Non-Traditional Housing - External Wall Insulation

Guest article from Joe Malone BSc(Hons) ICIOB Head of Asset Management ALMO Business Centre Leeds

It should be recognised that the majority of External Wall Insulation (EWI) systems are non-structural and therefore are fixed to load bearing fabric. There are times when a non-structural system is inappropriate for use such as in Crosswall construction.

The Triple Problem

Figure 1. Typical EWI System - Source: http://www.everwarmgroup.com

Non-traditional stock normally presents 3 major problems for registered providers, they are thermally inefficient, they may have structural defects and they are unattractive and often blight housing estates. Whilst each problem presents unique challenges, none are insurmountable and life cycles can be cost effectively extended for a further 30-40 years so long as investment decisions are evidence based and address the three problem areas under discussion.

Poor SAP Ratings

Since all non-traditionally constructed properties require an improvement in their SAP rating it makes sense to discuss external wall insulation. There are a number of EWI systems in use but unless you are looking or a structural system then there is little variation apart from the choice of insulation material.  Phenolic boards (PF), PUR (Rigid polyurethane foam), PIR (Polyisocyanurate) EPS (Enhanced polystyrene board) or mineral wool are all frequently used as part of these EWI systems.  Rigid phenolic insulation products offer best thermal performance when, compared with rigid polyurethane or extruded polystyrene. Its low thermal conductivity allows specified thermal performance targets to be achieved with minimal thickness of insulation. This is particularly significant where space saving is important. For this reason it is one of the most widely used products in the external wall insulation business. That being said Phenolic board comes with a few known and a few less well publicised issues:

1.   Demand for phenolic board has exceeded supply which has caused manufacturers to cut the 12 week curing period to 6 weeks in an attempt to keep the market supplied. There are some concerns with regard to the effect this decision will have on the quality of the product and there is some early anecdotal evidence regarding increased board shrinkage after system application. Phenolic boards were known to shrink which can occasionally cause gaps to open up in the building envelope. Will we now see an increase in the severity of this problem? Many commentators (myself included) believe that we will.

2.   Phenolic board has known acidic properties and should not be placed in direct contact with metal roof decks, wall cladding or stanchions. There are cases pending against the manufacturer where phenolic boards have caused corrosion of steel roof decks.

3.   Phenolic foam insulation has a significant environmental impact, exceeding that of other insulation materials. Significant amounts of petroleum and natural gas must be burnt during the manufacturing and refining processes, though the insulation industry has ceased to use chlorofluorocarbons (CFCs) in the manufacture of foam insulation products. In simple terms, it's nasty stuff, its manufacture was discontinued in the US in 1992 and you should consider whether continued use of phenolic board is a responsible business decision for you or your client.

4.     Phenolic foam insulation will deteriorate if it is exposed to moisture or sunlight for extended periods of time. While it will be safe from sunlight exposure inside your walls or between your floorboards, it is important to store phenolic boards correctly and apply render to walls within 48hr of fixing external wall boards.I have personally managed millions of pounds worth of EWI work and site management of this issue has been a consistent and ongoing problem.

Figure 2. A Wallsall scheme. No design work and bland results

PUR board comes with similar known environmental problems but is also known to suffer a loss of U value (thermal performance) with time. This is due to a combination of air infiltration and fluorocarbon gases diffusing out of the product (outgassing) over time. This rate of outgassing is unclear and varies from product to product but in all likelihood a property that has a PUR insulation system installed will have a significantly reduced U Value 10 years on from the date of installation.

The most cost effective and pragmatic choice of material from the 3 materials listed is EPS, in particular Graphite enhanced polystyrene (GEPS), which will give a significantly improved U-value over standard EPS. Long term performance gives significantly less concern than phenolic or PUR and whilst it could never be considered a ‘green’ product, it is in my opinion the more environmentally friendly and acceptable choice from the main types of rigid insulation board in main use. Mineral wool also provides a viable alternative choice to GEPS yet it one of the more underused options. Of course there are pro's and con's with every material choice and whilst I would recommend GEPS for low rise stock, consideration needs giving to fire performance when used in high rise stock.

Fire Safety Performance

A chemical called HBCD is often added to EPS or GEPS to improve fire performance and whilst you may not yet have heard about health concerns relating to the use of HBCD, it is on the verge of being banned or having its use restricted in Europe. In general terms EPS or GEPS has poor fire performance but can achieve a class E rating when enclosed with a laminated facing layer of the type seen in EWI systems. That being said, Class E isn't really acceptable for high rise applications and you would be looking to build in external fire breaks if such a system was used externally on high rise blocks. Whilst being the least environmentally friendly, PIR or PF insulation boards offer the best fire resistance having achieved a class O rating; this does raise their profile for use in high rise applications.

