What is Flexi Duct?

What is Flexi Duct?

In order to prevent mould, fresh air should always be introduced into your dwelling on a daily basis, as it helps to regulate temperature and reduce condensation. Introducing fresh air can be as simple as having a ventilation routine, or installing extractors or PIV systems.

A key component in ventilation is facilitated through ducting pipes, and for those who don’t know, we are here to answer the question ‘What is flexi duct?’

There is more than one form of duct, in shape, size and material it is made from, knowing the difference between them will help you know what type of duct is best suited to your needs.

We at I-Sells are here to provide the answers you need whilst also supplying you with all the information you need in order to combat mould and have a well-ventilated home.

Alongside mould control, our venting and ducting solutions also aid in the removal of condensation, retention (and in some cases addition) of heat, and the removal of stale air in your home.

Explaining ducting

Ducting System

The ducting system is a series of interconnected hollow tubes dedicated to the movement of air, ducting pipes can come in different sizes and as previously mentioned, in different shapes, each of which has its own benefit.

Ducting pipes can be as long or as short as required in accordance with the space it is occupying. Additionally, ducting can be created from different materials, as we will highlight below.

Ducting is used to facilitate the process of ventilation, ducting systems may vary, but generally are made from plastic or metal & made into the piping which intakes air from the room it is in & expels it outside.

The main purpose of ventilation is to introduce outdoor air into a dwelling, building or room being evenly distributed. Its general purpose is to allow “healthy” air for breathing & diluting pollutants that may be present in the dwelling, building or area.

Some ventilation purely extracts the air of the room it is in, whilst others extract the inside air and then recycle and return it.

No Ducting means more mould

Mould reproduces from tiny spores. The spores float through the air and deposit on the surfaces. Under adequate temperature, moisture, and nutrient conditions, the spores can form new mould colonies.

condensation is not the only cause of mould, it can occur from leaks in the structure of a building too. Ducting and ventilation can help prevent an environment necassary for mould to develop in.

Mould can severely affect your immune and respiratory system, as the spores are easily breathable and not obvious to see through the human eye.

If you have found mould in your home, you need to assess if this is your landlord’s responsibility (for example it could be from a leaking roof that needs repairing). Or if the damp is caused by improper ventilation. To assess your options, click here.

What is flexi duct?

“Flexi duct” is short for flexible ducting. Flexible ducting refers to ducting that isn’t set into place, its flexibility allows it to bend in areas where it is required to.

Flexible ducting would be appropriate for unconventional spaces that aren’t able to support typically rigid ductwork.

What kinds of ducts aren’t flexi?

Types Of Ducts

Round Pipe:

Round ducting has the highest airflow performance, this is due to a wider diameter increasing its free area. The lack of corners also decreases resistance within the pipe, which allows for higher airflow rates and higher rates of ventilation.

It is recommended for most applications where space is available for installation. Its main purposes are for long ducting runs, whole-house ventilation systems, HVAC systems, bathroom, toilet and utility room extract fan venting, or any occasion where high rates of extraction are required.

Round PVC ducting pipe can be used for venting exhaust odours and steam from cooker hoods, although flexible ducting may be easier to install in many instances. 

Sizes are available from 100mm, 300mm, 500mm and 600mm diameters, with sizes between these dimensions also available. Both round and flat ducting have smooth inner surfaces to reduce air resistance and noise.

Pros & Cons of Round Ducting



High Airflow

Difficult to conceal

Less friction

Poor results in low-pressure systems


More expensive than flat ducting



Better for high-pressure systems


Designed for HVAC systems


Flat Channel Duct:

Flat channel ducting is ideal when a low profile is required due to lack of space. It can also be concealed and flush fit against ceilings and walls or within voids.

Please bear in mind that as a result of a lower free area, there is greater pressure created within a duct run. It is therefore essential that you check the manufacturer information for the fan you are planning to install within the run. 

If the fan is extracting at a rate that is greater than the duct run can cope with, there is potential for the fan to overheat and burn out. This is both costly and unnecessary. 

Flat channel plastic ducting is generally used for installations requiring low extraction rates such as bathrooms, toilets, and utility rooms. Sizes available from 110x54mm to 310x29mm.

Pros & Cons of Flat Channel Ducting




Creates more noise within duct run


Poor results within high-pressure systems

Easy to flush fit and conceal

Increased air resistance

Ideal for low-pressure



Be mindful of where you place your ducting in order to understand what kind of ducting you need, and where it’s access doors will be, which are vital in order to maintain the effectiveness of your ventilation by cleaning the ducting buildup at regular intervals.

See our handy plastic ducting guide for additional information

Is flexible ducting better than rigid?

Realistically, it is not about if rigid ducting is better than flexible, as it depends on your needs & area of installation. Once you know this, it will be easier to assess which option works in accordance to what you have. So let us explain a little bit about each option.

Rigid Ducting

Generally made from metal, galvanized carbon steel or stainless steel, (as well as PVC plastic & polypropylene) this option is suitable for permanent ventilation as well as air-conditioning systems/heating, furthermore it can be applied to large scale exhaust/dust extraction.


  • As the name suggests, rigid ducting is structurally secure & durable, meaning they are built to last & as a result can be used as part of building infrastructure.
  • Little maintenance required beyond inspections to take place every few months or annually.
  • Suitable for all applications, extraction, exhaust fumes, cold & hot air transfer.
  • Weather resistant, you can use them for external mounting.
  • Less airflow resistance = increased efficiency.

Possible disadvantages

  • The installation process must be thoroughly planned out & can lead to be quite complex, therefore a qualified professional is the best option to avoid any mishaps during this process.
  • Consideration must be given to mounting infrastructure due to the weight of metal rigid ductwork (depending on the length & size you opt for)
  • The metal itself, along with labour & time taken to plan & install may make this option slightly more expensive.

Flexible ducting

Flexible ducting can be made from PVC, silicone, rubber, aluminium & PU polyurethane. As a result, this ducting is easier to manipulate and therefore can be used in cabinets, a variety of layouts and other small spaces. (this is also thanks to its accordion styled design.)


  • Flexible by nature which by default makes it easier to install & manipulate post installation if needs be.
  • Can be installed in tight or confined spaces.
  • Option for transparent ducting to be able to see potential blockages.
  • Can be used as a temporary ducting system


  • Due to the flexible nature of this ducting there is a higher likelihood of punctures
  • Regular maintenance checks should be carried out to guarantee efficiency.
  • Mostly suitable for internal mounting
  • Higher chance of noise than rigid ducting

Contact I-Sells

We at I-Sells endeavour to make sure our customers have all the information they need before choosing to invest in our mould solutions. Be sure to visit our blog page to gain knowledge on the wide array of factors and issues surrounding ventilation, mould, condensation, and much more.

We hope to have answered the question ‘What is flexi duct?’

We understand you may have more questions, Please do not hesitate to contact us for more information with regard to whatever you may need our help with. If you’d like to send us an email, click here. For other contact options, see below:

Call us on 020 8463 9696

Visit us at our showroom:


Monday – Friday: 8:00 am to 5:30 pm

Saturday: 9:00 am to 12:00 pm

Sunday: Closed

15 St John’s Parade

Sidcup, Kent

DA14 6ES

United Kingdom

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