Jump to content

School me on this whole mechanical ventilation thing...


Recommended Posts

We have a post war ('46) end of terrace, three bed, one bathroom that's block-50mm blown cavity-block construction with render and full internal plaster. We have double glazing (10yr old) and we currently have moderate condensation on the inside of the windows most mornings and during the day in the winter. The house is occupied by two people, one works normal hours and is away from the house 8-5, the other can sometimes be home all day.

 

We heat it to 20 deg all winter long and keep the windows shut because we don't want to let that precious heat out. Our windows do not have trickle vents.

 

I'm working on more air-tightness upgrades (re-fitting the windows that the cowboys installed, insulating and bagging the suspended floors) so now need to think about ventilation. I understand that full MVHR isn't worthwhile on an old property such as this, and that there are heat recovery extractor fans that can be installed in places like the bathroom and kitchen. At the moment the bathroom walls are like a river in the winter time (north and west facing external walls which I'm going to marmox board soon) so that needs to be addressed.

 

I do have access to the loft so I could install ceiling vents in the bathroom and the three bedrooms and some sort of 'fan box thingy' in the loft.

 

What should I be thinking of doing to improve things? 

Link to comment
Share on other sites

Sounds like you have normal condensation problems. Really a matter of how far you are willing to go to solve it.

First things though, are there any problems with drains, downpipes, gutters, window sealing, roof, walls etc that may be allowing water into the house?

Assuming there are not any, then it is a matter if controlling the ventilation.

As a house can be thought of as a box, the less air that can escape it the better. The problem is, people breath out and sweat water vapour (about 2 kg a day each).

If that water vapour is not removed, it will condense on the first surface it finds where the temperature is below the dew point temperature. This is usually windows, so quite visible. Thing is, not all the vapour will condense, some will work its way through floors and ceilings (generally via holes and gaps) where it can condense into timber joists, unnoticed.  This is where the real problems start as rotting can happen.

Old fashioned thinking was to allow cold, lower humidity air from outside to, in effect, carry the excess moisture away. 

Modern thinking is to contain the moisture laden air inside the building, then mechanically extract it. This is done with a vapour control layer, which is just a plastic sheet to keep the air in, an airtight bag if you like.

Because it is airtight, and people are inside it, eventually the air needs replacing. This is why trickle vents were fitted to windows, and air bricks, now we fit fans, and some of those fans have heat recovery on them. 

So to sum up. Old method was to allow uncontrolled ventilation, haphazardly installed by the builder i.e. holes and gaps, so lots of air changes when windy, none when hot and sultry.

New method is to have controlled ventilation which is at the correct rate to stop condensation and keep the air fresh.

Simple really.

Link to comment
Share on other sites

Noticed you posted in ventilation with heat recovery.  The maths work against heat recovery with leaky houses as cost to install isn't proportional to the heat recovery when all other ventilation heat loss is considered

 

Bottom line is if you are getting condensation, you need better ventilation.  Condition based MEV or dMEV is worth looking into, way simpler and cheaper to install and will only ventilate when it's required.

 

Some light reading on the merits or otherwise of different systems

Atamate_SDAR+Paper+2019+(1).pdf

  • Like 1
Link to comment
Share on other sites

1 hour ago, JohnMo said:

Noticed you posted in ventilation with heat recovery.  The maths work against heat recovery with leaky houses as cost to install isn't proportional to the heat recovery when all other ventilation heat loss is considered

 

Bottom line is if you are getting condensation, you need better ventilation.  Condition based MEV or dMEV is worth looking into, way simpler and cheaper to install and will only ventilate when it's required.

 

Some light reading on the merits or otherwise of different systems

Atamate_SDAR+Paper+2019+(1).pdf 2.47 MB · 22 downloads

MEV = mechanical extraction ventilation?

dMEV = dedicated MEV (per room)?

Link to comment
Share on other sites

Posted (edited)
2 hours ago, SteamyTea said:

Sounds like you have normal condensation problems. Really a matter of how far you are willing to go to solve it.

First things though, are there any problems with drains, downpipes, gutters, window sealing, roof, walls etc that may be allowing water into the house?

Assuming there are not any, then it is a matter if controlling the ventilation.

As a house can be thought of as a box, the less air that can escape it the better. The problem is, people breath out and sweat water vapour (about 2 kg a day each).

If that water vapour is not removed, it will condense on the first surface it finds where the temperature is below the dew point temperature. This is usually windows, so quite visible. Thing is, not all the vapour will condense, some will work its way through floors and ceilings (generally via holes and gaps) where it can condense into timber joists, unnoticed.  This is where the real problems start as rotting can happen.

Old fashioned thinking was to allow cold, lower humidity air from outside to, in effect, carry the excess moisture away. 

Modern thinking is to contain the moisture laden air inside the building, then mechanically extract it. This is done with a vapour control layer, which is just a plastic sheet to keep the air in, an airtight bag if you like.

Because it is airtight, and people are inside it, eventually the air needs replacing. This is why trickle vents were fitted to windows, and air bricks, now we fit fans, and some of those fans have heat recovery on them. 

So to sum up. Old method was to allow uncontrolled ventilation, haphazardly installed by the builder i.e. holes and gaps, so lots of air changes when windy, none when hot and sultry.

New method is to have controlled ventilation which is at the correct rate to stop condensation and keep the air fresh.

Simple really.

Gutters and down pipes are good, roof is good, windows are Badly installed but I’m sorting that (no foam, fist sized gaps, upvc trim falling off outside etc). The house is dry, it just gets condensation and is a bit stuffy.

Edited by HughF
Link to comment
Share on other sites

10 minutes ago, HughF said:

MEV = mechanical extraction ventilation?

dMEV = dedicated MEV (per room)?

MEV correct.

dMEV decentralised.  Fan in each wet room.

 

Both uses cross flow ventilation through dry rooms, to wet rooms need a ventilator (can be trickle vent or a controlled opening in the wall.  Condition based system will open the vent as required only.

 

Stuffy rooms are a good indication that the CO2 levels may be higher than ideal.

 

Link to comment
Share on other sites

PIV is an option also ( positive input ventilation ) but seems to have opportunistically doubled in price in the last few years. 

 

I installed one of these in my parents house last year as it was very damp.  It just sits in the attic above the upstairs landing extracting from there to an outside vent.  No ducts to any wetrooms. I did this on the impression that if the internal doors are opened occasionally then the humidity will equalise.  It seems to be proving right. 

 

image.thumb.png.a2e7968119ae23f44ae10c1ce329b4cf.png

 

 

It has helped to get rid of lots of the mould although they seem to think the house is colder with it running. Hard to say objectively. I did need to install a DIY silencer as it was noisy without one. 

 

I can't see where the price increase goes over something like this is however. 

image.thumb.png.c3cfc24dd6d61ce8fde4d3ca438455e9.png

 

 

Link to comment
Share on other sites

Posted (edited)
43 minutes ago, HughF said:

Thanks for all the helpful advice, I appreciate it.

 

We keep most/all internal doors open except when we sleep/use the bathroom. Would something like this be a good idea in the bathroom:

 

Decentralised MVHR unit Ø100 mm with switch - EER100S (ventilationland.co.uk)

Or the Partel Lunos - that looks like a good system

Edited by HughF
Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share

×
×
  • Create New...