Timber frame design principle ponderings…

[G and J]

Ok.  I’ve now spent many hours pondering this topic for our forthcoming build.  And something fundamental has been bothering me.

 

I don’t understand humidity as much as perhaps I’d need to to know to answer my own question, but it seems to me that UK building practice is based on the assumption that most houses don’t have MVHR and so have internal relative humidity (RH) much higher than external RH, so we put the vapour control layer (VCL) near the inside to stop our steaminess rotting our frame.  

 

But….

 

What if I put a big fat layer of non breathable PIR insulation outside of a super airtight VCL, then put the frame and maybe some breathable and compressible (maybe rockwool) insulation in frame inside that.  Plasterboard straight onto the frame, service void courtesy of the frame - no vapour permeable membrane needed?  Airtightness provided by a continuous and well protected VCL. 

 

My frame stays reasonably warm all winter, and as I’ve got MVHR keeping my humidity at reasonably low levels it stays dry enough not to rot.

 

What have I missed? Why wouldn’t this be a good idea?

[IanR]

1 hour ago, G and J said:

but it seems to me that UK building practice is based on the assumption that most houses don’t have MVHR and so have internal relative humidity (RH) much higher than external RH, so we put the vapour control layer (VCL) near the inside to stop our steaminess rotting our frame.

 

It's not (directly) related to MVHR, it's simpler. With all else equal, RH increase as air temp reduces. In the UK we generally want the internal temp higher than the external temp. If warm vapour gets into the a building's walls, as it moves through the wall from inside to the outside it's temperature will drop and the RH will increase. If/when the RH reaches 100% the dew point is met and it will condensate. In a timber structure, if condensation occurs within the structure and its not able to dry out, the structure will rot. Even if it doesn't rot, the insulation performance will dramatically reduce.

 

The Vapour Check Layer is to reduce the amount of vapour getting into the structure. Additionally it's typical to also ensure that vapour permeability of construction layers increases from inside to outside to aid the water vapour natural movement out of the structure. So VCL towards the inside and progressively more vapour open materials as you move out.

 

1 hour ago, G and J said:

Airtightness provided by a continuous and well protected VCL.

 

Don't confuse airtightness with the VCL. The air tightness layer will happily exist within the outside construction layer, as long as that layer is vapour open. It is also fine to combine the air tightness and VCL on or close to the inside layer, as a self-builder you'll ensure that cold air can not breach the insulation and contact the back of the plaster board (or air tightness layer), but volume builders aren't always so successful.

 

While it's good to understand the physics, hopefully you plan get some professional support with your wall/roof build up.

Edited July 19 by IanR

[Nick Laslett]

This is a real can of worms, but I would suggest that you are basically describing an External Wall Insulation (EWI) system. 
 

With EWI you move the cold layer further towards the outside of the house, reducing the condensation problem. 
 

https://ewipro.com/services/system-build-ups/mineral-wool-insulation-system-build-ups/#mineral-wool-timber-frame

 

 

 

Weber have specifier tool for U0.15 wall. 
 

https://www.uk.weber/facades/selector

 

 

[G and J]

21 minutes ago, Nick Laslett said:

This is a real can of worms, but I would suggest that you are basically describing an External Wall Insulation (EWI) system. 
 

With EWI you move the cold layer further towards the outside of the house, reducing the condensation problem. 
 

https://ewipro.com/services/system-build-ups/mineral-wool-insulation-system-build-ups/#mineral-wool-timber-frame

 

 

 

Weber have specifier tool for U0.15 wall. 
 

https://www.uk.weber/facades/selector

 

 

Good grief, in hindsight it’s obvious that in many ways I’m describing EWI.   So there must be known and defined practices including near to boundaries.  

[G and J]

25 minutes ago, IanR said:

Don't confuse airtightness with the VCL

I’m not, but as you point out, the VCL can be the airtight layer, in this concept a nicely protected one.

