Insulating (internally) a 1930s solid brick wall

[Ben Blaukopf]

I would like to insulate a solid brick wall in a room composed partly of solid walls with lime mortar (and probably plaster) and partly of cavity walls in the extended part.

Googling suggests that

a) Allowing lots of air flow past a (now colder) external solid wall will cause damp problems, mould, and the house will disintegrate, and zombies will appear. So doing anything that allows air flow later past the bricks is a Bad Thing.

b) I ought to use breathable material throughout to allow vapour to escape out of the building.

Now, I can see point a). Point b) though, seems a bit odd. Modern houses use non-breathable plaster, and have trickle vents. I have replaced the window in this room with trickle vents. Why do I need breathable material?

What I would like to do is batten out the solid wall, stick a bit of board insulation between the battens, then VCL, plasterboard, skim layer. Partly because this will make the finished surface of the solid wall flush with the finished surface of the cavity wall.

I would also like to do the same on the bay windows which are only single skin. There are no trickle vents on those windows, though of course I could fit some (and probably ought to anyway!).

[The Natural Philosopher]

No, its a GOOD thing. As long as te air isnt internal moisture laden air, but proper deep breathes fresh outside air.

No, a brick wall IS breathable

You dont. The modern solution to insulation ventilation and damp is to build an airtight shell inside the structure, punctrured with specific vents to the outside, and then make that shell moisture proof, before adding insulation round it, and then, finally a structural wall/floor/roof, whch is, from the insulation outwards, permeable, so that water from OUTSIDE doesn't build up in it.

Sounds perfectly fine to me. You may be able to get foil and celotex or kingspan backed plasterboard that you can just glue to the inside of the solid wall. Its not cheap, but it saves a lot of hassle. And does a better job than battens - no cold bridges

Trickle vents are pretty superfluous unless you really have a house that is hermetically sealed.

[Theo]

No, the airflow past the outside dries off moisture. Some traditional architecture is designed to optimise the airflow, especially on north faces that don't get sunshine.

In a house in the winter it's warmer on the inside than the outside. People inside breathe out moisture (and fires, cooking and bathing) which goes into the air because it's warm. In a traditional house, the walls are permeable. This differential temperature/moisture profile pushes the moisture through the walls and is dried off by the wind outside. All is well. Similarly when it rains and the wall is soaked, the moisture has a letout the other way. You end up with a smooth moisture profile because it can always equilibrate with the layer next to it.

Modern finishes, synthetic insulation and paints aren't breathable, so what happens when somebody applies that to a traditional wall is you end up with a barrier in the wall where moisture gets trapped halfway. Then there's nowhere for it to go, causing a damp problem.

Basically, either you seal everything up so the moisture doesn't enter the wall (modern finishes, vapour barriers), can escape (vented cavity walls) or you make everything breatheable so the moisture doesn't get trapped.

I'd go to ubakus.com (demo mode) and create the stackup you want, including your current wall construction. See what it reckons about condensation within the wall. Adding insulation can make damp problems worse because now things are colder than they were before, causing more condensation - that's why it's worth modelling it first.

If you want to keep the wall permeable, wood fibre insulation seems quite good and is breatheable.

some useful videos on installation and a training course:

Trickle vents are mostly about room air quality (CO2 and VOC buildup) - they don't do a lot for moisture in the room.

Theo

[Fredxx]

Condensation occurs where warm moist air passes over colder bricks.

No, you want a barrier between the warm moist air and anything colder to stop condensation. That is why you get foil backed plasterboard.

Warm air with the same moisture content as the outside cold air won't from condensation. Unless it's raining.

That is an effective way of improving insulation.

[John Rumm]

With traditional walls moisture was expected to pass through in both directions. Having some heat inside the building kept that side relatively damp free, and any condensing of water from inside moist air would occur somewhere inside the wall - probably nearer the outside - making that somewhat damp. Air flow on the outside combined with some heat leakage from the inside would help this evaporate away.

The more permeable lime mortar would tend to carry most of the water, so the bricks would tend to stay drier (and less likely to suffer spalling from frost damage - at least up until someone re-points them with a cement based mortar!)

In cases as described above the whole thing is to an extent breathable.

The problems can start to arise when an impermeable layer is applied to the outside (say a cement render). While that stops water getting in, it also traps water trying to get out.

Modern houses will usually try to stop water flow in both directions, and will try to ensure that moist air from inside can't reach cold areas of masonry. This means you need a vapour control barrier on the warm wet side of the insulation.

The diagram here shows it quite nicely:

So long as you have a vapour barrier on the warm side of the insulation, moisture ought not be able to get through to the (now colder) masonry, and hence condense in the wall.

[RJH]

Why do you need a VCL if you're using impermeable insulation materials?

[Brian Gaff]

I don't know but the problem seems to me that this is very hard if like myself there is stuff screwed to the walls. I'd prefer the fix to be outside. Some kind of cladding that insulates. However the powers that be are all talking about knocking these type of houses down and rebuilding with more insulation. I hope I'm gone by the time they can just take your paid for house and shove you somewhere else in a modern house. Brian

[The Natural Philosopher]

Bollocks. I lived in a single 4" thick thick brick house. The water used to run down the walls inside in winter.

