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Blocking up extract fan vents for MVHR retrofit

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Question from a novice (not sure where to post this). How would you go about blocking up 3 extract fan vents? 2 bathroom extracts (100mm) and 1 kitchen extract (150mm)


- I'm having an MVHR system installed next week (retrofit) so I want to remove and block up these 3 vents. (Obviously I'll work on airtightness elsewhere)

- Walls are brick and block.

- Bungalow so it's all accessible

- I'll then have the failed CWI extracted and replaced with EPS beads

- overall objective of retrofit is to get rid of condensation and make my home more sustainable (use less energy)


Do I need to try and leave a cavity? Would people just use expanding foam? How do I prevent a thermal bridge? Any recommendations of products please?

The vents are close to the soffits.

bath extract.JPG

kitchen extract.JPG

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One of the best foams by far ilbruk FM330 foam. No problem with going full wall thickness but depends what the finish is inside, if plaster/plasterboard then put a piece in first and you can foam up behind it, externally brick is best but would require taking out any damaged bricks, hopefully 2 but could be 4 and replacing otherwise you end up with a horrible circular patch 

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  • 3 weeks later...

Thanks @markc. Apologies, I've been distracted by the MVHR install!

That's good to know about illbruk FM330 foam - it's been mentioned on lots of window discussions. I'll try and do the replacing bricks method. If I do this then how do I block up the hole inside? It's a 1960s house so the cavity walls are block on the inside with plaster (not dry-lined). I assume I need to put a patch of plasterboard (or wood?) in the hole, leaving enough space for a coat of wet plaster. But how do you secure and stop the plasterboard patch from falling down the cavity? And does something need to go behind the plasterboard?  Once everything else is done I'll be having the CWI sucked out and replaced with EPS beads.

One of the bathroom extracts (not in photo) is in the extension so this one is dry-lined on timber studs - I think I'll do this one with foam as it's not visible and not having replacement CWI.

I hope this makes sense - I don't know all the correct builder terminology.

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  • 2 weeks later...
  • 2 months later...

@Conor and @markc a belated thank you for your help! I managed to block up my extract vent holes - first time for everything. The cooker hood one was nasty - the builder years ago had hidden his shoddy job and there was a massive double hole to fill. I stuck some ME508 airtightness tape at the back of the inner leaf, filled with FM330 and then plastered over the top. Now all covered up with a new recirculating cooker hood. Cheated slightly on the brick work outside and happened to find a builder to come and brick up the holes. Thanks again.

hole 1.JPG

hole 2.JPG

hole 3.JPG

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