EWI Thickness Guide: What Insulation Depth Do You Need?

There’s something oddly satisfying about understanding how things really work. When it comes to insulation, one of the most common questions we’re asked is how thick insulation should be and whether going thicker actually makes a difference.

 

At EWI Store, we stock a wide range of expanded polystyrene (EPS) and mineral wool insulation thicknesses and we can also supply bespoke thicknesses manufactured specifically for your project. The right choice depends on whether you’re working on a retrofit or a new build, the material you’re using and the U-value you need to achieve to meet building regulations.

 

In this guide, we’ll explain which insulation thicknesses are most popular, why certain sizes are used and when it makes sense to go beyond standard stock sizes.

When people talk about “standard” insulation thicknesses, they’re often referring to different applications. Insulation products can be manufactured in thicknesses ranging from approximately 20mm to 200mm or more. However, for external wall insulation projects, the most commonly used and stocked sizes typically fall between 90mm and 150mm.

 

Thinner boards are typically associated with internal wall insulation or specialist applications. Thicker boards are specified when higher thermal performance is required to meet modern U-value requirements. This is why 90mm and 100mm EPS are among the most popular choices for retrofit projects with thicker insulation often used on new builds.

 

Here at EWI Store, we carry both mineral wool and expanded polystyrene (EPS) insulation in stock, with mineral wool available up to 120mm and EPS up to 100mm as standard. We also carry 90mm and 100mm EPS in store as a standard product, and they are our most popular sizes. This is because, on a typical retrofit project involving an existing 9-inch brick property, 90mm of EPS insulation is enough to exceed the 0.3 W/m²K U-value required by building regulations. To achieve the same performance using mineral wool, around 110mm is usually needed.

If you have a new build project where you are using EPS to get a single skin wall up to the required U-value, then you will need more than 90mm. This is because new builds have a higher standard of insulation thickness required. This table gives an idea of the U-values you can achieve with graphite EPS on a 220mm brick wall:

If you want to achieve the same performance with Rockwool, we would recommend going for 200mm insulation. This thickness on the Thermalite block or similar should give you a U-value of 0.18. As with all these calculations, make sure you get a U-value analysis tailored to your project before you order your materials, as you don’t want to be told you are a few mm off of being compliant once the job is done!

You may be surprised to know that the cost of cutting these extra-thick boards is less than you would expect. Depending on the amount you are going to order, we can provide you with your EPS at a similar cost in terms of volume, compared to other stock sizes. If you would like a quote for a particular thickness of board, just give us a call and we will be happy to help.

 

We also get asked for mechanical fixings for these large boards. For most oversized boards, we carry the appropriate fixings or can get them ordered. For extremely thick insulation over 200mm, you may need to recess your fixings into the insulation to ensure they are long enough to bite into the brick below. Ask us if you are not sure about the right fixings for your job.

 

Remember, if you're not sure about anything, just call EWI Store and we can talk you through all of your requirements.

 

Frequently Asked Questions About Insulation Thickness

What thickness of insulation meets UK building regulations?

Is thicker insulation always better?

Can I order insulation thicker than standard stock sizes?

Do thicker insulation boards need special fixings?

Should I get a U-value calculation before ordering insulation?