You cant get they water out as fast as its put in, especially in a bathroom.

We dont sew ourselves into our underwear for winter any more.

You make everywthing breathable by opening a window.

But of course you dont. There is a reason why modern houses are not made breathable except with especially calculaed ventilation - trickle or otherwise.

Bollocks. They are strictly defined to allow the rotation of air within the building, and nowhere is CO2 buildup mentioned in the regs or VOCs. They are there to remove moisture mainly. Submariners regularly go to CO2 levesl 100 times those in air.

In the limit trickle ventilation is the largest form of heat loss in the house, which is why heat recovery ventilation is necessary for really passive houses.

The modern system of seal, insulate, and ventilate precisely is in fact the best way to keep a house warm dry and have decent air quality inside.

All the old fashioned methods may just about work with an open coal or wood fire. But they are not as good.

[The Natural Philosopher]

The short answer is you dont. VCLs are normally applied outside as short term barriers to heavy rain that nevertheless can still evaporate any that gets past, long term Or in a roof as a windproof layer that can still 'breathe'

[RJH]

Retrofitting ventilation is something I don't see much - or at all.

I understand the 'open a few windows', but how to achieve the balance between a healthy home and minimising heat loss?

And are there any real world examples of retrofitting heat recovery ventialtion systems?

[RJH]

On external cladding - yes - far easier if it can be done. Why where it's possible isn't it done? A friend has just had external cladding fitting to all three sides of her end-terrace FOC under some grant or other. But the cost isn't trivial for those of us that would have to pay, and certainly needs to be thought about in terms beyond economic returns.

[The Natural Philosopher]

I think that has been done, but until and unless you have gone for the 'hermetically sealed triple glazed, 12" thick celotex' in floor walls and ceiling, then the losses from trickle vents are not going to be significant.

In short, as you go for ever greater levels of insulation, different items become the dominant heat loss. Once you have draught proofed a house an insulated roof comes next, after that the walls are an issue, once you have done those the floors are an issue, and when those are sorted your dominant heat loss is ventilation.

So you wouldn't 'retrofit' HRV until and unless you had 'retrofitted' all the above.

And, frankly, it would have been cheaper to knock the bastard down and start again.

[RJH]

:-) Ta, noted.

[John Rumm]

That would assume the insulation materials are impermeable... If just installing it between battens then you have a break in the continuity of the VCL at each batten even if the insulation board itself is foil faced. (you also have a cold bridge at each batten)

If you were using foil backed PB, then you would not need an additional VCL.

Personally I would stick the insulation board to the wall, then batten over it, and fix the PB to the battens - that eliminates the cold bridge and gives a continuous VCL on the warm side of the wall.

[Mathew Newton]

My house! ;-)

We extended the ground floor and made an open plan kitchen/diner/lounge area, and one thing that my wife was keen on was that the kitchen didn't look too 'kitcheny' in what would become our main living space so we looked at avoiding the use of any wall cupboards and also a conventional extractor hood that we'd previously had... The latter sowed a seed that led to me exploring retrofitting a whole-house MVHR system which, looking back, was a hell of a lot of work however the end result has been fantastic. Warm fresh air throughout, towels and wet washing drying quickly indoors, lower dust levels and a slight cooling effect in certain conditions in the summer. As I say though, hard work and that's even with the opportunity of having ceilings down, having a house built with I-beams running in a fortunate direction etc that we had.

[Theo]

How airtight is your house? I've occasionally seen them go by on ebay for not-too-much, but wondered if having a chimney, a leaky loft etc might mean there's not as much point.

What was the major work? Ductwork to all the rooms?

Theo

[Animal]

Discussions of permeable materials and trickle vents are not relevant to interior wall insulation. Put up polystyrene or PIR sheet, skip the battens unless you want to hang very heavy stuff on the wall. Cover with foil unless the insulation is already foiled, plaster & skim.

[Mathew Newton]

Hard to quantify as whilst it was built in 2007 and so ought to be reasonably air tight in terms of expected spec it was built by Persimmon and so its anyone’s guess how much practice might differ to theory! Subjectively speaking however I’ve found the house to be very well built and no obvious major leaks, and no chimneys either. Many say/assume MVHR is no use in anything less than a Passivhaus but I can categorically say this is not true.

Yeah pretty much. As like most installations I used 63mm semi-flexible radial ducting and whilst it’s fairly straightforward to use it was still quite a task routing one to every (or most) rooms and then bringing it all together in the loft to the manifolds connected to the main unit, which as it happens I did buy off eBay as surplus stock.

[RJH]

Um. How would you fix the battens without cold bridging?!

And while I have stuck insulation to one wall in my house, then battened over, I do wonder how long the glue keeps its 'stick' . . .

[RJH]

Apparently it makes for a very comfortable home - never experienced it myself though.

Impressive! It'd be too much in my Victorian terrace - although creative use of the unusing chimney stacks might be one solution.

Shame Passivhaus isn't free, but they've clearly thought about retrofit: