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There is no evidence-based universal “best” rigid insulation washer: the right choice is the washer-and-fastener system specified for your insulation, substrate, and project loads. For a 2-inch solid-cap plastic option, TRUFAST’s Thermal-Grip CI Prong Washer is a relevant example; Rawlplug’s KCX is an integral-cap option with a manufacturer-stated thermal-bridge figure. Neither claim establishes a universal thermal winner.
What insulation washers do—and what they do not do
A washer spreads the fastener’s load over a broader area of insulation. It works as part of an attachment assembly with a compatible screw, nail, or anchor; the washer alone does not provide structural attachment or create an air or weather barrier. TRUFAST and EJOT both describe washer-and-fastener combinations for rigid insulation. TRUFAST Thermal-Grip CI Prong Washer; EJOT ejotherm STR H.
Continuous insulation is intended to form an uninterrupted layer across the building envelope and minimize thermal bridging. Fasteners nevertheless penetrate that layer, so washer design is only one part of the assembly’s thermal performance. The cited product information does not establish a comparative whole-wall result for the washers below. TRUFAST’s continuous rigid insulation overview.
Documented washer and fastener options
These examples have different designs and stated uses; they are not interchangeable by default. Dimensions identify particular products, not a universal sizing recommendation.
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#1 Best Overall
- Reliable Materials: These plastic washers are made of polypropylene material, waterproof, not easily deformed, and have good insulation, corrosion resistance, and temperature resistance, providing reliable working performance for various installation projects
- Precise Dimensions: Each plastic washers flat measures 45mm/1.77 inches in diameter with 5mm center hole, maintaining consistent thickness and weight of 1.7 grams per unit
- Adequate Quantity: The package contains 100 plastic washers, offering sufficient supply for foam board fasteners installation and multiple construction applications
- Practical Function: These thermal insulation washers are suitable for fixing and supporting rigid foam insulation foam board, foamed polystyrene board, extruded polystyrene and many types of foam board, which help to distribute pressure and prevent cracking. Plastic washers is a very practical and convenient ideal tool
- Simple Installation:These plastic washers are round with smooth edges, beautiful and easy to use, easy to fix with screws or nails, and easy to disassemble. They are suitable for foam board insulation, stabilizing gypsum ceiling, installing polystyrene board, hanging foam board, etc
| Product | Documented design and size | Stated use or compatibility | Thermal evidence |
|---|---|---|---|
| TRUFAST Thermal-Grip CI Prong Washer (TGP-2000) | 2-inch polypropylene solid-cap washer, with a stiffened center ring and flexible perimeter. | For rigid insulation and mineral wool. TRUFAST lists compatible Grip-Deck self-drilling and HiLo screw options for relevant substrates. Manufacturer product page. | No comparative thermal figure is stated on the cited page. |
| SPAX Plasti-Grip CI Prong Washer | 1-3/4-inch plastic tapered prong washer. | For rigid insulation attachment with suitable screws or nails; SPAX recommends particular screw types and a minimum head diameter. Check the product instructions for those requirements. Manufacturer product page. | No comparative thermal figure is stated on the cited page. |
| Rawlplug R-KC / KCX | Integral-cap insulation washer; KCX is listed as a 105 mm flange. | Rawlplug describes stated wood and sheet-metal uses with EPS, mineral wool, and polyurethane boards. An optional larger holding plate is available for soft insulation. R-KC guide; KCX product page. | Rawlplug states 0.001 W/K for the KCX integral-cap design; the page does not provide peer-product comparisons or enough test context for a whole-wall ranking. Manufacturer product page. |
| EJOT ejotherm STR H | Fastener assembly with galvanized-steel screw, included 60 mm washer, and EPS sealing plug. | Specified for timber and some steel substrates. EJOT indicates a 30–40 mm screw-in depth; confirm the applicable system instructions. Manufacturer product page. | No comparative thermal figure is stated on the cited page. |
| EJOT DDT 70 | 70 mm galvanized-steel washer. | Used with the DDS anchor to spread load for rigid insulation boards. Manufacturer product page. | No comparative thermal figure is stated on the cited page. |
How to choose a washer for your assembly
1. Match the insulation and its compressibility
Confirm that the manufacturer specifies the washer for your insulation type and thickness. The documented examples include rigid insulation, mineral wool, EPS, and polyurethane, but that does not mean every product is suitable for every board. For soft or compressible insulation, check whether the system calls for a larger holding plate or another measure to prevent the washer from sinking into the material.
2. Match the fastener to the substrate
Identify the actual substrate—such as wood or sheet metal—and use only the screw or anchor options documented for that substrate and insulation system. A washer’s diameter does not establish that a particular fastener will hold in the substrate. Verify screw or anchor type, embedment, and spacing against the product documentation and project requirements.
