Wet rooms ventilation and IWI installation
This post is for people involved in domestic retrofitting of energy efficient measures. It is an overview of what constitutes a wet room and how this effects ventilation and IWI installation to properties that have Kitchen/diners, through lounges, open plan properties, etc, and a bit more…
This is a grey area where Building regulations, IWI installation guides and PAS2035 don’t help when we have properties that are not made using a “Cookie cutter”. You will find reference to seek advice from a professional system designer, but still this has its own issues. When an auditor compares your robust specification against an IWI installation guide or Part F of the building regs they may deem it “Non-compliant”.
That being said I’ll try my best…
Firstly, lets have a brief description of different types of rooms you may find in a home.
What is a wet room?
A wet room is typically a room with a bath or a shower in it, a room where food is prepared, or a room that has a sink or appliance that produces significant quantities of water vapour. These rooms are typically the kitchen, utility room, bath/shower room and sometimes the WC.
What is a kitchen when doing ventilation and IWI installation
A kitchen is where food is prepared and does not have to have a sink. If food is prepared using a microwave, cooker or stove then the space is a kitchen. It may also have other kitchen appliances such as dish washers and washing machines, but it doesn’t have to have these to be classed as a kitchen.
What is a kitchen diner?
A kitchen diner is a kitchen with an addition area for eating food.
What is a Utility room
A utility room just has to have a sink or an appliance that gives off significant quantities of water vapour. It doesn’t have to have a sink, but if there is a sink used for cleaning or washing, a washing machine, a dish washer, tumble drier, etc, present in the room or space then it is classed as a utility room.
What is a bathroom?
A bathroom is a room that has a bath or a shower installed. An ensuite with just a WC in it would be a WC. An ensuite with a shower or bath in it would be a bathroom.
When is a WC a wet room?
A WC is a room with a toilet pan and cistern in it. A WC is a wet room if it does not have an openable window. If it has a window that opens then it is not classed as a wet room.
What is a habitable room?
A habitable room is a room you are likely to spend time in for living, sleeping or eating that is not a wet room. It will generally have natural light, an openable window, electric lighting and heating. These rooms would generally be a bedroom, living room, lounge, dining room, home office, morning room, sun room, etc, and meets the minimum space standards. e.g. minimum ceiling height of 2.3m.
What is a store room when doing ventilation and IWI installation?
A store room as any space that you may store things in that does not fall into the wet room or habitable room categories. They may not have lighting, heating or ventilation and you would not normally use the room for relaxing, eating or sleeping.
What is a conservatory?
A conservatory is not a habitable space. It is used for leisure. A conservatory must have windows for least 50% of the walls and the roof must be at least 75% glazed. If less than 50% of the wall are glazed or less than 75% of the roof is glazed then it cant be labelled a conservatory.
What ventilation is usually installed?
When retrofitting energy efficient measures to homes the ventilation is usually upgraded to constant flow mechanical ventilation to all wet rooms (usually dMEV) and 4000mm2 back ground ventilation to all habitable rooms (usually window trickle vents)
What walls are usually treated with IWI?
All external facing walls to all rooms and spaces are insulated with internal wall insulation (IWI). In most projects the wet rooms are omitted. This is usually because a wet room such as a kitchen or bathroom would have to have appliances, fixtures and fittings removed then reinstated which can be very costly. We do get the odd project where wet room is insulated, but not many.
Were IWI is omitted from wet rooms, the BIES IWI best practice guide states there needs to be a constant flow extract fan and a safe heating system (fixed heater) to ensure the room can reach a minimum of 18C when occupied.
A standard house with separate kitchen, dining room and living room would have a dMEV installed to the kitchen and 4000mm2 window trickle vents to the dining room and the living room. See Plan 1.
IWI would be installed to the living room, dining room and front door areas, omitting the kitchen.
What about when people start removing walls between rooms?
Well now this is were it starts to get interesting.
When someone removes a wall between a kitchen and the dining room then this is a kitchen diner. They may be shown on a floor plan as distinct rooms but the two areas are now classed as one room, and as part of the room is a kitchen then the whole kitchen diner is now a wet room.
When you install a dmev to the room (kitchen/diner) you can’t have any back ground ventilation in the same room.
As you cant install IWI to only some of the walls within a room, you can’t insulate the dining are and not insulate the kitchen area, so no wall can be insulated in the kitchen/diner.
In Plan 2 the kitchen/diner is now classed as a wet room. A dMEV is installed but no trickle vent or IWI.
If you install a vent or IWI to the dining areas then these will be raised as a non-conformity on audit.
Open plan properties – ventilation and IWI installation
In plan 3 you can see that more walls have been removed so the whole floor is open plan. Now think of the ventilation and IWI installation to be installed
This now starts to be tricky as you could define the wholes space as a wet room. Imagine if it was a very small kitchen, dining room and living room with a total area of 4m x 4m then this would be easy to define it as a wet room, install a dMEV, don’t install background ventilation and not install any IWI.