 

26 minutes ago, IanR said:

It's not (directly) related to MVHR

The presence of MVHR would appear to deliver lower humidity internally.  I guess my question could be rephrased as “if we start from that premise can we build thinner and more thermally and cost effective walls?”

[IanR]

Just now, G and J said:

The presence of MVHR would appear to deliver lower humidity internally. 

 

Yes, but so does a leaky structure that is exchanging warm moist air with colder air. It's the function of the cold air being warmed up that reduces the RH, not the MVHR.

[Nick Laslett]

14 minutes ago, G and J said:

Good grief, in hindsight it’s obvious that in many ways I’m describing EWI.   So there must be known and defined practices including near to boundaries.  

The Weber specifier tool has 2 fire specs

 

A2-s1,d0 reaction to fire, B-s1,d0 reaction to fire. 
 

Searching “ewi a1 fire rating”, seemed to bring up a lot of hits. 

Edited July 19 by Nick Laslett

[JohnMo]

26 minutes ago, G and J said:

presence of MVHR would appear to deliver lower humidity internally.

You think so and are lead to believe that, but it's not exactly true.

 

Our house has MVHR and our heated summer house dMEV. Both have a sound ventilation strategy. As an example mid winter humidity on 16 Jan

 

House

 

Summer house

 

Both are heated to a similar temperature, and the humidity is at a similar figure, with the dMEV slightly lower.

Edited July 19 by JohnMo

[G and J]

OK, I am very ready to accept that my thinking is muddied, and very slowly clears through our debate, so apologies for seemingly daft assertions and questions…. but…

 

Our houses are designed to withstand peeps who don’t have MVHR and don’t ventilate, those happy with condensation and mould and fug.  Such houses are designed to dry outwards for good reason.

 

But my house won’t be like that so why can’t my frame dry inwards?

[Iceverge]

2 minutes ago, G and J said:

But my house won’t be like that so why can’t my frame dry inwards?

 

Of course it can, otherwise every car and train in the land would be full of mushrooms! 

 

Good luck finding a certifier willing to stake their indemnity insurance on the workmanship on a building site and the variable ventilation behaviours of the general public though. 

 

[G and J]

2 minutes ago, Iceverge said:

 

Of course it can, otherwise every car and train in the land would be full of mushrooms! 

 

Good luck finding a certifier willing to stake their indemnity insurance on the workmanship on a building site and the variable ventilation behaviours of the general public though. 

 

And perhaps that’s the answer to ‘why not?’

 

Shame that potentially good engineering (which I’m not sure if inward drying actually counts as, mind) has to conform to such norms.

[IanR]

2 minutes ago, G and J said:

Shame that potentially good engineering (which I’m not sure if inward drying actually counts as, mind) has to conform to such norms.

 

It's not good Engineering, it's attempting to work against physics.

 

Vapour flow travels from a high vapour pressure to a low vapour pressure.

 

Vapour pressure is a measure of water saturation in air, ie. absolute humidity rather than relative. Colder air outside is able to carry less moisture than warm air inside. In most circumstances, humans living inside a house put more moisture into the inside air than the climate does outside. So, generally there is a higher vapour pressure internally than externally. Vapour, as do gasses, moves in the direction from high pressure to low pressure, until equilibrium is achieved.

 

The natural order of things is for the vapour with the higher absolute humidity to move towards that with the lower. The majority of time in our climate that is in to out. If you then try and interrupt this by not stacking your materials progressively more vapour open from in to out, you will either slow down or block altogether the vapour flow.

[Iceverge]

I have come across external layers of PIR on TF structures like this one but with an active vapour control layer included. 

 

https://passivehouseplus.ie/magazine/new-build/longford-self-build-goes-certified-passive-on-a-budget

 

They in effect dry inwards. 

