140mm Rockwool Flexi Insulation Slab 1.2m x 400mm
Delivery
We'll get your order to you fast, keeping you in the loop every step of the way.
Most stocked items reach mainland UK in just 3 working days, and many arrive sooner.
On orders over £500 ex VAT. Under that? Just £50 ex VAT, or only £35 ex VAT if you're within 35 miles of our Rotherham depot.
After ordering to confirm your delivery day. No guessing, no chasing us.
On highlighted products. Order before 12pm and we'll have it with you tomorrow.
Available on selected products. Your options and pricing are calculated automatically at checkout.
We deliver nationwide, with Moffett mechanical offload taking care of the heavy lifting.
Sometimes possible, just contact us to find out.
Deliveries are made kerbside, Monday to Friday.
Dates are an estimate, not a guarantee. We deliver to UK mainland addresses. Please contact us before ordering if your postcode starts KW, IV, AB, PH, PA, DD, KY, BT, TR, PL, EX, TQ, HS or ZE, as these may cost more, take longer, or fall outside our delivery area.
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Product Description
Product Description
ROCKWOOL Flexi is a stone wool insulation slab used across a wide range of domestic and commercial construction projects. It goes between studs in partitions and external walls, between joists in floors, and between rafters in roofs — and it stays put without slumping, cutting or fixing. The slab is made from natural volcanic rock spun into stone wool fibres, and it is manufactured with a flexible edge along one side that compresses on installation and then expands to grip the frame tightly from within.
This is the 140mm slab in the 400mm width — the primary specification choice for timber frame external walls, and the point in the Flexi range where the lambda value improves from 0.038 W/mK to 0.035 W/mK. That improvement matters in practice: at 140mm with a 140mm stud and a standard breather membrane, the build-up achieves a U-value of 0.28 W/m²K.
The slab is non-combustible to Euroclass A1 and is suitable for cold frame, warm frame and hybrid frame constructions. It is also used in suspended floor applications, where 140mm Flexi achieves a U-value of 0.22 W/m²K across all standard P/A ratios.
Benefits
- Primary specification for timber frame external wall insulation to meet Part L
- Improved lambda of 0.035 W/mK at 140mm and above — better thermal performance per millimetre
- U-values from 0.28 down to 0.20 W/m²K achievable depending on membrane and stud selection
- Flexible Flexi Edge for a tight friction fit — no cutting to width required
- Will not slump, even if timber studs shrink after installation
- Euroclass A1 non-combustible — the highest possible reaction-to-fire classification
- Also suitable for suspended floor applications — achieves 0.22 W/m²K at all standard P/A ratios
- Stone wool is vapour permeable, reducing the risk of condensation and mould
Good to Know
- Size: 1200 x 400 x 140mm; weight: refer to ROCKWOOL datasheet for pack details
- Thermal conductivity: 0.035 W/mK at 140mm and above, to BS EN 13162:2012 + A1:2015
- Reaction to fire: Euroclass A1 to EN 13501-1
- U-values range from 0.28 to 0.20 W/m²K depending on stud depth, breather membrane and VCL specification
- Install in accordance with current ROCKWOOL guidelines — visit rockwool.com/uk
- Always refer to the ROCKWOOL Flexi technical datasheet before installation
Technical Datasheets
Technical Datasheets
Product FAQs
Product FAQs
What is ROCKWOOL Flexi used for at 140mm?
What lambda value does ROCKWOOL Flexi achieve at 140mm?
What U-values can be achieved with ROCKWOOL Flexi 140mm in a timber frame wall?
Is ROCKWOOL Flexi 140mm suitable for a warm frame or hybrid frame wall?
Does the 140mm Flexi need to be cut to fit between studs?
Is ROCKWOOL Flexi 140mm non-combustible?
Can the 140mm Flexi be used in a suspended floor?
What size does the 140mm Flexi come in and how is it packed?
Customer Information
Customer Information
Free delivery on orders over £500 ex-VAT. All orders below £500 (ex-VAT) are subject to a £50 delivery charge.
Ordering & Delivery
How do I place an order with Insulation UK?
