Best construction method to avoid mould growth

[SimonD]

3 hours ago, goatcarrot said:

the nourishment that mould needs is dead organic matter so wood/cellulose is the perfect food, perhaps concrete/stone/block/foam it is even less likely.

Not necessarily, there are many examples across the country right now of newly built houses using concrete/stone/block/dryway/plasterboard-skim etc.developing bad mould problems very soon after occupation. I know someone who moved into a newbuild on a development only to find mould growing in the utility room within 3 months. The problem is more down to relative humidity and ventilation. Below a certain amount of humidity, mould will not grow on timber or cellulose or other materials. The wood fibre and cellulose (actually technically both cellulose) are treated, often with a borax solution to prevent fungal attack and for fire protection.

 

Essentially what you need to do is ensure that your fabric is well built and detailed properly - so sufficient insulation, means of protection from ingress of damp and means of escape if damp does got into the fabric of the building. Internally you need to ensure good airtightness together with sufficient ventilation. Sufficient ventilation should ensure relative humidity remains within acceptable limtis within the house, but in addition it has been found in research that building fabrics that buffer moisture (i.e. absord excess moisture in the air and then release it at another time) can reduce relative humidity within the house by between 10-25%.  Breathable fabrics include wood fibre, hemp,or sheepswool insulation but the whole wall system needs to work in concert.

 

Hope that helps.

 

 

[ProDave]

1 hour ago, SimonD said:

Not necessarily, there are many examples across the country right now of newly built houses using concrete/stone/block/dryway/plasterboard-skim etc.developing bad mould problems very soon after occupation. I know someone who moved into a newbuild on a development only to find mould growing in the utility room within 3 months. The problem is more down to relative humidity and ventilation. Below a certain amount of humidity, mould will not grow on timber or cellulose or other materials. The wood fibre and cellulose (actually technically both cellulose) are treated, often with a borax solution to prevent fungal attack and for fire protection.

 

 

The issue there is developer houses, built as cheap as possible, often to poor standards, and certainly with none of the measures we have been talking about like good insulation, good detailing, and things like mvhr.

[goatcarrot]

@SimonD thanks, yes I agree within the living space itself but how do you control the RH within the cavity if it’s raining and windy for days on end? Is ventilation enough? 

maybe a weatherproof outer layer like zinc siding is one answer. 

[tonyshouse]

Passive House - definitely no mould

[goatcarrot]

9 hours ago, tonyshouse said:

Passive House - definitely no mould

@tonyshouse is yours a timber frame?

[Mr Punter]

1 hour ago, goatcarrot said:

@tonyshouse is yours a timber frame?

 

How dare you!  Wash your mouth out!

 

Brick & block with extra wide cavity.

[ProDave]

11 hours ago, goatcarrot said:

@SimonD thanks, yes I agree within the living space itself but how do you control the RH within the cavity if it’s raining and windy for days on end? Is ventilation enough? 

maybe a weatherproof outer layer like zinc siding is one answer. 

It is the INTERNAL air tight and vapour control layer that is your friend, on the inside of all the insulation.  So even if somehow a bit of mould developed somewhere withing the frame (it should not) then it cannot get into the living space.

[SteamyTea]

Regarding the MVHR failure risk, you can always open windows.

[SimonD]

13 hours ago, ProDave said:

The issue there is developer houses, built as cheap as possible, often to poor standards, and certainly with none of the measures we have been talking about like good insulation, good detailing, and things like mvhr.

I agree completely, but it also shows that it is not the building material in and of itself that is the problem, which was the question about cellulose.

[goatcarrot]

1 hour ago, Mr Punter said:

 

How dare you!  Wash your mouth out!

 

Brick & block with extra wide cavity.

I don’t understand this comment, what’s wrong with TF compared to brick and block, do you consider it inferior?

[goatcarrot]

1 hour ago, ProDave said:

It is the INTERNAL air tight and vapour control layer that is your friend, on the inside of all the insulation.  So even if somehow a bit of mould developed somewhere withing the frame (it should not) then it cannot get into the living space.

Yes I understand that, just trying my utmost to guard against that possibility 

[SimonD]

12 hours ago, goatcarrot said:

@SimonD thanks, yes I agree within the living space itself but how do you control the RH within the cavity if it’s raining and windy for days on end? Is ventilation enough? 

maybe a weatherproof outer layer like zinc siding is one answer. 

