What forms of construction are of sufficient risk that assessment using WUFI is needed?

The recently strengthened Building Control Regulations - under S.I. 9 (2014) & ancillary instruments - have highlighted the fact that the Architect and Design Certifier (DC) need to take greater responsibility for the specification of all apects of the construction works (be they new build or retrofit). Clearly some proposed wall, roof and floor buildups have greater hygrothermal risks than others, as do some climates and exposure levels, etc. Those risks need to be evaluated and sober careful judgments formed by experts who are competent, trained users of the relevant type of evaluation tools (underpinned by a good knowledge of building physics) and experience of construction systems and practice. That may be the architect him or herself or an external provider. The architect can then use such reports to influence and support their overall specification and the DC can use it to underpin the relecant aspects of the design certification.  The following are constructions that we understand should only be given desktop hygrothermal risk evaluation using WUFI or Delphin numerical software (under IS EN 15026) not Glaser Method software (under IS EN ISO 13788). The list is not exhaustive:

SOLID WALL & INTERNAL WALL INSULATION

Any buildups where capillary action can deliver liquid water from driving rain deep into a construction such as a solid masonry wall, even more so internally insulated solid walls, and even more again for internally insulated solid walls that have an outer surface that is highly absorptive, such as brick.

Note 1: where the vapour pressure at the centre of the wall is greater than that within the room the presence of a vapour barrier can result in greater mould and freeze-heave risk, as it will reduce the solid wall’s ability to dry to the room (as it did before the retrofit).

Note 2: the water absorption characteristics of the outer surface (known as the a-value) of one wall compared to another can vary by as much as 500 times, a range of 30 between common renders and bricks is usual. That is an extraordinary range for a key characteristic that is seldom discussed or measured in the UK.

GAPS AND CRACKS

Any buildups where vapour convection (through gaps and cracks) can be significant (such as a ceiling where insulation is laid on flat as described in IP 5/06), and systems where airtightness and thermal bypass is not clearly addressed (such as insulation on dabs on masonry, or light gauge steel frame constructions) - regardless of whether the insulation and other layers are considered to be of low hygrothermal vulnerability or not.

Note 1: This is because vapour convection can deliver as much as a hundred times more moisture than will occur through diffusion: in buildings where extract ventilation may be compromised and moisture loads high there is particular vulnerability. While WUFI Pro is not a computational fluid dynamics (CFD) software it can model the moisture impacts of different levels of airtightness.

Note 2: An example of a buildup that would appear to have low hygrothermal vulnerability is a rendered cavity wall with unfilled cavity and wet plastered internal finish).

EXPOSURE DUE TO LOCATION OR HEIGHT

Most constructions in areas of greater driving rain (shown as Zones 3 and 4 in in BR 262).

Note: Table 1 of BR 262 gives a very good sense of how hygrothermal risks increase for cavity walls depending on the outer surface, the width of cavity, the location and type of insulation and the width of cavity. The study pre-dates computer simulation and is based on the experience of building control officers over many years. It corresponds closely to results using hygrothermal numerical simulation under BS EN 15026. The Table should be reviewed in assessing which constructions may be considered ‘at risk’.

Most constructions experiencing locally exposed conditions such as the upper floors of tower blocks. WUFI Pro can model the impact of increased wind speeds at height.

POTENTIAL IMPACT OF DRIVING RAIN PENETRATION BEHIND A LAYER OF COMPONENT

Where there is a clear potential that gaps could be present or open up in the outer layer of a component allowing driving rain to penetrate behind. A resilient buildup is one that can successfully withstand likely levels of imperfect construction and water penetration and dry out after. Some constructions will prove to be sufficiently resilient when assessed, others could show failure.

Note 1: There is a large body of case studies from the US, Canada and Sweden of failure of EPS EWI on timber frame: these failures occurred only after water penetration: the buildup may have been hygrothermally acceptable up to the point that water penetrated behind the EWI depositing water on the hygroscopic racking board, timber studs or base plates, then everything changed. It is the worst case that should be evaluated, not the best. 

Note 2: WUFI Pro can model the impacts of different levels of water penetration: ASHRAE 160 states that simulating the impact of 1% driving rain penetration behind the outer layer of ther assembly is enough.

SPRAY FOAMS

The retrofit of open- or closed-cell spray foam onto a pitched roof of what had been a ‘cold attic’ (i.e. a ventilated attic where some level of insulation is still present on its floor). Based on simulation and site testing we have carried out we are aware that the timbers of north facing roofs can be quite vulnerable.

Note 1: As a general principle insulation should only every be located on one plane, it should be continuous, air should not be allowed pass between insulation layers and the air and vapour control layer (AVCL) should be on or near to the warm side of the insulation.

Note 2: It has become common practice for spray foam to be sprayed on the pitch between rafters, sometimes onto 50mm deep vent spacers isolating it from the original sarking felt, sometimes not (there are BBA certs showing both conditions). An AVCL is generally not used on the attic side of the spray foam despite the fact that it is exceptionallyvapour open. The continuity of an AVCL at ceiling level is depended upon despite their widespread absence in reality and the guidance of IP 5/06.

Note 3: The resulting condition is actually exceptionally complex hygrothermally because a third climate is created between the internal and external climates. Due to the difference in area of the surface that gains heat and moisture from below (i.e. the ceiling) compared to the surfaces that loses heat to the external climate above (i.e. the two sides of the pitched roof)) WUFI Pro, 2D and Plus must all be used together to estimate the impact and acceptability of such a fast and cheaply installed buildup.

GREEN ROOFS

Flat roofs with 'cold deck' buildups are quite reliant on reverse diffusion to evaporate any inter-stitial condensation that has formed and drive it away from the vulnerable roof deck. (Reverse diffusion generally occurs when external surface temperatures are higher than internal ambient temperatures.) This allows decks to last many years longer than they might otherwise. The soil/medium and planting of green roofs isolate the roof structure from the sun's radiant heat thereby weakening the valuable impact of reverse diffusion. As Ireland also has higher levels of ambient relative humidity and lower standards of construction than Germany (the country where many green roof systems come from) Irish architects and DC should act with care when specifying  or certifying green roofs: risks are elevated.  

Information about WUFI

Visit http://www.wufi.ie/architecture/wufi for more information on hygrothermal risk evaluation, on training or purchasing the software, and on the risk evaluation service that Building Life Consultancy offers. You can also call Joseph Little at 01-8746573.

Date

Friday, June 20, 2014