Zero energy for all?

From 2020 onwards all newly built or renovated houses in NI and ROI will have to comply with the Nearly Zero Energy Building (NZEB) standard. How that requirement will be put into practice remains unclear but there are some general rules we already know will have to be followed…

Europe has been driving the recent changes to the building regulations in both NI and ROI, through its Energy Performance of Buildings Directive (EPBD).
The EPBD’s aim is to get to a stage where all buildings are of such a high thermal performance that they require nearly zero energy to be comfortable to live in. The EPBD definition also states that the “nearly zero or very low amount of energy required should be covered to a very significant extent by energy from renewable sources”.
The changes envisaged in the EPBD are far reaching; new buildings and any existing buildings undergoing major renovation (defined as any work affecting more than 25 per cent of the floor, or façade, area) will have to comply with this standard, Europe-wide, from the end of 2020.
There is also a requirement on member states to plan for the retrofitting of the entire existing building stock with a view to making it too conform to the NZEB standard. This is likely to require an agreed plan for each building which can be phased over several interventions, such that one measure does not preclude, or add to the cost of, a subsequent measure in the plan.

In practice, NI and ROI haven’t as of yet published detailed regulations showing how the standard is to be applied.
However, in ROI, the Irish government is proposing a minimum target for newly built houses of 45kWh/sqm/yr primary energy use, whilst deep renovations would be required to use no more than 150kWh/sqm/yr. (before the addition of renewables energy systems) to meet the NZEB standard.
Work done by the Zero Carbon Hub suggests a similar effective residential NZEB target will be introduced in 2016 in the UK, albeit measured in carbon emissions savings rather than energy.
Make no mistake, the 45kWh/sqm/yr NZEB new-build target is quite a challenge: it is about the same as the Passive House standard in terms of U-values and thermal bridging. The 150kWh/sqm/yr standard is close to current building regulations requirements for “new thermal elements” defined in NI Technical Booklet F1 or “backstop” U-values defined in ROI Part L, 2011.
However, as the EU is targeting an 80 per cent emissions reduction by 2050 from all sectors, it would appear that the 150kWh/sqm/yr retrofit figure may not be accepted by the Commission as sufficient to achieve long-term carbon emission reductions and energy savings. This is especially so as at least 70 per cent of our current buildings will still be in use in 2050.

Getting NZEB ready 
To complement the National Retrofit Strategy of the Irish Government, a detailed Code of Practice for Retrofit (S.R.54:2014) has been published by the National Standards Authority of Ireland.
This outlines the kinds of fabric upgrade measures that will deliver the projected energy savings envisaged in the EPBD. It’s also got a lot of examples of energy upgrade measures and is a document homeowners, specifiers and contractors should consult before undertaking an energy upgrade.
Given that the NZEB targets have yet to be defined, and may be subject to upward revision, a range of four targeted U-values are included, with wall U-values of as low as 0.15 W/sqmK accommodated. Roof and floor U-values are also considered, commensurate with each of the four wall insulation levels. This document is freely available and can be downloaded from the website.
The Society for the Protection of Ancient Buildings publishes a very good interactive on-line Responsible Retrofit Wheel which may be used in conjunction with S.R.54.  A conservative adherence to the advice contained in both documents, in combination, will be required to confirm that any proposed retrofit solution is safe.

A SIP house under construction in Co Limerick. John Hayes

A SIP house under construction in Co Limerick.
John Hayes

Regrettably, few of the major insulation suppliers have taken up the challenge of providing holistic retrofit advice beyond the limits of their own product ranges. Government supports for single measures has also not helped in this regard.

The specifier therefore needs to exercise caution when following supplier advice and always seek collateral warranty, Agrément certification and actionable product liability insurance.

Cavity wall upgrades 
Full-fill pumping with a bonded bead, which will not retain moisture but allow it to percolate to the base of the cavity and drain to outside, is the most popular cavity wall solution in ROI, whilst blown fibre is preferred in the UK Approved Document L1B. Other forms of cavity fill are available but some suffer from an increased risk of moisture crossing the cavity.
Filling the residual cavity is not suitable for brick outer leafs in exposed sites where the volume of moisture absorbed by the brick is such as to require ventilation on both sides of the outer leaf to achieve long-term durability.
It may be possible to upgrade such exposed, or absorbent, brick leaves by the application of a vapour permeable, but hydrophobic, coating (which has to be reapplied periodically). Rendered block outer leafs in good condition do not generally pose a moisture loading, or frost heave, risk.

However, full-fill cavity wall solutions may only take you part of the way as the U-value is largely dependent on the width of the cavity available. To achieve a specific U-value target may require an external or internal insulating layer in addition to a fully filled cavity.
External insulation is generally better as it eliminates more thermal bridging (particularly around openings) and, because it draws the dew-point outwards, reduces the risks from interstitial condensation. It also retains the thermal mass of the wall which can help reduce the risk of overheating in summer.
Such additional benefits, and peace of mind, come at an inevitable cost premium and may, in the wrong hands, impact on appearance.

Solid wall upgrades 
There is sufficient evidence of failure now to indicate that external wall insulation provides the most robust, long-term solution to upgrading solid walls.

Internally insulating solid walls is inherently problematic and should only be considered after careful modelling using hygrothermal simulation software compliant with EN 15026:2007.
Any internal wall insulation solution that relies on a polythene or foil vapour control layer (VCL) on the warm side of the insulation should be treated with caution and be subjected to specialist hygrothermal modelling for the effects of both solar driven moisture in summertime and any possible puncture, or discontinuity, of the VCL on site.
In particular, it may be necessary to remove all timber, or gypsum, trapped on the cold side of the insulation as its moisture content may exceed safe limits, promoting mould growth and even structural decay. The mould risk posed by interstitial condensation even extends to existing wallpaper and wallpaper paste trapped behind any new insulation!
This may not leave many safe internal wall insulation options, especially for protected structures, but there is little point denying building physics. Humidity monitors, placed behind insulation, can be useful to keep track of moisture levels in walls where modelling fails to rule out a risk of mould.