You have an external layer, let's just call it a 'rain screen' for simplicity. This forms the outside protective layer against the weather, which could be your outer leaf of a brick & block, metal cladding, or timber cladding. Behind this, you'll typically design in a way for any water ingress to get back out of the building. Typically this is some kind of free space. However,it may  also be achieved through use of building materials like a wood fibre external wall insulation with a lime render where is moisture does penetrate the outer skin, the material can buffer that moisture ingress and then naturally dry out when the weather improves.

 

With a timber frame, which will typically sit behind such a screen,  build method and detailing is important. The traditional and basic way to build up a timber frame is, from the inside:

 

Plasterboard
Vapour check - usually a membrane sometimes just a plastic sheet sealed at the seamsand sometimes a more high tech membrane that has a variable vapour check behaviour.

Timber studs with insulation between the studs
Frame sheathing - OSB/Plywood
Breather membrane
ventilation cavity - minimum 25mm

outer skin.

 

Now, there's a myriad of variations to this so this is just a basic illustration.

 

The vapour check prevents warm moist air entering the timber frame/insulation layer. If it was allow to get in there, at some point the wall temperature would be cold enough to allow the warm moist air to condense, which would cause damp within the frame. The OSB/Plywood and breather membrane are there to allow some moisture to pass from within the frame to the outside, to prevent a harmful buildup of moisture while also providing some protection against ingrees of excess moisture from the outside.

 

The problem as has been found in many timber frame buildings built in this traditional way is that the vapour check gets pierced during 1st and 2nd fix and not sealed up properly again. This allows the vapour to pass into the frame, causing excess moisture through condensation that can't be dealt with by typical insulation materials like glass wool. This moisture stays in the frame and causes rot over a long period of time.

 

So your basic protection for the timber frame is proper detailing of the vapour check layer and your external membrane.

 

With materials like wood fibre and sheepswool, for example, research has shown that because they're hygroscopic, they can soak of excees moisture in the frame (glass wool can't so the moisture condenses onto the material) and then pass it out to the atmosphere to even out humidity levels (a little like a hot item will cool down to atmospheric temperature). Because these material regulate this moisture in the frame better, it is better for the frame and actually reduces moisture levels with the frame long-term. This type of system build is approached slightly differently but again detailing is key.

 

I've attached a document by BRE that talks about these natural insulations and how they can prevent bacteria and mould build up. The paper focusses more on vapour permeability but hygroscopicity is just as important.

 

 

BRE Information Sheet Natural Fibre Insulation - IP18_11.pdf

[Mr Punter]

9 minutes ago, goatcarrot said:

I don’t understand this comment, what’s wrong with TF compared to brick and block, do you consider it inferior?

 

Sorry.  @tonyshouse built his place using brick and block and is a very strong advocate.  It is worth having a read as his place was very carefully detailed and performs very well.

 

I don't have a preference for build method.

[goatcarrot]

9 hours ago, Mr Punter said:

 

Sorry.  @tonyshouse built his place using brick and block and is a very strong advocate.  It is worth having a read as his place was very carefully detailed and performs very well.

 

I don't have a preference for build method.

Ok no worries, got you.

what’s the best way to find his content, sorry not familiar with the site yet

 

[joe90]

Try this. http://tonyshouse.readinguk.org

[SimonD]

15 hours ago, joe90 said:

Try this. http://tonyshouse.readinguk.org

I'm most impressed by his ground heat bank. I played around with building a prototype out the back of my old house about 15 years ago but life, children and other things got me distracted so never developed it to work very well at all so had to bin it all. Wish I get the funds and time to try again someday.

[SteamyTea]

2 minutes ago, SimonD said:

never developed it to work very well at all

Probably because in the UK, the climate climate is against you.

[SimonD]

24 minutes ago, SteamyTea said:

Probably because in the UK, the climate climate is against you.

Yeah, that's usually the assumption although with sufficient collector area (a lot),  storage size and energy efficient house, many UK locations receive enough solar radiation to provide enough energy. There also isn't actually that much high quality research available specifically for the UK. My main problem was that I made the whole system myself - flat plate collector and storage tank included - and I made a lot of mistakes. Essentially they were pretty crap and I need to rebuild the whole thing.

 

There's an interesting piece of research published on Science Direct in about 2018 looking at this which confirms there is some feasability of these systems. I don't want to hijack this thread but can dig out the link and post if you're intersted.