp environments where metal corrosion is a concern
- Most standard residential and light commercial retrofit insulation work
Plastic fixings have a much lower thermal conductivity than steel. This means they create far less of a thermal bridge through the insulation layer. For most residential EWI projects, this makes them the preferred choice where energy efficiency is important.
Metal Insulation Fixings Explained
Metal insulation fixings use a steel or stainless steel pin inside a plastic body or sleeve. Some are fully metal. They are stronger than all-plastic fixings and can handle heavier loads.
The most common type in UK construction is sometimes called a metal pin hammer fixing or a steel-pin disc anchor. It has a plastic plug body with a metal pin hammered into it. The metal pin expands the plug for a secure mechanical fix.
Fully metal fixings, such as long stainless steel screws with insulation washers, are used for fixing insulation boards to light gauge steel or timber framing. These are commonly used in HVAC duct insulation and specialist construction applications.
What Are Metal Fixings Good For?
- Heavy insulation boards that need greater pull-out resistance
- Buildings above 18 metres where higher structural performance is required
- Substrates such as dense concrete or engineering brick where a stronger fix is needed
- Applications requiring Class A1 non-combustible fastener classification
- Fixing insulation to light gauge steel or metal framing
Metal conducts heat far better than plastic. A metal pin fixing creates a thermal bridge through your insulation layer. This can reduce the effective U-value of your wall. Always check whether your project requires a specific chi-value (point thermal transmittance) before choosing metal fixings.
Plastic vs Metal Insulation Fixings: Side-by-Side
Here is a direct comparison of the two fixing types across the factors that matter most.
| Feature | Plastic Fixings | Metal Pin Fixings |
|---|---|---|
| Material | Nylon or polypropylene | Steel or stainless steel pin with plastic body |
| Thermal bridging | Low. Minimal heat loss through the fixing | Higher. Metal conducts heat and creates a cold bridge |
| Load capacity | Good for standard residential insulation work | Higher pull-out resistance for heavier applications |
| Fire classification | Not Class A1. May not suit all fire regulations above certain building heights | Class A1 non-combustible. Complies with BS EN 13501 |
| Corrosion resistance | Excellent. Will not rust in damp or coastal conditions | Depends on material. Stainless steel is suitable. Standard steel requires coating |
| Typical embedment depth | Minimum 25mm into structural substrate | Minimum 35mm into structural substrate |
| Best application | EPS, XPS, mineral wool on masonry up to 18m | Heavy boards, high-rise, steel frame, HVAC ductwork |
| Cost | Generally lower cost per fixing | Generally higher cost per fixing |
Many modern metal-bodied fixings use a plastic collar around the pin to reduce thermal bridging. These are often called thermally broken fixings. They offer better structural performance than all-plastic fixings while still limiting the cold bridge effect. Always check the product datasheet to confirm the chi-value.
Thermal Bridging: The Key Difference
Thermal bridging is one of the most important factors when choosing between plastic and metal insulation fixings.
A thermal bridge is a path where heat travels more easily through a building element. When a metal fixing penetrates your insulation layer, it creates a point where heat can escape. These are called point thermal bridges.
Each fixing creates a small but measurable amount of additional heat loss. This is measured as a chi-value (the Greek letter X). Multiply the chi-value by the number of fixings per square metre and you get the total extra heat loss from the fixings in your wall.
Plastic fixings have a much lower thermal conductivity than steel. This means their chi-value is far smaller. Over an entire building, this difference can have a meaningful effect on the wall's U-value and the energy rating of the building.
UK Approved Document L requires buildings to meet specific U-value targets. Point thermal bridges from fixings must be accounted for in the U-value calculation. Choosing plastic fixings with a low chi-value can help you meet these targets without adding extra insulation thickness.
Always check the system BBA certificate or ETA for your insulation fixing to confirm its thermal performance data.
What About Thermally Broken Metal Fixings?
Some metal fixings are designed to reduce thermal bridging. They use a plastic sleeve or insulating collar around the metal pin. This breaks the direct metal-to-metal contact path through the insulation.
These thermally broken fixings perform better than standard metal fixings. They are a good option when you need the higher load capacity of a metal pin fixing but still want to limit heat loss.
