Underfloor ventilation for hob extractor on Island

[paro]

Hi All - I'm trying to figure out how/where to install the flat ducting from an island hob extractor to outside. I can't find anything useful on what is an acceptable way to pull this through the floor.

 

The build up for the floor will be a 80mm PIR with 50-65mm screed + hardwood floor. The flat ducting needs to be 90mm so not sure how best to install. I was originally thinking about going as close to the block and beam as I can but the concern becomes whether the air flow would be stifled by the temperature change it meets when colder. Either way it is going to be in both parts of the substrate just not sure which one gets the lions share and why.

 

If anyone can point to an example someone doing this that would be appreciated.

 

Thanks, Paul

[G and J]

Questions (having noted you are doing a refurb and therefore I’m assuming you won’t have MVHR).  
 

If you are laying a wood floor why do you need a screed?   You could have a floating wooden floor on thicker insulation.  
 

Why not use a recirculating hood?

[paro]

Hi - This is going into a new extension on the back of the cottage. The build up was specified by the architect. I am expecting to have dMVHR or PIV (this will be another set of questoins as I wan't to avoid trickle vents). My experience of recirculation hasn't been great when it really matters. I bought a new fancy aeg hood a few years ago with recirculation and it really did nothing to stop the things you really need gone from coming back through the filters.

 

[G and J]

50 minutes ago, paro said:

Hi - This is going into a new extension on the back of the cottage. The build up was specified by the architect. I am expecting to have dMVHR or PIV (this will be another set of questoins as I wan't to avoid trickle vents). My experience of recirculation hasn't been great when it really matters. I bought a new fancy aeg hood a few years ago with recirculation and it really did nothing to stop the things you really need gone from coming back through the filters.

 

Methinks that stuff like that floor build up often gets dumped down onto drawings without thought - more or less standard formula.   Don’t assume that everything on your drawings is there first a good reason.  You are the client - it is your right to question.  If you are installing UFH it would be different. 
 

If you need air removed from your kitchen then you could still catch the grease with a recirculating hood and have a boost to your chosen ventilation system.

 

I did once link a previously recirculating hood to an external vent via a flat-ish rectangular duct along the top of some kitchen wall cupboards.   Sort of worked but I think the flow rate wasn’t what it could have been and on full it sounded like a hovercraft.  Plus I suspect the guests rarely turned the bloody thing on anyway.

[JohnMo]

1 hour ago, paro said:

My experience of recirculation hasn't been great when it really matters

Did you have a normal extract fan such as dMEV in the kitchen as well, if you only had extract in bathrooms it will never work.

 

Recirculation fans are fine, with decent grease removal performance and activated charcoal filters, but you need the kitchen dMEV fan as well or work.

 

1 hour ago, paro said:

am expecting to have dMVHR or PIV (this will be another set of questoins as I wan't to avoid trickle vents)

You can't avoid trickle vents, your ventilation will not work without cross flow ventilation. Air  is swept across dry room by extract from wet rooms.  You can install humidity activated trickle vents, so ventilation is only where it needs to be. Add that to auto boost Greenwood dMEV CV2 or CV3 fans. Trickle vents should not be installed in wet rooms.

 

 

9 hours ago, paro said:

The build up for the floor will be a 80mm PIR

That seems very thin, does it even meet building regs?

 

 

[paro]

1 hour ago, G and J said:

Methinks that stuff like that floor build up often gets dumped down onto drawings without thought - more or less standard formula.   Don’t assume that everything on your drawings is there first a good reason.  You are the client - it is your right to question.  If you are installing UFH it would be different. 
 

If you need air removed from your kitchen then you could still catch the grease with a recirculating hood and have a boost to your chosen ventilation system.

 

I did once link a previously recirculating hood to an external vent via a flat-ish rectangular duct along the top of some kitchen wall cupboards.   Sort of worked but I think the flow rate wasn’t what it could have been and on full it sounded like a hovercraft.  Plus I suspect the guests rarely turned the bloody thing on anyway.