You can place an order for delivery anytime online. If you want to collect from our distribution centre in South Yorkshire then please email us your requirements at sales@insulationuk.co.uk
and we will send a payment link and collection instructions. If you can’t see what you are looking for or need more information, then call our team on 03003 034 578 and we can sort your order from there.
How do I know what’s happening with my order and when I will receive it?
When you have placed your order, you will then receive an email confirmation, giving you an order reference number and asking you to double check everything is correct on your order such as delivery address, quantities etc. If there are any amendments to be made, then please reply to your confirmation email with any details.
Once the team has processed the order you will receive an email with a BY delivery date, this is the latest date you will receive your delivery.
Your final email will be a 24/48hr notification, on the day of the delivery if you need a more accurate ETA then contact the office on 03003 034 578.
Do you offer next-day delivery?
Yes, we do on certain lines, you will see our next day delivery products marked as NEXT DAY. Order before 12pm for next day delivery (charges will apply)
How much does delivery cost and how long will it take?
Standard delivery is £50 + VAT for all orders below £500 excluding VAT, above £500 excluding VAT delivery is free. Bespoke deliveries can be made but will incur charges. Each product page gives you an estimate delivery by date. For more accurate information call the team on 03003 034 578
Can I choose a specific delivery date or time slot?
We will endeavour to try and accommodate a preferred date, if you need a delivery on a certain day, then please call us on 03003 034 578.
Do you deliver nationwide?
We deliver across most of the UK; however, some postcodes are restricted due to their location. Check out our delivery map for more information. If you find that you are in area we don’t deliver to, we have access to a wide range of haulage options, please call the team on 03003 034 578 to discuss your requirements further.
Will the driver help unload the insulation products?
Most of our deliveries are made with vehicles with mechanical off-load (known as a moffett or fork-lift), the driver will leave them in a safe place of your choice. They will help as much as they are allowed but are not insured to bring anything into a property.
What happens if my order arrives damaged or incomplete?
In the unlikely event your order arrives damaged, then please contact within 48 hours at sales@insulationuk.co.uk with any images of the damage and the team will be in touch. If there is anything missing, then notify the team within 48 hours on 03003 034 578
What if my order doesn’t arrive and I have paid for trades to be on site, can I claim anything towards the costs I have incurred?
We advise not to have trades booked in and around the day of your delivery. Things can happen beyond our control, and your delivery may be delayed or abandoned. We cannot be held liable for any consequential loss such as tradesman wages etc.
Product Advice & Support
How do I know which insulation product is right for my project?
With such a variety of insulation products, we fully understand making sure you have the right one can seem daunting. Our team have over 50 years of experience in the construction industry and there isn’t a question they haven’t been asked! Speak to our technical team on 03003 034 578 and they will be happy to offer some help and guidance.
Do you offer technical advice or support before I buy?
Yes we do! Just call 03003 034 578 and ask for our technical team and we will be happy to support you.
Product Information & Suitability
Product descriptions on this website are provided as general guidance only and are not a substitute for the manufacturer’s Technical Data Sheet (TDS), fire classification information, and the project’s design/specification requirements. Suitability depends on the full build-up and intended use (including any requirement for encapsulation, coverings, and compliance with Building Regulations and the project fire strategy).
It is the customer’s responsibility to confirm, before ordering, that the product specification and performance are suitable for the intended application by reviewing the relevant TDS and taking professional advice where required. If you are unsure, contact us before purchase and provide your intended build-up so we can confirm what information we can support.
Nothing in this notice limits or excludes your statutory rights
Can I speak to someone to confirm product specifications?
Of course you can! There is also a range of data sheets and guides available on the product pages under Technical Information.
Are your products suitable for both DIY and trade customers?
Yes, they are. If you are new to DIY projects involving insulation,
then we advise to always do your research. We have a handy guides section which may help.
Do you offer fire-rated or eco-friendly insulation options?
Yes, we do. Fire ratings are indicated in the specification table on the product page, and our sustainable range is continually growing.
Pricing & Payments
Do you offer trade or bulk-buy discounts?
We always aim to offer the best price online; however we do offer project pricing for larger orders, speak to our sales team with your requirements on 03003 034 578.
