Insulating Guide

Insulation options

The three most important factors that contribute to energy efficiency are as follows.

  • The insulation and airtightness of the exposed walls, roofs and floors; good insulation and airtightness reduce heat loss.
  • The choice of fuel and the efficiency of the heating system; this affects the amount of fuel required to satisfy the heat loss.
  • The efficiency of lights and electrical appliances; this affects the demand for electricity.

For people planning a house extension  some guidelines for implementing the best practice energy efficiency features into the design and specification of domestic extensions are listed below.

The main areas to consider are;

  • energy efficient extension shapes;
  • insulation of external walls, exposed floors and roofs;
  • specifying energy efficient, high-performance windows;
  • limiting thermal bridging and air leakage;
  • providing controlled ventilation;
  • providing efficient heating;
  • specifying energy efficient lighting.

Extension Form

Efficient forms reduce heat losses by reducing the ratio of heat loss area (i.e. the area of exposed walls, roofs and floors) to floor area. It is a good idea to make theextension as compact as possible. A two-storey extension is inherently more efficient than a single-storey extension. An extension with some south-facing pitched roof will permit the use of renewable energy technology such as solar water heating or photovoltaic (PV) panels to generate electricity. These will significantly reduce carbon dioxide emission


Another important factor is the amount and orientation of glazed openings (windows, roof windows and glazed doors). Windows fulfil several functions: they provide views out, let daylight in and assist with ventilation. However, the heat loss through one square metre of a modern, high-performance double-glazed window is nearly six times greater than the heat loss through one square metre of new external wall, and between ten and twelve times greater than the heat loss through one square metre of new insulated roof. Where glazed openings have a southerly orientation (south – + 30°) they can trap some useful solar heat gains on sunny days during the heating season. At other times the openings will contribute to heat losses; and in the summer, unless they are shaded, they may contribute to solar overheating. Glazed openings with northerly orientations increase heat losses (compared with an equivalent area of wall) without trapping any compensating solar gains. Conservatories seem to offer inexpensive accommodation with the bright, ‘airy’, almost external feel of a highly-glazed space. However, solar heat gains are more than offset by the high rate of heat loss through glazing, especially if the conservatory does not have a southerly orientation ( – + 45°). An unheated, southerly-oriented, highly-glazed conservatory will provide comfortable accommodation during spring and autumn, and on a few sunny days in winter. At other times in the winter it will be cold, and in the summer it will overheat. Comfortable periods can be extended by the use of shading, blinds and ventilation, but conservatories should never be heated because this leads to excessive fuel use, fuel costs and carbon dioxide emissions.



Three types of insulation are commonly used in domestic extensions:

  • rigid insulation/ thermal lining boards;
  • flexible insulation
  • foam insulation

Rigid insulation is usually a form of plastic foam board, e.g. polyisocyanurate board. Examples of the flexible type are glass fibre and mineral fibre quilts. Foam (bead or expandable cell) insulation is freform foam often applied via a Spray foam Insulation Systems.

Insulating Floors

New ground floors should be insulated to the Best Practice standards

(i.e. to achieve maximum U-values of 0.20W/m2K).

The most common types of new ground floors are:

  • ground-bearing concrete (slabs)
  • suspended pre-cast concrete ‘beam-and-block’ floors
  • suspended timber floors

Ground-bearing concrete floors can be insulated by placing insulation beneath the slab or above the slab, beneath a screed or timber floor. In the case of insulation beneath the slab, rigid insulation should be used, and if the insulation thickness exceeds about 75mm it may be necessary to include some steel reinforcement mesh in the slab itself. Depending on the size and shape of the floor, up to 100mm of high-performance insulating material. The perimeter of the slab should also be insulated. If the insulation is placed above the slab, and the floor is finished with timber, a vapour control layer should be included beneath the timber finish. Again rigid insulation should be used. Suspended pre-cast concrete ‘beam and block’ floors are usually insulated above the floor.  Again rigid insulation should be used. The thickness of the insulation should not exceed about 100mm, so a high performance insulating material (i.e. one with low thermal conductivity) may be required to meet the Best Practice standard.

Suspended timber floors can be insulated by placing insulation between the joists, usually to the full depth of the joists. The insulation may be supported either on timber battens fixed to the sides of the joists or on netting placed over the joists.


