Managing moisture the key to healthy internal wall insulation retrofits of solid walls

Transcription

1 Managing moisture the key to healthy internal wall insulation retrofits of solid walls Joseph Little- MRIAI, BArch, MSc Arch AEES Beñat Arregi - BArch, EHU/UPV 10 North Great George s Street Dublin, Ireland Tel: consult@buildinglifeconsultancy.com 17 th International Passive House Conference April 2013, Frankfurt am Main

2 Introduction Internally insulating solid walls to the Passivhaus standard: how appropriate is it? We focused on lower cost internal insulation options: Cellulose blown through a gauze: low-carbon approach Cellulose with an AVCL: best practice approach PIR with foil face taped: commercial approach Using numerical hygrothermal simulation (under EN 15026), we assessed: 48 variations of a solid brick wall build-up using WUFI Pro 18 variations of a built-in joist end detail using WUFI 2D associated risk of mould growth on original substrate? greater likelihood of timber decay at built-in joist ends?

3 1D simulation outputs summary We ve established what appears to be the maximum effect changes in each variable have on relative humidity and thus mould risk at original plaster finish behind IWI Variations Cases assessed Max. RH Masonry thickness 275 mm / 447 mm 3% Amount of driving rain London climate / Dublin climate 9% Water absorption of masonry kg/m² s / 0.11 kg/m² s 41% Internal insulation type cellulose / cellulose + AVCL / PIR + foil 17% Insulation thickness 50 mm / up to U-value of 0.15 W/m²K 13% Key findings: a) The variables that cannot be changed (masonry thickness and external climate) appear to be the least significant b) The water absorption of the masonry ( a-value ) is by far the most significant factor c) No build-up is successful on high absorption substrates unless the uptakeof driving rain is reduced (e.g. by means of a render or impregnation) d) The type of internal insulation is also a major issue

4 1D simulation outputs: risk of mould growth on original substrate Mould growth is likely to occur if RH at a surface exceeds 80% for a prolonged period

5 1D simulation outputs: risk of mould growth on original substrate The impact of a low absorption outer surface (due to an impregnation, good external render or rainscreen-blue vs. orange) is dramatic for vapour-closed strategies; and has a smaller (yet still very significant impact) for a vapour-open strategy

6 1D simulation outputs: risk of mould growth on original substrate An IWI buildupfeaturing an AVCL or vapour-resisting PIR boards appears to be appropriate onlyfor low absorption substrates (blue). For high absorption surfaces (orange), cellulose with no AVCL achieves the best results

7 1D simulation outputs: risk of mould growth on original substrate For low absorption surfaces (blue), AVCLsappear to be advantageous where higher (i.e. non-ph standard) U-values are used (though a sheltered climate allows more latitude) But 2D simulations for joist ends add further complexity to this ìssue

8 2D simulation outputs: risk of timber decay at built-in joist ends 2D simulations of built-in joist ends paint a more restrictive picture than 1D simulations

9 2D simulation outputs: risk of timber decay at built-in joist ends Timber should not exceed 20 mass-% of moisture during a prolonged period The fibre saturation point (~25 30 mass% in most timbers) is a clear threshold for decay

10 2D simulation outputs: risk of timber decay at built-in joist ends Location plays an essential role:for low-absorption walls (blue), moisture content at joist ends doubles in Dublin, compared to the same cases in London

11 2D simulation outputs: risk of timber decay at built-in joist ends A vapour-open approach (with no AVCL) appears to be the best option for protecting joist ends in all cases

12 2D simulation outputs: risk of timber decay at built-in joist ends Buildupsfeaturing low absorption & AVCLs, which looked acceptable in 1D, can reach critical moisture levels at joist ends (especially in severe climates)

13 2D simulation outputs: risk of timber decay at built-in joist ends Insulation thickness is critical(light vs. dark blue), particularly in wet climates: it can often separate failure from success

14 Conclusions When internally insulating solid walls, dealing with moisture uptake from driving rain is the most critical issue. There are standard ways to measure this uptake (using Karsten or Rilem tubes): this should be promoted The presence or absence of a vapour resisting layer (whether integral to insulation or an AVCL), which may reduce vapour ingress from room but inhibits the drying out of the solid wall substrate, may be the most significant question to ask about the insulation strategy For high absorption surfaces, a vapour-open approach achieves the best results. Onlywhen used with low absorption exterior surfaces do AVCLsappear useful in reducing risk of mould growth, but not useful in preserving built-in joist ends For protecting joist ends, an air-tight but vapour-open approach appears to be the best option in all cases Ireland has perhaps three exposure zones, none of them as sheltered as the London region, or indeed Frankfurt am Main. Insulation strategies (whether green roofs, pitched roofs or internal wall insulation, etc.) need to be proven for each climate they re used in: one size does not fit all

15 Managing moisture the key to healthy internal wall insulation retrofits of solid walls Joseph Little- MRIAI, BArch, MSc Arch AEES Beñat Arregi - BArch, EHU/UPV Thank you 10 North Great George s Street Dublin, Ireland Tel: consult@buildinglifeconsultancy.com 17 th International Passive House Conference April 2013, Frankfurt am Main