What payment methods do you accept?
We accept all major cards including American Express, as well as Apple Pay, Google Pay and PayPal
Can I set up a trade account with credit terms?
We do offer trade accounts, however there are certain criteria we ask for, and any credit account application is subject to credit checks. Call the team on 03003 034 578 to discuss any credit applications.
Do your prices include VAT?
The website has a toggle at the top right of the page so the prices can be displayed including and excluding VAT. VAT, where applicable, is charged at checkout.
Returns & Cancellations
Can I return unwanted insulation products?
Contact the team via email sales@insulationuk.co.uk with the items you wish to return and any other information. Please note if we you need us to collect any unwanted items,
a collection fee will apply.
How do I arrange a return or exchange?
Contact the team via email sales@insulationuk.co.uk or check out our refund policy.
Are there any restocking fees?
Restocking charges will apply in certain instances, please contact the team via email at sales@insulationuk.co.uk check out our refund policy.
What if I’ve ordered incorrectly or need to cancel my order?
Contact the team via email as soon as possible. The best way to get in contact is to reply to any of the emails we have sent you, they will come straight through to the team and will have any relevant order information already on there.
Installation & Compliance
Do you offer installation services or just supply materials?
We do not offer any installation services; we are a supplier of insulation and building materials.
Can I get a datasheet or certificate of performance for your products?
Yes, data sheets can be found in the Technical Information drop down on the product page.
How should I store insulation products before installation?
There are a variety of insulation products each with their own storage instructions. Generally, please keep them off the ground, in a dry area, away from the elements.
Sustainability & Quality
Are your insulation products environmentally friendly?
This depends on the individual products, data sheets are available,
and any environmental benefits are noted on the product pages.
Do your products help reduce energy bills?
The correct insulation properly installed can reduce heat loss from a building and therefore help reduce energy costs.
Are your products made in the UK or imported?
With such a wide variety of products some are made in the UK and some are imported.
Customer Service & Support
How can I contact your support team?
By phone on 03003 034 578, email sales@insulationuk.co.uk or via our
webchat online
What are your opening hours?
Monday to Friday 8am to 5pm for sales and technical support, orders can be placed on line 24/7
How quickly can I expect a reply to my enquiry?
We aim to respond to our customers as quickly as possible.
Technical Details
- Brand:
- Rockwool
- Material:
- Mineral wool
- Manufacturers Part Number:
- 123329
- Length:
- 1.2 m
- Width:
- 400 mm
- Thickness:
- 140 mm
- Unit Coverage:
- 0.48 m²
- Qty Per Pack:
- 4 Items
- Sqm Per Pack:
- 1.92 m²
- Applications:
- Walls, Ceilings, Floors, Roofs
Thermal performance helps show how effectively the product reduces heat transfer.
Thermal conductivity measures how readily heat passes through the material, so lower figures generally indicate better insulating performance.
R Value shows the material’s thermal resistance at that thickness, so higher figures generally mean stronger insulation performance.
These figures relate to the insulation product itself and should be considered alongside the full wall, roof or floor build-up when checking project requirements.
- Thermal Conductivity:
- 0.035 W/mK
Acoustic performance helps show how effectively a product or build-up can reduce sound transfer.
Impact performance relates to contact noise, such as footsteps or dropped objects. Where shown as Ln,w, lower dB figures generally mean better impact sound performance.
Airborne performance relates to sound travelling through the air, such as voices, music or traffic. Where shown as Rw, higher dB figures generally mean better sound reduction.
These figures should be checked against the product datasheet and considered as part of the full floor or ceiling build-up.
- Airborne Performance:
- 42 dB
Euroclass reaction to fire is the European EN 13501-1 classification used to show how a construction product behaves when exposed to fire.
The main scale runs from A1 and A2 at the highest end, through B, C, D and E, down to F.
A1 is non-combustible and A2 is limited combustibility, while lower classes indicate a greater contribution to fire.
Euroclass F is the lowest rating on this scale and indicates the weakest reaction-to-fire performance.
This is different from fire resistance, which relates to how long a complete wall, floor or roof build-up can resist fire.
- Reaction to Fire:
- Euroclass A1