Sorry should have added that there will be UFH. 

[Nickfromwales]

+1 on needing trickle vents. Won’t pass b regs, and would make a horrible home to live in. 

[JohnMo]

3 minutes ago, paro said:

Sorry should have added that there will be UFH. 

Then 80mm is just not good, you either have no insulation or lots. No insulation, will not pass BR, so not an option. 80mm will just loose so much heat to ground. Unless you meant to type 180mm then that is ok. You need a minimum of 150mm ideally 200mm if PIR or equivalent.

[Nickfromwales]

9 hours ago, paro said:

The build up for the floor will be a 80mm PIR with 50-65mm screed + hardwood floor. The flat ducting needs to be 90mm so not sure how best to install. I was originally thinking about going as close to the block and beam as I can but the concern becomes whether the air flow would be stifled by the temperature change it meets when colder. Either way it is going to be in both parts of the substrate just not sure which one gets the lions share and why.

You could stack 2 lots of 225x25, you’d just need to get inventive on the connections to/from, but doable. 
 

As far as temp is concerned, flow rates won’t suffer but you will get condensation I expect. That means the ducts need to run downhill to the outside wall so they can drip dry. With 2 lots of 225, you’ll have space for some insulation under the lowest one so that issue would be reduced significantly if avoiding using the full depth of the insulation to fit the duct.

[Nickfromwales]

2 minutes ago, JohnMo said:

Then 80mm is just not good, you either have no insulation or lots. No insulation, will not pass BR, so not an option. 80mm will just loose so much heat to ground. Unless you meant to type 180mm then that is ok. You need a minimum of 150mm ideally 200mm if PIR or equivalent.

If a refurb or addition, folk don’t have the luxury. 100mm minimum if heated, 120-140 would be much better.

 

@paro  I’d ditch the screed here and beef up the insulation. Fit an overlay UFH system (see Wunda website, good vids/info there) and then bond your hardwood floor directly to that. 

[paro]

1 hour ago, JohnMo said:

Then 80mm is just not good, you either have no insulation or lots. No insulation, will not pass BR, so not an option. 80mm will just loose so much heat to ground. Unless you meant to type 180mm then that is ok. You need a minimum of 150mm ideally 200mm if PIR or equivalent.

This is real concern then as this was specified by the architect in the BR drawings submitted to BC. My biggest issue here is that that this has been designed/built to allow a seemless transiiton from the main cottage into the extension on the same level. I can maybe scrape another 15-20mm out of the 65mm screed but was rather hoping to use that for additional celing height as they are only 2250. I literally just finished the roof so there is absolutley nowhere to go height wise. 

 

I'll have to ask BC, but really don't want to hear there answer. 

 

 

 

 

[paro]

I am hoping that the architect is soundly using note 4 from 4.2 of part L1(a) 'If meeting such a standard would create significant problems in relation to adjoining floor levels, a lesser provision may be appropriate.'

[paro]

1 hour ago, Nickfromwales said:

You could stack 2 lots of 225x25, you’d just need to get inventive on the connections to/from, but doable. 
 

As far as temp is concerned, flow rates won’t suffer but you will get condensation I expect. That means the ducts need to run downhill to the outside wall so they can drip dry. With 2 lots of 225, you’ll have space for some insulation under the lowest one so that issue would be reduced significantly if avoiding using the full depth of the insulation to fit the duct.

Thanks. I like this idea as an option. Need to check my levels for the though with the telescopic air bricks. 

[paro]

1 hour ago, Nickfromwales said:

+1 on needing trickle vents. Won’t pass b regs, and would make a horrible home to live in. 