Rank #2
- Reliable Material: these washers are made of polypropylene, a plastic material, featuring with good insulation, low profile and good durability, which are easy to hold
- Size Information: the plastic washers are about 45 mm/ 1.77 inch in diameter, and the middle hole is about 5 mm; Each plastic washer is about 1.7 g
- Sufficient to Use: you will get 800 washers for screws and nails, enough for your daily replacement and use in foam board insulation and other repair
- Fixing and Supporting: these plastic washers are designed to use for fixing and supporting expanded polystyrene sheets, extruded polystyrene and many types of foam boards
- Easy to Install: the round washer is easy to fix with screw or nail, and also you can remove it easily; The extra washers can be stored for the next time to use
3. Choose the design for the application, not just the material
Plastic prong washers, solid-cap washers, integral-cap facade washers, washer-integrated screw assemblies, and galvanized-steel load-spreading washers serve different system designs. Consider how the washer seats, whether it uses a cap or plug, and which compatible fastener the manufacturer specifies. Do not substitute one type solely because its diameter appears similar.
4. Verify the project’s loads and approvals
Fastener spacing, embedment, and capacity must suit the substrate, insulation assembly, and project loads. TRUFAST’s cladding specification cautions that fastener type, embedment, and spacing need verification against substrate and project wind load. Check the full project specification, applicable approvals, and current manufacturer instructions; washer diameter alone is not proof of structural capacity or code compliance. TRUFAST cladding fastening systems specification.
Rank #3
- Enough for Use: you'll get 200 pcs of plastic washers for screws, enough for your daily replacement and for most types of medium soft foam boards and other repairs
- Size Information: plastic washer is about 60 mm in diameter, 5.2 mm in overall depth, only 4.2 mm when low profile, 5.5 mm in the middle hole, suitable for screw sizes of 4, 5 or 6 mm (5 mm recommended); Each is approx. 5.5 g, light and easy to carry
- Quality Material: these washers for screws are made of polypropylene (a plastic material), safe, sturdy, with good insulation, and can serve you for a long time; Plus, the reinforced design makes them easy fixation, rigidity and balance
- Easy Installation: the washer kit is easy to fix with screws or nails, and can also be easily disassembled for repeated use; Excess washers can be stored for next use
- Wide Range of Applications: these plastic washers are designed to hold and support expanded polystyrene panels, extruded polystyrene and many types of foam panels, bringing more convenience
What the thermal-performance claim can tell you
Rawlplug states that the KCX integral-cap design reduces thermal bridges to 0.001 W/K. The cited product page does not provide enough test context to interpret that as a whole-wall result, nor does it compare KCX with the other products here. Treat it as a product-specific manufacturer figure, not proof that KCX—or any other washer—is the best thermal performer in every assembly. Rawlplug KCX product page.
A washer cannot guarantee a particular energy result by itself. Thermal performance depends on the complete wall or roof assembly and its installation. Choose a documented system that fits the insulation, substrate, attachment needs, and project requirements rather than selecting on an isolated thermal figure.
Quick Recap
Best Value
- Enough for Use: you'll get 100 pcs of plastic washers for screws, enough for your daily replacement and for most types of medium soft foam boards and other repairs
- Size Information: plastic washer is about 60 mm in diameter, 5.2 mm in overall depth, only 4.2 mm when low profile, 5.5 mm in the middle hole, suitable for screw sizes of 4, 5 or 6 mm (5 mm recommended); Each is approx. 5.5 g, light and easy to carry
- Quality Material: these washers for screws are made of polypropylene (a plastic material), safe, sturdy, with good insulation, and can serve you for a long time; Plus, the reinforced design makes them easy fixation, rigidity and balance
- Easy Installation: the washer kit is easy to fix with screws or nails, and can also be easily disassembled for repeated use; Excess washers can be stored for next use
- Wide Range of Applications: these plastic washers are designed to hold and support expanded polystyrene panels, extruded polystyrene and many types of foam panels, bringing more convenience
Rank #4
- Reliable Material: these washers are made of polypropylene, a plastic material, featuring with good insulation, low profile and good durability, which are easy to hold
- Size Information: the plastic washers are about 45 mm/ 1.77 inch in diameter, and the middle hole is about 5 mm; Each plastic washer is about 1.7 g
- Sufficient to Use: you will get 500 washers for screws and nails, enough for your daily replacement and use in foam board insulation and other repair
- Fixing and Supporting: these plastic washers are designed to use for fixing and supporting expanded polystyrene sheets, extruded polystyrene and many types of foam boards
- Easy to Install: the round washer is easy to fix with screw or nail, and also you can remove it easily; The extra washers can be stored for the next time to use
Installation checks before fastening
- Confirm the exact washer, screw or anchor, substrate, and insulation combination in the manufacturer’s instructions.
- Check insulation thickness and compressibility, and use any required cap, plug, or holding plate.
- Follow the specified embedment and fastener spacing for the project’s loads and substrate.
- Seat the washer as directed; do not assume a larger washer or a tighter spacing is automatically suitable.
- Use the project specification and system documentation to verify approvals and structural requirements.
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