Timber frame 
Additional internal insulation should normally only be added to a timber frame construction directly to the existing insulation layer.  A vapour diffusion balance of at least 5 to 1 is recommended with the outermost diffusion-open layer facing a well ventilated cavity.
Any interruption in the 5:1 rule, like retention of a pre-existing VCL, or racking board (which can be plywood, OSB or gypsum board) can result in interstitial condensation being trapped within the upgraded wall.  As the racking layer is structurally important, its relocation will require the input of a structural engineer.

External insulation before rendering. Larsen Contracts

External insulation before rendering. Larsen Contracts

Alternatively (and preferably), it may be possible to remove the outer rain screen, add external wall insulation and then reinstate a ventilated rain screen, provided the 5:1 diffusion balance is achieved.
Where a timber frame is enclosed within a masonry rain screen external leaf, external insulation may be possible by a combination of external wall insulation outside the masonry leaf in combination with pumping the cavity but this again will require specialist hygrothermal analysis to select the right combination of hygroscopic and hydrotropic insulation materials.

In detached properties, it may even be worth considering removing the external masonry leaf entirely and applying a suitable, diffusion-open mineral fibre, or wood fibre, external wall insulation solution fixed directly to the timber frame.
In the absence of specialist hygrothermal modelling, the requirement for a 50mm well-ventilated cavity on the outside of a timber framed wall should never be compromised.

Structural insulated panels (SIPs) 
The inherent airtightness of structural insulated panels makes them relatively easy to upgrade.  However, successful upgrading demands careful control of air leakage.

Before and after air permeability testing is advisable to ensure that the upgrade has not introduced additional air leakage. Foil-faced insulation boards with taped joints are easily installed and can then be finished with a services cavity and plasterboard layer.
Similar concerns exist for progressive vapour diffusion through the wall based on a 5:1 outside to inside diffusion potential, it’s just that in this case, the internal layer will have to be extremely diffusion closed to achieve the 5:1 ratio.
Solar driven moisture is not generally a concern in this form of construction because of the ventilated external rain screen construction.

Steel frame 
Lightweight steel frame construction has the potential for repeat thermal bridging reducing its overall thermal performance and inducing interstitial condensation.

Internal insulation can be effective but the diffusion profile of the wall must follow the principles outlined for timber framed construction above. In this case, whilst the steel studs are unlikely to be structurally weakened by interstitial condensation forming as a result of the additional insulation, the linings are as likely to promote mould growth as with timber frame.
If internally insulating, the existing lining and vapour control layer may have to be relocated to inside the additional insulation layers to avoid the risk of interstitial condensation forming on (or being held within) any trapped lining/racking layers.
In preference, if an external insulation can be added, this will help neutralise the thermal bridging effect and reduce the risk of interstitial condensation. A diffusion-open insulation board can be particularly suitable in that instance.

Insulation above the ceiling is easily done, cheap and effective, provided that sufficient air circulation can be maintained in the roof space above the insulation. There should be a noticeable draught at all times, a requirement that can present difficulties for blown insulation materials.

Approved Construction details are available on the ROI Department of the Environment website showing how to maintain the ventilation at eaves level all around. Care is needed to ensure that the roof space insulation overlaps the wall insulation and any possibility of air infiltration into the roof space insulation is avoided. This is best done from above by removing the lower rows of roof tiles.
Insulation between rafters, beneath rafters and above rafters are all possible but each has a different condensation risk profile. Some combinations may also have town planning implications, if for example, the ridge height is increased or the eaves line extended.
Battening and counter battening with a breather membrane is required above the insulation to prevent condensation forming. Old style bitumen sarking, or any impermeable sarking, should not be used as this will “sweat” with condensation and cause mould growth. Ultimately structural damage can occur as rot takes hold, fed by the condensation and lack of air circulation.

It is not advisable to insulate at both ceiling and rafter level as this creates confusion in assessing the moisture risk profile.

Raised timber ground floors are best insulated from below or by removing and replacing the floorboards. A wood fibre board should be securely fixed to the underside of the joists and the void between filled with mineral wool, wood fibre or cellulose.  Care is required to insulate between the wall and the first/last joist, even if it is only 20mm. Expanding foam can be useful to fill this gap.  It is vital to produce an air-tight seal of the insulated raised floor.

Solid floors in new build work should be insulated with at least 150mm of suitable board insulation with at least 50mm edge insulation all round. All walls in new construction which pass through the floor insulation will require a thermal break of foamglass block or a structural polystyrene foam blocks. If light weight masonry is used, it must be fully waterproofed below dpc level. Often a fully insulated raft foundation is cheaper.
Very good results are achievable in existing solid ground floors by extending the external wall insulation all the way down to the top of the foundation. It is a little appreciated fact that Irish floors lose most of their heat horizontally out through the perimeter, not downwards.
Extra thick external insulation, up to 300mm, below ground level can dramatically improve overall floor U-value without the need to replace the floor, with all the disruption that might entail. A Thermal Modeller will be required to determine the resulting floor U-value to be applied in compliance documentation and heat loss calculations.

Given the cancer risk posed by radon gas, no floor upgrade should be considered without achieving a durable seal against its passage from the ground into the dwelling.

In summary, NZEB new-build or retrofit is not something that should be undertaken without careful consideration of hygrothermal (condensation) impacts. In other words, mould growth is a real risk if the designer and installer don’t have the skills set to avoid it.

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