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Insulating your self build: What you need to know

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31 May 2017

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Gone are the days when your mammy wouldn’t let you out of the house on a cold day without a thick, warm jacket, woolly hat and gloves.

You’re a bit old now for these kind of reminders, but your mam was pretty spot on all the same. If it wasn’t for her words of wisdom, you would have been shaking in the fierce weather.

Think of insulating your build as adding those essential accessories to your house to keep the heat in and get through cold, bitter days. A properly insulated home will not only reduce dreaded heating bills, but will also improve your home’s comfort and make it a quieter place to live in. Plus, you’ll be helping the environment too.

Here’s everything you need to know about insulating your self build to live in a comfier and more efficient home.

Requirements

When it comes to insulation, Technical Guidance Document Part L Conservation of Fuel and Energy 2011 (TGDL) sets out the U-value targets you should be achieving for your new home. However, levels of insulation higher than those required are in many cases economically justified.

It is also important that insulation should be well distributed around the building shell. It’s better to have a good overall level of insulation than, for example, a highly insulated floor with no roof insulation.

Insulating floors

The maximum elemental U-value for floors is set at 0.21m2K. Heat loss through the ground floor of a typical two-story house accounts for 10 percent of total heat loss, so it’s important to pay attention to your floors.

If you are insulating underfloor heating, a higher rate of insulation will also be required. Heat loss through floors varies according to ground conditions: houses built on wet soils lose more heat than those on dry soils, and detached houses lose more heat than semis or terraces.

The typical thickness of insulation for a detached two storey should be 103mm of rigid insulation board covering the full floor area. A detached bungalow’s floor insulation should be at least 90mm. If you have underfloor heating installed, add an additional 300mm to avoid increased heat loss from the warmer ground floor.

Floor insulation should be laid at the same time as the slabs and foundation are poured so it’s essential you maximise thickness and thermal efficiency at the start as it’s the hardest area to retrofit or upgrade.

Insulating walls

Up to half the heat lost from a house occurs through the walls. Wall insulation may be placed on the outside, in the cavity or on the inside of the wall.

The majority of walls in new Irish homes consist of two ‘leaves’ with a cavity between them to resist rain penetration. It’s standard practice in most new builds to insulate this cavity as well as the wall, either with full-fill or partial-fill boards.

The maximum elemental U-value for walls is set at 0.21 Wm2K. For cavity walls, a minimum of 78mm rigid insulation board, with a thermal conductivity of 0.21W/m2K, must be used, unless you are planning to use additional insulation outside the cavity with thermal blocks or similar.

When full-fill cavity insulation is being used, your architect should assess the walls to ensure that this won’t leave open the prospect of wind-driven moisture crossing the cavity and creating dampness.

Proper insulation within the cavity requires great diligence during building to ensure clean wall-ties, continuous cover and no cold spots. If any change in the vertical thickness of the cavity occurs, a horizontal damp-proof cavity tray should separate each thickness change.

Material used for the cavity insulation include mineral wool (glass or rock), polystyrene beads or granules and cellulose fibre. The cost of cavity wall insulation depends on a number of factors including the width of the cavity. The width is typically about €7 per square foot. The payback period is about 4-7 years.

Internal insulation

Increasingly, Irish builders are opting for internal insulation, where slabs of insulated plasterboard/dry-lining are affixed inside the house, on the inner leaf or external walls. Make sure the insulation has a vapour barrier to prevent condensation on the wall next to the insulation.

The upside is that walls feels warm to the touch and heat loss is reduced, however, the downside is that rooms are reduced internally in size by the thickness of the insulation. This is carried out before all skirting, electrical sockets and switches and other wall attachments are completed.

Types of internal insulation used include polystyrene, fibreglass and polyurethane boards.

Polyurethane tends to be the more expensive option, but its thermal performance is much better than others.

A 25mm thickness of polyurethane will have roughly the same insulating performance as a 38mm thickness of polystyrene or fibreglass.

External insulation

External insulation involves fixing insulation materials such as expanded polystyrene slabs to the outer surface of a wall. The ‘outsulation’ is covered with a special cement-based render to provide weather resistance.

A steel or fibreglass mesh is embedded in this render to provide strength and impact resistance. A thin layer of insulation can be applied around the edges of windows and doors to avoid thermal bridging.

In terms of cost, external insulation is expensive and tends to be used more on retro-fits than new builds. But if your house is likely to be subjected to poor weather conditions, external insulation may be the solution to the problem.

The cost of external insulation is estimated at about €150 per square metre, leaving the payback period at about 30 years.

Insulating roofs

About a third of heat loss in the average home goes through the roof space if it’s not insulated properly, so concentrate initial efforts on roof/attic insulation.

Heat rises, and it flows from warm to cool, so for maximum efficiency, think of insulation in terms of priority from the roof down to the floor.

Under TGD Part L, up to 300mm of typical product insulation, giving a U-value of 0.16W/m2K, should be used in pitched roofed homes at ceiling/attic joist level, or 0.20W/m2K if insulated on the sloped roof section. For a flat roof, a U-value of 0.22W/m2K must be achieved.

To get up to the 300mm level, you may need a number of layers of 100mm/150mm insulation between the joists, or one or two thick layers as an alternative.

As rolled insulation is less effective when compressed, larger joists must be than normal must be employed or alternatively fixed battens to standard-sized joists if you want to run access floorboards over the insulation without squashing it.

Recessed lighting at exterior points of your building shell should be avoided as it can cause gaps in the fabric. All recessed lighting should be covered at the back. This can be done with special clay ‘flowerpots’. Leave a clearance around the lights to avoid overheating and don’t bury or disguise electric cables.

Insulating a 50 square metre attic should cost around €500 and could save you more than €130 a year. Insulating a flat roof of the same size will cost around €1000.

While this is a lot of info to digest, insulation is an important part of your build, so you need to get it right – otherwise you may be lumbered with unnecessarily high energy bills.

Are you thinking of building your own home?

Check out this handy guide to building your home in Ireland complete with stories from EBS customers who have already built a home.

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The content of this blog is expressed in broad terms and is limited to general information purposes only. Readers should always seek professional advice to address issues arising in specific contexts and not seek to rely on the information in this blog which does not constitute any form of advice or recommendation by EBS d.a.c.

EBS d.a.c. neither accepts nor assumes any responsibility in relation to the contents of this blog and excludes all warranties, undertakings and representations (either express or implied) to the fullest extent permitted under applicable law.

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