Thanks. I was going to raise this as a separate Q but since you mention it.  My objection to trickle vents is simply that in my area the farmers always have some kind of fire on the go which just stinks the house out. I was hoping to go with some kind of ducted MVHR/MEV or PIV combo to control the air in and out. Are you saying that even with a properly sized system that trickle vents will still be needed. I can certainly get them as I haven't ordered the glass yet but was hoping the mechanical option would be acceptable.

 

[Nickfromwales]

17 minutes ago, paro said:

I am hoping that the architect is soundly using note 4 from 4.2 of part L1(a) 'If meeting such a standard would create significant problems in relation to adjoining floor levels, a lesser provision may be appropriate.'

You are allowed “deviations”, so that may well be in check. 
 

We do all often jump too quickly to the pinnacle, but life is life, and 80mm will have to suffice if that’s all you have. 
 

At this stage id defo explore doing away with the screed as you do not “need” it. Then the entire build up can be insulation. 

[paro]

36 minutes ago, Nickfromwales said:

You are allowed “deviations”, so that may well be in check. 
 

We do all often jump too quickly to the pinnacle, but life is life, and 80mm will have to suffice if that’s all you have. 
 

At this stage id defo explore doing away with the screed as you do not “need” it. Then the entire build up can be insulation. 

Thanks. I'll take a look at direct to insulation option you mentioned. 

[Nickfromwales]

7 minutes ago, paro said:

Thanks. I'll take a look at direct to insulation option you mentioned. 

There are options for 22mm chipboard routered out, or insulated panels routed out (for the pipes), so get some googling done and check the responses here for related content   

[paro]

BCO has been today and it turns out my thinner insulation is not an issue as my application went before the 22 reg changes. 

 

Re ventilation, he was happy with no trickle vents provided I can provide a design for a properly speced mechanical system for the rooms. He went on to say that a mechanical system could handle the hob extraction if designed correctly, that there was actually no need to have an extractor above/next to.  He also said recirculation alone wasn't sufficient. 

 

A good result really. Thanks all for your help. 

Edited Thursday at 14:31 by Nickfromwales
"no need" vs "need"?

[JohnMo]

6 minutes ago, paro said:

no trickle vents provided I can provide a design for a properly speced mechanical system for the rooms

So you need mechanical ventilation in all rooms. So MVHR or two fan systems one supplying and one extract with no heat recovery.

 

If a farmer is burning stuff all the time, complaint to environmental health. Because he shouldn't be.

[Nickfromwales]

1 hour ago, paro said:

BCO has been today and it turns out my thinner insulation is not an issue as my application went before the 22 reg changes. 

 

Re ventilation, he was happy with no trickle vents provided I can provide a design for a properly speced mechanical system for the rooms. He went on to say that a mechanical system could handle the hob extraction if designed correctly, that there was actually no need to have an extractor above/next to.  He also said recirculation alone wasn't sufficient. 

 

A good result really. Thanks all for your help. 

Your BCO makes little sense.....not surprised as so few actually do when it comes to ventilation. Serious lack of education here or 'gettting with the times' .

 

Trickle vents being omitted is fine, but then you need MVHR to provide fresh air to each habitable space, and extracts to all other wet/stinky areas. Any type of ventilation system that doesn't recover heat is just perverse AFAIC, who wants a system on 24/7/365 that blows cold air from outside into all the bedrooms and living spaces all day and night in the winter.....feck that.

 

If you spoke to a ventilation system provider, worth their weight, then they'd tell you under no circumstances should that recirculation system be subject to all the cooking stream/grease etc as it will kill it off at a rapid rate of knots. You defo need to extract the grease etc at source, with a powerful local recirculating cooker hood, and leave the MVHR to change the cleanest possible air in the house, boosting when applicable.

 

Recirculation alone isn't sufficient, unless you have mechanical ventilation. Bloody great help when some professional gives you just 'some' or worse/wrong information, when you are beholden to them.....

