internal insulation of brick walls

[peter smithdale]

Having proposed internal wood-fibre based insulation and lime render to the solid brick walls of a large Victorian listed building, I have been presented with the following objections:

1) it is expensive

2) although the condensation risk calcs indicate there will be some condensation but no annual build-up (due to hygroscopic drying to the inside and outside), any peculiarities (at corners, reveals, other odd details) will result in saturation of the wood fibre board and possible undetected spread of rot throughout the walls and into vulnerable parts of the structure

3) any minor failure of the brickwork allowing rainwater ingress will have similar results

4) any future occupant painting over the wall with the 'wrong' paint will negate the 'breathability' with results as above

5) ditto airbricks being blocked, fans failing etc leading to high humidities in the building

6) why not use thermal laminate on metal framing because, not being in contact with the brickwork, it does allow it to 'breathe'; it is cheaper; it is more effective at preventing moisture from inside the building reaching the walls; and you get better U values for less space loss.

complete heresy I know and whilst the author does not agree with some / all points it does beg the question as to whether perishable internal insulation systems should remain the preserve of highly managed and monitored buildings (National Trust properties, eco-builders own homes) whilst more robust systems should be used in the vast majority of buildings whose occupants do not care about their old buildings nor understand how they work.

[Mark Siddall]

Peter,
1) Agreed. Though if you were to consider ecological costs associated with other insulation i.e. PIR, PUR etc. then chances are that it is cheaper. (I appreciate that it’s hard to convince a client about this though!)

2) Can you design out the problem areas? To vulnerable junctions could you not introduce a membrane taped to the boards that could flash the weak details? Would mitering the joints help?

3) As you are dealing with a well established existing building one would expect that any minor failures allowing rainwater ingress should be have been identified and rectified at some point in the buildings history (any further remedial maintenance would obviously need to be recommended within your scope of works). Given that you are insulating internally the insulation should be well protected from rainwater ingress. Domestic plumbing leaks could be the biggest thread, but as these are unpredictable it would be inappropriate to allow this factor to dictate the selection of insulation type, besides leaks would destroy insulation if they remain unaddressed.

4) 'Breathability' is a problematic concept. Pete Warm has done some analysis of  the diffusion of water vapour through the building envelope (presented at the Slaithwaite AECB Workshop). The calcs demonstrated that the principle means of removing water vapour is ventilation not diffusion. Traditional breathing wall concepts rely upon the vapour barrier being five time less vapour open than the breather membrane. In Canada they use traditional air barrier solutions where the vapour barrier is fifteen time less vapour open than the breather membrane. On this basis provided that the exterior of your building is not painted with an impermeable paint, I can’t see you having any problems.

5) It is suggested that breathing walls would not provide suitable levels of vapour diffusion to satisfy all the requirements in the first place. Adequate ventilation (inc. manual operation) will avoid long term high humidity in the building (thus avoiding mould etc).
Obviously air bricks should not be blocked without performing the appropriate analysis and fans should be maintained. If the occupant does not undertake approach matters with due diligence i.e blocks airbricks, does not maintain fans, then that is their problem and it’s not something that the designer can control (at best they can advise).

6) I don’t quite follow you here “thermal laminate”?

A concern that I’d have would be the cold bridges through the insulation and likelihood that condensation could occur at these points. Assuming impervious internal paint generally there is likely to be a higher vapour pressure. Would lead to greater condensation at these points? The cold bridges are likely to be timber members as a consequence if there is higher internal vapour pressure there is a potential that they could rot. (This problem is a by product of you electing to insulate internally rather than externally.) I believe that adequate ventilation could help to avoid this potential issue but couldn’t say so with great confidence (it’s just a hunch).
 

Surely any truly sustainable system will be suitably robust and offer the required performance. As always it’s a balance and without all the data before us it ends up being something of a subjective one (albeit informed subjectivity).

Mark

[peter smithdale]

Dear Mark,

many thanks for the detailed response.

4) surely a combination of the two - diffusion is necessary to bring the moisture through the materials to the surface where ventilation can remove it?

6) Thermal laminate = sandwich of rigid insulation board and plasterboard. This is spaced away from the wall on metal studs.

I am dealing with a solid brick structure with joists built in to the walls. The floor voids will be a cold and poorly ventilated area (particularly if we are trying to achieve airtightness) and the joist ends both structurally essential and vulnerable to condensation. My feeling is the thermal laminate system is far more robust because it interferes less with the drying out of the original structure. In one way improves the situation by preventing vapour reaching the original structure. The original structure may be colder, by virtue of heat not reaching it, but probably no colder than it was in the days before central heating (which, for this particular building, is now). The floor voids and joist ends will be no colder than the walls and so will be less of a hotspot for condensation; in fact they may be warmer, as it is not possible to insulate in there.

I'm not convinced of the idea that hygroscopic insulation materials are necessarily more friendly to old buildings. I think that insulation is inherently dangerous to old buildings, but we are driven to look for the least risky ways of putting it in. I think the idea that the danger is minimised by the use of material of similar character to old ones seems right but may be wrong, and may be driven by the fact that people who are enthusiastic about insulation and old buildings tend to be the same people who are enthusiastic about natural materials.

[Mark Siddall]

Peter,
4) As I understand it the vapour pressure will alway be from inside to out (during the winter when it is needed most). Thus diffusion to the interior is not strictly necissary (though could be benefitial in the summer).

6) Ah.

Hmmm. To state the fairly obvious I'd be careful that you (or the on site labour) don't block underfloor air bricks in the name of airtightness.... mould and rot could abound. Using the thermal laminate I'd look at where the dewpoint occurs and plan accordingly (goes for any internal insulation really). Unless you get some suprising airtightness I reckon that the internal warm humid air will get into your cavity (esp. at floor/wall junctions...so detail carefully and contol the site).

On the whole I agree. I'm not convinced of the idea that hygroscopic insulation materials either. I can see some of the potential benefits in the arguement but I'm not sold on the concepts.

To my mind the best solution is external insulation as noted above....the issue is getting the planners on board for the change in aesthetics.

Mark

[David OLIVIER]

I totally agree. External insulation is usually a far superior solution. It makes the building structure warmer and dryer, not colder and wetter. It can also be applied to a considerably greater thickness without "getting in the way".

David.

[peter smithdale]

Many thanks for your replies.

The building is listed Grade 2 so external insulation is out of the question.

The floor voids I referred to are upper floors so I don't think they will be ventilated to the outside - a point to watch.

[fostertom]

3" of hemcrete sprayed internally then