EWI System Failures

There are a number of EWI high rise system failures in Scotland and Birmingham and the North West; I'm sure there are many more that to date I am unaware of but the point is made. One spectacular failure in the North West was captured by a passerby on their mobile phone:

In Scotland court cases are pending but in Birmingham the issue has been turned into a dispute over whether the products or the standard of installation were the cause of these failures What is clear is that insulation boards have moved or become partially detached from the external building façade and the I have some concerns that systems have not been adequately wind tested for installation at height. Another known problem is that mechanical fixings into no fines concrete have been very poor or completely inappropriate for the circumstances. There have been a significant number of hammer fixing failures into concrete and particularly no fines concrete.

In general terms, the majority of EWI system installers are satisfied with mechanical hammer fixings alone whilst a minority of installers believes this is a problem and adopt a belt and braces approach to installation by both gluing and mechanically fixing boards to the external façade. I agree with this approach for low rise stock but would still issue a note of caution with regard to specifying a standard non-structural system for medium to high rise stock.  If the thermal solution is not given adequate consideration and fails then I have experienced first hand how extremely difficult it is to get contractors to resolve these failures when high rise access alone (mast climbers or scaffolding) can cost them somewhere in the region of £100k to £250k per block.

EWI Structural Systems v Non-Structural Systems

It should be recognised that the majority EWI systems are non-structural and therefore are fixed to load bearing fabric. There are times when a non-structural system is inappropriate for use such as in Crosswall construction. Crosswall construction takes all the building loads from floors, roof etc. on the gable walls. The front and rear facades of these properties are non-load bearing and therefore unsuitable for fixing a standard EWI system. In these cases you would choose a structural EWI system such as Structherm. Moreover, a structural EWI system has several other potential applications when considered for use on non-traditional stock.

·       Full cladding of defective buildings (reduces need for difficult remedial work)

·       Fully designed structural cladding for non-traditional medium to high rise structures. Designed to account for wind loadings etc.

·       Full over-cladding of defective or inefficient system built structures (improving structural safety and thermal continuity)

·       Enclosing balconies and walkways (converting external space into usable internal floor area)

·       Forming new or extending existing parapets (improving safety at roof level)

You should also note the failures of mechanical fixings into concrete (particularly no fines) and ensure a system is designed to bypass this issue. This is a note of warning that should apply to all system design but you should note that bad site management during any installation process will negate any effective design process. I have visited site on many occasions and seen the wrong size hammer fixings used or more commonly, operatives not using depth stops attached to their drills and often even punching straight through walls when drilling for hammer fixings. The length of hammer fixing is critical to the design process yet I am convinced that they often don't account for the depth of existing render applied to some non-traditional properties and therefore hammer fixings can be fixed to insufficient depth in the structural panel. It is all too easy to make assumptions about the depth of existing render and I often insist on having patches chiselled out to expose the substrate. This allows us to make a more informed decision about the required length for hammer fixings.

Pre-Existing Concrete Defects

In part one I discussed the defects relating to carbonation and chloride attack of concrete non-trads. The problem is of particular concern when dealing with high rise stock because the repair of cracks and spalling can significantly add to your refurbishment cost. Each small localised repair can cost circa £30-40 per repair. There can be hundreds of such repairs on each high rise and this doesn't account for access costs or the cost of an anti carbonation coating. Rusting is of course an expansive reaction and treating rusted rebar is a key part of the concrete repair process to prevent future spalling of the concrete. There is an argument that says overcladding with an EWI system cuts off oxygen required for the rusting process and therefore prevents any further deterioration of the rebar. This could to a degree mitigate the requirement for concrete repairs but where structural engineers are involved in the design process then they are less likely to accept this argument.

Is EWI Appropriate for All Properties?

In short, No! There are concerns about the potential for EWI systems to cause damp and this stems from two issues:

1.     Traditional properties built on the ‘overcoat’ principle and using traditional stone or lime mortars are meant to breathe. Adding EWI would affect the walls breathability and is completely inappropriate for use on these properties. An ex-colleague of mine has been trying to find a solution for improving SAP on single skin stone properties in Cumbria, they went with an internal wall insulation system and whilst pragmatically this raises less concern it will still affect the walls breathability.

2.     I’ve seen a number of EWI systems badly installed and bridging the dpc. The installers may well claim that their materials will not wick moisture but you’d be wise to discount their claims. 

Design

Figure 3. A well designed Nottingham City Homes scheme in Bulwell, Nottingham.

Well designed EWI systems have the ability to completely transform our estates but if design is not given adequate consideration then completed estates will remain as bland or even be damaged, particularly by the choice of gaudy colour schemes. Housing providers who give residents too much choice in the design of their estates are often those that end up with a series of ill matched pink, green and violet housing.

Whilst colour choice of external render systems is one consideration, all EWI systems offer a choice of features that will add architectural detailing, particularly effective can be the use of bricks slips. On the last scheme I was involved with we gave a great deal of consideration to all aspects of design, even to the extent of having artists illustrations done so we could make informed choices on design. To many organisations, EWI is nothing more than a technical solution to deal with thermal efficiency but they are failing to protect the future of their estates and communities by not giving adequate consideration to the design process.

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