[Nickfromwales]

On 09/07/2025 at 10:29, paro said:

Thanks. I was going to raise this as a separate Q but since you mention it.  My objection to trickle vents is simply that in my area the farmers always have some kind of fire on the go which just stinks the house out. I was hoping to go with some kind of ducted MVHR/MEV or PIV combo to control the air in and out. Are you saying that even with a properly sized system that trickle vents will still be needed. I can certainly get them as I haven't ordered the glass yet but was hoping the mechanical option would be acceptable.

 

Anything sucking air in from outside will pull the smoke/BBQ smells etc into the house. That includes trickle vents, PIV, MVHR anything.

 

Trickle vents are required any time you drop below 3ACH, eg if you have not installed trickle vents/have made your home reasonably airtight or completely airtight to PH standards etc; and are then subject to choking on your own output overnight in the bedrooms, and wafting the stench out of a window after other activities, etc.

 

For one client in the sticks, same scenario, I just put a timer contactor on to the A/C power supply to the MVHR unit to give them a 1-2 hour hit of no incoming air from atmosphere (time delay user-definable), and obvs explained that the vegan curry better 'stay in' until the MVHR system comes back on etc.

[paro]

1 hour ago, Nickfromwales said:

Anything sucking air in from outside will pull the smoke/BBQ smells etc into the house. That includes trickle vents, PIV, MVHR anything.

 

Trickle vents are required any time you drop below 3ACH, eg if you have not installed trickle vents/have made your home reasonably airtight or completely airtight to PH standards etc; and are then subject to choking on your own output overnight in the bedrooms, and wafting the stench out of a window after other activities, etc.

 

For one client in the sticks, same scenario, I just put a timer contactor on to the A/C power supply to the MVHR unit to give them a 1-2 hour hit of no incoming air from atmosphere (time delay user-definable), and obvs explained that the vegan curry better 'stay in' until the MVHR system comes back on etc.

 

I may just be making more complicated than needed.  So just been looking at the environvent 160/300 MEV. The space I'm covering is a kitchen/dinner with a utility and pantry. If I run ducting from MEV to around the hob location as well as the utility room and tie that in with a humidistat voc sensor it should automatically increase the flow when cooking or it smells bad stuff (not sure how accurate and smart these sensors are). I'll call them tomorrow to find out. 

 

It's a really old cottage and leaks like a sieve I am not really concerned about being too airtight yet. The extension opens directly into the main cottage as well.

 

[JohnMo]

What is your overall strategy for ventilation for the whole house. Doing bits here and there isn't the way to do it 

 

Ventilation duct by hob isn't correct, you will just fill the whole system with grease, then dust, then grease etc. 

[SteamyTea]

On 08/07/2025 at 23:09, paro said:

concern becomes whether the air flow would be stifled by the temperature change it meets when colder

Not sure I understand your concern here.

You will need some access to it for cleaning, even in a well run kitchen, things still managed to find their way in.

In commercial kitchens, baffles are used to catch all the fat and grime. Not seen them on domestic extractions. They are easy enough to make.

[Nickfromwales]

2 hours ago, paro said:

 

I may just be making more complicated than needed.  So just been looking at the environvent 160/300 MEV. The space I'm covering is a kitchen/dinner with a utility and pantry. If I run ducting from MEV to around the hob location as well as the utility room and tie that in with a humidistat voc sensor it should automatically increase the flow when cooking or it smells bad stuff (not sure how accurate and smart these sensors are). I'll call them tomorrow to find out. 

 

It's a really old cottage and leaks like a sieve I am not really concerned about being too airtight yet. The extension opens directly into the main cottage as well.

 

Adding so indiscriminately to your extraction setup will make it a very cold house during winter. I'd steer away from mechanical centralised ventilation and just go for fans where you want them, used when you need them, only. Having the above arrangement means you're extracting from the utility regardless if there's any need to or not.

 

Having 1x extractor on at any one time should allow the cold air in > heated air from the whole house out to become diluted enough for the cold air infiltration to be of negligible impact.