[JohnMo]

50 minutes ago, G and J said:

Our houses are designed to withstand peeps who don’t have MVHR and don’t ventilate, those happy with condensation and mould and fug.  Such houses are designed to dry outwards for good reason

Not that sure it makes any difference. By the time most houses have had a couple of coats of emulsion the building structure is pretty much separated from the internal environment (except for air leakage, which comes from inside or most likely outside). So MVHR or no ventilation the structure needs to perform on its own merits.

 

The mould and fug come from two things, cold bridging and/or poor ventilation. Not really related to a rotten structural frame.

[Iceverge]

27 minutes ago, IanR said:

It's not good Engineering, it's attempting to work against physics.

 

Vapour flow travels from a high vapour pressure to a low vapour pressure.

 

Surely all that matters is that no localised condensation occurs.

 

From this point of view I think EWI is beneficial, even one of low vapour permeability, as it keeps the structure above the dew point, therefore no condensation. 

 

In any case if you have a good airtighess layer, no internal vapour of note will get anywhere near the dew dew point of the structure regardless of the vapour permanently of the materials.

 

Moisturise damage through diffusion just doesn't happen, it's all through poor airtighess. 

 

 

 

 

 

 

 

 

 

 

 

 

[IanR]

42 minutes ago, Iceverge said:

They in effect dry inwards. 

 

Where about does it indicate that? It's a big document to read on my phone.

 

22 minutes ago, Iceverge said:

Surely all that matters is that no localised condensation occurs.

 

Even localised condensation can occur, as long as there's sufficient drying capability for it not to stay wet long enough for mould to grow. Accepting that wet/damp insulation is ineffective.

 

22 minutes ago, Iceverge said:

From this point of view I think EWI is beneficial, even one of low vapour permeability, as it keeps the structure above the dew point, therefore no condensation. 

 

Above the dew point in a constantly heated home. What if heating is periodic,  ie. mornings and evenings only.

 

However, I agree that a low vapour permeable EWI can work. I considered PIR sheet external to SIP (before I saw the light) and the SIP supplier did the WUFI analysis before they'd approve it... and it passed. But perhaps that was helped by minimal moisture getting into the structure.

 

There maybe sufficient drying capability in the structure, there may be sufficient evaporation that condensation dries quickly enough. For me though, no condensation risk is better. But if there is, and it's a non-standard build up that's not already been analysed you need to get the WUFI analysis done as Building Control will/should want to see it.

Edited July 19 by IanR

[MikeSharp01]

This whole topic is frightening - only thing that let's me sleep nights is the wufi analysis we paid for. All technical membranes high / low vapour permeable materials makes my head spin.

[G and J]

15 minutes ago, MikeSharp01 said:

This whole topic is frightening - only thing that let's me sleep nights is the wufi analysis we paid for. All technical membranes high / low vapour permeable materials makes my head spin.

On the bright side, I’m already in my seventh decade, and whilst in decades to come I might ask one of my carers to ladle some more polyfilla into the latest soggy corner of my rotting from the inside timber frame (by then) old fashioned house, I’ll not be around to see any really big bits crumble.  That’s got to be good news.   🙂

[G and J]

2 hours ago, IanR said:

 

It's not good Engineering, it's attempting to work against physics.

Should I infer from this that you are asserting that frames can’t dry inwards?

[MikeSharp01]

I am sure I don't know. Isn't that what the trick membranes are for in the modern era?

[Iceverge]

1 hour ago, IanR said:

Where about does it indicate that? It's a big document to read on my phone.

 

 

If you look towards the end of the article it shows the wall buildup. 

 

100mm block

50mm vented cavity 

50mm PIR

Breather membrane 

11mm OSB

220mm studs full fill with mineral wool. 

Intello Membrane

100mm service cavity with mineral wool 

Plasterboard and skim. 

 

Any moisture, even from construction, won't be drying outwards through a PIR sheet with two foil facings. It will have no choice but to go inwards through the Intello. I expect the buildup was approved on the basis that as a passivhaus the membrane sealing should be excellent. 