But what if is a property is 12m x 6m? This now an open plan property with clearly define zones. A kitchen area, a dining area, and a living area and a lot of heat loss wall that one would want insulated.
In this situation it is best to install a dMEV to the Kitchen area and install background ventilation at the furthest practical point away from the dMEV. In this scenario that would be installing a trickle vent to the living room window. You may then be able to install IWI to the living room and dining room as where the IWI in the dining room stops is at an internal wall between the kitchen area and the dining area. At this internal wall you can seal any vapour control layer so moisture laden air can not pass behind the IWI and condense on the cooler solid wall. On some projects we have been able to terminate the IWI at a floor to ceiling bifold door. There is always a solution…
If there was not an internal wall to terminate the IWI then you would also have to omit the dining area from IWI.
So will all of vents and IWI above be passed on audit?
Yes, if presented correctly, but there are a few obstacles.
Ventilation
Firstly consider ventilation and IWI installation, namely the ventilation for open plan building is easier to be compliant when installing natural ventilation as you can have background ventilation in the same space as intermittent extract ventilation (IEV). The down side to this is that the ventilation be a minimum of 8000mm2 for all habitable rooms and kitchens and 4000mm2 for other wet rooms. It is difficult to install 8000mm2 of trickle vents to a single window so you may have to install core vents or air bricks. Very messy and there is also a requirement for a minimum amount of vents in a property.
When installing a dMEV as constant flow extract ventilation you are not supposed to install background ventilation in the same space, but where is the air going to come from to extract to outside? With open plan homes I always advise background ventilation to be installed at the furthest point away from the dMEV on an opposing elevation so fresh air can be drawn into the property, pass through all zones and extract out of the dMEV.
Contact your dMEV manufacturer
If you are installing a dMEV with background ventilation in an open plan property, I advise that you contact the manufacturer of the dMEV you are installing, explain the situation and ask them for a ventilation specification for dMEV’s and background ventilation for your project. This will then support your ventilation upgrades and be compliant.
Air permeability tests
If the property has a high air permeability rate, lots of air can pass through the fabric of the building, then you may not need background ventilation. This will have to be supported with an air permeability test (door fan blower test or air pulse test) to measure the available air changes in the property and habitable spaces to prove additional ventilation is not required.
Internal wall insulation (IWI)
Although in Plan 3 you can see IWI has been installed to the dining room and living room and the front door area, we have be pulled on audit for not insulating all of the external facing walls to the same space. Quite some time ago but then every day is a learning day.
With extensive communication including an explanation of building physics and how stopping IWI at an internal wall to a kitchen diner is no more of a risk than stopping at an internal wall between separate kitchen and dinning room they refused to budge.
Contact your IWI manufacturer
If you find yourself in the same situation then contact your IWI provider and they will be able to confirm you have installed there IWI system compliantly. It is our advise that if you have an open plan property then ask you IWI manufacturer to provide you with a specifications and include this within your retrofit design.
Recap of ventilation and IWI installation
Separate wet rooms and habitable rooms
If you have separate wet rooms and habitable rooms, install dMEV to wet rooms and 4000mm2 background ventilation (trickle vents) to all habitable rooms. You may omit wet rooms providing they have a dMEV and a fixed heater. No back ground ventilation to be installed in the same room as a dMEV
Kitchen/diner
If you have a kitchen/diner then install dMEV to the kitchen diner and all other wet rooms and 4000mm2 background ventilation (trickle vents) to all habitable rooms. You may omit wet rooms providing they have a dMEV and a fixed heater. No back ground ventilation to be installed in the same room as a dMEV and if you omit IWI from the kitchen side of the kitchen/diner then you have to omit the dining side also.
Open plan
If you have an open plan property, install a dMEV to the wet room areas (Kitchen in the plans above) and 4000mm2 background ventilation as far away from the dMEV as practical. Install IWI up all external facing walls but you may stop at an internal wall then omit the kitchen area.
To support this installation contact the manufacturer of the dMEVs and the IWI system and ask for a specification for your project.
Conclusion – Wet rooms ventilation and IWI installation
This post offers only a brief glimpse into the complexities of specifying ventilation and internal wall insulation in domestic properties—especially those with unconventional layouts or construction types. Ensuring compliance through standard methods can be challenging, and the deeper you explore the various normative documents—such as PAS 2035, Building Regulations, BS 5250: Management of Moisture in Buildings, manufacturer specifications, and BIES guidance—the more it becomes clear that these resources often raise more questions than they answer.
Many homes simply don’t fit neatly into the frameworks these documents provide when installing ventilation and IWI installation. Instead of offering definitive solutions, they often lead you down a rabbit hole of interpretation, requiring professional judgment and a tailored approach to each unique building.