Insulating Internal and External Walls

Exposed walls should be also be  insulated to the Best Practice standards. The external walls of domestic extensions are usually constructed by one of two methods:

  • masonry cavity construction
  • timber-framed construction

Masonry cavity construction consists of an outer leaf of brickwork, a ‘cavity’ that is fully or partially filled with insulation, an inner leaf of concrete blockwork, and a plasterboard lining. The thermal performance of this type of construction varies with the thicknesses of the cavity and of the insulation, the type of blockwork used for the inner leaf, and the type of lining board.

Timber-framed construction usually consists of a structural timber frame with insulation placed between the framing members, lined internally with plasterboard and externally with a sheathing board and a waterproof breather membrane. The timber frame is often clad externally with a skin of brickwork, separated from the frame by a cavity.

External insulation system involves mechanically fixing a minimum of 120mm of mineral fibre insulation, rockwool, to the external wall to achieve a U-value of 0.26 W/m2k which exceeds the Home Energy Saving scheme standard. A modern silicate epoxy waterproof render system is applied to the rockwool which consists of a 4mm base coat with an integral PVC reinforcing mesh followed by a further 4mm of undercoat. When these base coats are dry a bonding agent is applied and then the final finish coat is applied. The finished render is self-coloured. Around 130mm is the thickness of the insulation including the plaster (plaster 10mm)on finishing. The system would have a U Value of around 0.26 which is better than the required standards.

Foam or bead insulating is also used for external walls. Polystyrene bead or polyurethane foam which is produced from the chemical reactions of a free forming foam and is useful for insulating hard to reach (internal wall cavities/roof areas) or spacious areas (groundfloor) at low cost. When sprayed in place the foam insulation expands completely filling all cavities and voids creating a sealed building envelope which eliminates air leakage/infiltration. Air leakage/infiltration accounts for up to 50% of heat loss from a building. Foam formulations now can be 100% water blown and therefore contain fewer harmful agents, volatile organic chemicals, HCFCs, HFAs, HFCs or formaldehyde.

The general method of installation is as follows; 22mm holes are bored into the external walls of the building, they can also be drilled internally if required this method is commonly carried out on new builds. Pumped-wall-insulationCavity Wall Insulation ImageA boroscope test is carried out to assess the suitability for cavity wall insulation. The holes are approximately the size of a one euro coin.
Bead or foam ingredients along with an adhesive are pumped under pressure into the cavity. Cavity full fill is measured.The wall Cavity Coin Size HoleHoles are filled with matching mortar.The walls are warmer after insulation which in most cases results in the elimination of condensation.

Internal Wall insulation usually involves a dry lining internal wall insulation solution. This involves a qualified technician installing up to 82.5 mm of insulation coupled with airtight membrane leaving a skimmed plaster finish. The typical application will achieve a U-value of 0.27 W/m2K which is required under the SEIA guidelines and for HES Scheme grant approval. This reduces the space and dimensions of the rooms and where necessary all electrical and plumbing services are removed and re-positioned by registered contractors. A new skim plaster finish is then applied, new or existing skirting boards and window boards are fitted and the house is left clean and tidy after works have been completed.The maximum HES Grant available for dry-lining works is €2,500.



Insulating Roofs / Attics

Roofs should be insulated to the Best Practice standards. There are three common methods of insulating the roofs of domestic extensions:

    • insulating at ceiling level (with an unheated loft above)
    • insulating within the pitch of the roof (between the rafters) like attic insulation
    • insulating a flat roof.

Fibreglass insulation is rolled into the attic space between and across the joists, difficult to reach areas can be insulated with blown shredded fibreglass. Cold water pipes and cold water tanks are lagged and sealed.

Insulated walkways are installed for safe access from your attic hatch to your water tanks. atticinThe minimum requirement is for 300mm of insulation under SEI requirements and a U Value of around 0.16 is achieved. The SEAI and Woodies have a good graphical leaflet and Youtube video on How to Insulate Your Attic Click here to see this document!

As most companies are SEAI registered, all Better Energy Home Scheme grants are available to all customers. This facility enables Ireland to become a greener and more cost and energy efficient country.

We list below an example of grants available for each type of work:

Attic Insulation € 200.00
Internal Insulation €1,800.00
External Insulation €2,700.00 – €3,600.00
Bolier Upgrade €560.00
Solar Heating €800.00
BER €50.00