However, even thermally broken metal fixings typically have a higher chi-value than a good quality all-plastic fixing. For projects where thermal performance is the priority, all-plastic fixings remain the standard recommendation.
Fire Resistance
Fire performance is a major consideration for insulation fixings in multi-storey buildings.
Metal insulation fixings comply with BS EN 13501 as a Class A1 non-combustible fastener. This means they do not burn and do not contribute to the spread of fire. This is important on taller buildings where fire regulations are stricter.
Plastic fixings are not Class A1. They can melt or burn at high temperatures. This does not mean they are unsuitable for most projects. For standard residential buildings below 11 metres, plastic fixings are widely used and accepted.
For buildings above 11 metres in England, the Building Safety Act 2022 and associated guidance has tightened the rules around combustible materials in external wall systems. Always check the specific requirements for your building type and height before specifying fixings.
Always check the fire performance requirements for your specific building. For residential buildings above 18 metres, the guidance around combustible materials in EWI systems is strict. Seek advice from a qualified fire engineer or your insulation system supplier before specifying fixings.
When to Use Plastic vs Metal Insulation Fixings
Here is a practical guide to help you choose the right fixing type for common UK insulation applications.
| Application | Best Choice | Why |
|---|---|---|
| EPS boards on brick or block walls, up to 18m | Plastic hammer fixing | Low thermal bridging. Good pull-out resistance in masonry. Corrosion resistant. |
| Mineral wool boards on dense concrete | Plastic or thermally broken metal | Check pull-out test results. Plastic is preferred for thermal performance. Metal gives extra security on hard substrates. |
| EWI on buildings above 18m | Metal pin fixing (Class A1) | Fire regulations require non-combustible fasteners on taller buildings. Always verify with your system supplier. |
| Insulation on light gauge steel framing | Stainless steel screw with insulation washer | Plastic plugs cannot grip thin steel sections. Self-drilling stainless steel screws are the correct choice here. |
| HVAC ductwork insulation | Self-adhesive stick pins or metal hangers | Stick pins bond directly to the metal duct surface. Metal hangers support heavier insulation on larger ducts. |
| Coastal or high-humidity environments | Plastic or stainless steel | Standard steel corrodes in damp or salt-laden air. Plastic or stainless steel fixings are both suitable in these conditions. |
| Hollow or lightweight block substrate | Specialist expanding anchor or longer fixing | Standard fixings may not give adequate pull-out resistance in hollow blocks. A pull-out test is strongly recommended before specifying. |
The substrate determines how well a fixing holds. Dense solid brick and concrete are predictable. Hollow blocks, lightweight blocks, and aerated concrete are not. A pre-installation pull-out test tells you whether your chosen fixing will achieve the required resistance. Do not skip this step.
Browse JALFT insulation fixings for masonry, block, and steel substrates.
Common Mistakes to Avoid
Most problems with insulation fixings come from choosing the wrong type for the application, or from poor installation. Here are the most common mistakes.
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Using metal fixings where thermal bridging matters Every metal pin fixing creates a small cold bridge. On a large project with many fixings per square metre, this can meaningfully reduce your wall's thermal performance. If energy efficiency is your goal, use plastic fixings or thermally broken alternatives.
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Not checking fire performance for taller buildings Plastic fixings are not Class A1. Using them on buildings above 11 metres without checking fire guidance first is a serious risk. Always verify fire requirements for your building type and height.
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Using standard steel fixings outdoors without corrosion protection Standard carbon steel fixings will corrode when exposed to moisture. In external wall insulation systems, this causes staining and weakens the fix over time. Use plastic or stainless steel fixings for all external applications.
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Under-specifying the embedment depth Plastic pin fixings typically need at least 25mm into the structural substrate. Metal pin fixings typically need at least 35mm. Embedment into the insulation board or render layer does not count. Check the product ETA or BBA certificate for confirmed minimum depths.
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Over-driving the fixing pin Hammering a fixing pin too far crushes the insulation board around the disc. This weakens the load-bearing area and can crack the render surface above. Drive the pin flush with the disc. No more than that.
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Using too few fixings per board Insulation boards in EWI systems face significant wind loads, especially at the edges and corners of a building. Under-fixing is a common cause of system failure. Always follow the fixing pattern specified by the system designer or BBA certificate for your project.
Frequently Asked Questions
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