Deliberately poor U-Values for permeable IWI?!

[JonoMarshall]

We've sold our Passivhaus and are now renovating a larger victorian terrace property, our previous learnings don't seem to be translating to a solid wall victorian conversion!

We're considering permeable (no gap) timber fibre IWI as we're in a conservation area and can't do EWI. We have solid walls and are currently removing the wallpaper from lime plaster.

Pavatex said that reducing U-values beyond 0.4 was inadvisable as would have too significant effect on rate of drying of the (exterior) wall, hence frost damage. This seems to be confirmed by Jospeh Little in Breaking The Mould: "It seems reasonable to deduce from this that once driving rain is dealt with internally-insulated walls appear to work best with less insulation than 0.45 W/m2K" https://www.buildinglifeconsultancy.com/­content/breaking-mould-5 (link not secure)
 

However, I wondered if there was an alternative to accepting a worse U-value (for IWI) by reducing humidity (heat recovery from bathrooms for example) as the improved interior environment might provide better external wall performance during dry spells? Or is this madness?

Should I be less focussed on U-values?

Edited November 16, 2021 by JonoMarshall

[ProDave]

I think that is a technical analysis of an issue I have observed before.

 

A plain brick wall of a house will absorb moisture from rain.  In a typical house there is enough heat leaking out through that wall to keep the surface temperature of the wall above 0.

 

Insulating the inside too well results in the outer surface of the wall getting below 0, and any moisture in the wallfreezing, expanding and the bricks failing.

 

This is very noticable with garden walls, that have no heat escaping through them, and the fail due to frost damage.  A near neighbour has re rendered his garden wall several times and within 2 years the render has failed and fallen off due to this effect, that would not happen with a house wall.

[SteamyTea]

It takes a lot of energy to evaporate water from a brick.

So anything that stops it getting into it, the better.

If you create a dry atmosphere  inside, as long as it is dryer that the partial vapour pressure that the inside brick temperature governs the evaporation rate, forced ventilation will help.

 

Is there not a treatment that can be applied to bricks to stop them absorbing moisture.

 

 

Edited November 16, 2021 by SteamyTea

[Gordo]

43 minutes ago, SteamyTea said:

It takes a lot of energy to evaporate water from a brick.

So anything that stops it getting into it, the better.

If you create a dry atmosphere  inside, as long as it is dryer that the partial vapour pressure that the inside brick temperature governs the evaporation rate, forced ventilation will help.

 

Is there not a treatment that can be applied to bricks to stop them absorbing moisture.

 

This is a very interesting subject where the modern drive for thermal performance is applied to a solid wall masonry walls. I have considered this and have read some guidance of refurbishment works on the subject. My take on it is that so long as the interstitial condensation occurs closer to the exterior of the wall there should be no detriment to the wall. Spalling in a facing brick should only be a concern if the mortar mix is too strong or it is totally saturated like a half brick garden wall.
 

I have seen mould issues behind IWI though which concerns me. I would strongly advise against external sealer which may trap moisture from naturally evaporating.

Edited November 16, 2021 by Gordo

[Gordo]

If anyone wants some practice advice on renovating an older property here is a link to a guide from the energy saving trust. While it may be simplistic it is invaluable 

 

http://inglehome.co.uk/wp-content/uploads/2015/07/CE184-practical_refurbishment_of_solid-walled_houses.pdf

[Iceverge]

0.45 isn't too bad, especially in a terrace. 

 

Have you run done a run through PHPP or Jeremy's heat loss spreadsheet here ? It's pretty good and will give you a feel for the balance of varying U values. 

 

Concentrate on MVHR, airtightness and high performance windows and you'll be fine. 

[Gordo]

3 hours ago, Iceverge said:

0.45 isn't too bad, especially in a terrace. 

 

Have you run done a run through PHPP or Jeremy's heat loss spreadsheet here ? It's pretty good and will give you a feel for the balance of varying U values. 

 

Concentrate on MVHR, airtightness and high performance windows and you'll be fine. 

Yeah it’s not the best but as mid terrace not end of world. Id suspect high ceilings which would leave all the heat at ceiling level likely a problem but not likely to want to change that feature. I’d concentrate on draughty proofing, insulating roof then floor then windows followed by PIV ventilation when reasonably draft proofed. Retrospect MVHR would be prohibitively expensive but better than PIV. Walls would be next on list 

Edited November 16, 2021 by Gordo

[SteamyTea]

7 hours ago, Gordo said:

I would strongly advise against external sealer which may trap moisture from naturally evaporating.

There is liquid water tight and vapour tight, different thing, should have been a bit clearer about that.