 

1 hour ago, IanR said:

Even localised condensation can occur, as long as there's sufficient drying capability for it not to stay wet long enough for mould to grow. Accepting that wet/damp insulation is ineffective.

 

I would prefer no condensation in my wall and a continuous layer of EWI is a nice method to achieve this as it keeps the dew point outside the structure.  It also makes a tremendous difference to the structures U value by mitigating thermal bridging. 

 

1 hour ago, IanR said:

 

 

 

 

 

 

 

1 hour ago, IanR said:

Above the dew point in a constantly heated home. What if heating is periodic,  ie. mornings and evenings only.

 

 

Either the house cools rapidly through uncontrolled ventilation in which case there's no differential vapour pressure or else when unoccupied there's no moisture being added internally. I don't see the realistic issue. 

 

1 hour ago, IanR said:

However, I agree that a low vapour permeable EWI can work. I considered PIR sheet external to SIP (before I saw the light) and the SIP supplier did the WUFI analysis before they'd approve it... and it passed. But perhaps that was helped by minimal moisture getting into the structure

 

These calculators assume that there's zero faults in construction and zero construction moisture. I would be wary of the layer of OSB sandwiched between the insulation and the PIR foil facing. 

 

In any case PIR performs poorly at low temperatures regarding its thermal resistivity. Some like EPS would be just as good on a freezing winters day. However it's susceptibility to fire has moved me to prefer rockwool or woodfiber. 

 

 

 

[Iceverge]

1 hour ago, IanR said:

But perhaps that was helped by minimal moisture getting into the structure.

 

 

This is a massively important point.

 

Good airtighess is hugely hugely important to stop moisture laden simply blowing through cracks and condensing inside the wall.  

 

When you have good airtighess and planned ventilation you can get away with murder!

[IanR]

2 minutes ago, G and J said:

Should I infer from this that you are asserting that frames can’t dry inwards?

 

There are no absolutes, so I've not been so definitive in what I have said.

 

If there's sufficient air leakage though the structure, allowing plenty of evaporation, it could dry inwards. If it's warm and humid outside and there's good cooling internally with low absolute humidity, then it's likely to dry inwards for a period.

 

As building performance improves though, with lower air infiltration and better insulation, "The natural order of things is for the vapour with the higher absolute humidity to move towards that with the lower", which in our climate is generally in to out.

[IanR]

1 hour ago, Iceverge said:

 

If you look towards the end of the article it shows the wall buildup. 

 

100mm block

50mm vented cavity 

50mm PIR

Breather membrane 

11mm OSB

220mm studs full fill with mineral wool. 

Intello Membrane

100mm service cavity with mineral wool 

Plasterboard and skim. 

 

 

I've just done inside out to the vent cavity in Ubakus, and had to make some approximations, but chose products that were close in vapour permeability terms. 

 

It tries to dry outwards, as I would expect it to, and Ubakus highlights a risk at the foiled PIR (PUR in my build up below). Insufficient drying reserve.

 

I can't select a sd=7.5m VCL, so used a 10m. I assume they used a higher def condensation analysis tool that showed something different.

 

 

 

 

 

 

 

Edited July 19 by IanR
Changed images - missed the Service cavity was insulated

[G and J]

10 hours ago, Iceverge said:

100mm block

50mm vented cavity 

50mm PIR

Breather membrane 

11mm OSB

220mm studs full fill with mineral wool. 

Intello Membrane

100mm service cavity with mineral wool 

Plasterboard and skim. 

So, I’m trying to understand the logic behind this build up.  
 

A skin of PIR externally (carefully taped?) presumably does a brilliant job of alleviating cold bridging, and means the frame has to dry inwards.  
 

Presumably the breather membrane is to keep rain out, but could be non breathable given it’s proximity to the non breathable PIR.

Is the intello membrane primarily for air tightness?