Just About Everything You Need To Know About Attic Floor Insulation
According to the SEAI, without attic insulation, up to 30% of the heat you pay for could be escaping through your roof. As it is relatively cheap to top up this insulation to the recommended minimum depth of 270mm, (or ideally as much as 450mm), it is the most cost effective way of achieving a warmer home, generating major savings in heating bills.
In addition, poorly insulated attics combined with solar gain during summer months can make rooms at the top of your home prone to overheating, so insulating the attic will also combat this summertime problem.
The R-Value measures the effectiveness of insulation material in resisting heat flow. The higher the R-Value, the better the insulation’s ability to retain heat, making it more energy-efficient. It’s primarily used in the context of insulating materials themselves.
The U-Value measures the rate of heat transfer through the material (or an assembly of materials). It’s often used in the context of entire building elements, such as walls, windows, or roofs. Correctly installed insulation will create a low U-Value for the building element you are insulating, meaning that only small amounts of heat will pass through, thus keeping your home warmer for longer.
In summary:
- Higher R-Value = Better insulation
- Lower U-Value = Better insulation
When planning to improve your home’s insulation, it’s good to consider both values to get a complete picture of its thermal performance.
A U-Value of 0.16W/m2K is required for an insulated attic floor in order to meet current building regulations and secure the SEAI attic insulation grant. The thickness of insulation required to achieve this U-Value will vary according to the type of insulation used.
A 270mm depth of mineral or glass fibre should be capable of achieving this 0.16W/m2K U-Value requirement, but if you prefer to use natural materials like sheep’s wool or hemp, a slightly deeper thickness of 300mm should do the trick. Your installer will be able to advise you on the best choice of material and required thickness for your specific case.
In order to lay the rolls of insulation, the homeowner must first clear the attic space of any stored items. This is the only time-consuming part of the job as the laying of insulation can be done by the installer in a matter of hours.
If the attic space is already floored without insulation installed, (or the existing insulation requires additional layers), the floor boards must be taken up and cleared out of the way. If the homeowner is not able to do this, then they can talk to the installer who may offer this service (for an added fee of course).
The insulation (mineral fibre, glass fibre, hemp or sheep’s wool material) is brought into the attic in rolls and first laid in the gaps between all floor joists. A second layer is then laid on top of this in a perpendicular direction to the first layer. It is important to ensure the insulation is installed in two layers in different directions in order to eliminate cold bridging (weak points which allow heat to escape more easily), so that the maximum insulation benefit is achieved.
Thermal studies have shown that compressing your insulation can reduce its thermal properties by more than 50%. If you need to create a floored area for storage (or a floored pathway from your attic hatch to your water tank for example), you can use 175mm loft legs fixed to the top of your attic floor joists to provide a lightweight, cost effective solution for raising the floor panels and avoiding insulation compression. Click here to see them in action.
If 175mm loft legs are too small to accommodate the height of your new attic floor insulation, 300mm loft legs are also available. Talk to your installer about this.
It is not recommended to lay attic floor insulation directly over downlights protruding into the attic floor from the ceiling below because this can be a fire hazard. However, you can use loft lids, (also called downlight hoods), to cover over any downlights and create a protective barrier so that attic floor insulation does not directly contact the light fitting. This also acts as a dust cover and provides sufficient ventilation around the light for heat dissipation.
Electric cables give off heat when current flows through them, particularly if they are supplying large loads (to electric showers for example). Therefore, to avoid a potential fire hazard from covering these with insulation, move the cables above the insulation (to the cold side) or alternatively sheathe groups of cables in large diameter (100mm) conduit so that any heat generated can more easily dissipate.
Attics are meant to be draughty and unheated above the insulation level. Once your attic floor is fully insulated, the room warmth should stay where it belongs – in the rooms below. As a result, the attic space will now be colder, so any warm moist air that gets in can condense on cold surfaces, especially in the winter months. This can saturate the insulation and potentially cause damage to roof timbers and ceilings over time.
To combat this, a good airflow is needed to take any moisture out of the attic space before it can condense and cause problems. This is achieved by either small vents installed at the eaves, ridge tile vents, vent tiles or gable wall vents. Sometimes a combination of different types of vent is needed. Ensure any installed insulation does not block these.
When attic floor insulation is being laid, care must be taken to ensure the insulation is not pushed right up over the eaves and block the ventilation path there. By installing loft rafter vent trays in between the rafters at the eaves and then laying the insulation all the way up to the base of these vent trays to hold them in place, a channel can be maintained to allow sufficient air ventilation into the roof space. Click here to see an example.
It is essential that all pipework and water storage tanks are insulated, as the attic area will become much colder once the recommended levels of attic floor insulation are installed, (as the heat escaping from your warmer rooms below will be greatly reduced).
The best advice is to lay insulation across the whole attic floor including the area directly under any water tanks. The tanks should then be fully insulated, including their base, as well as any pipework exposed to the cold side of the attic floor insulation. Gaps around pipes where they enter the home below should also be sealed for airtightness with silicone sealant.
The cost mainly depends on the size of the floor area to be insulated, the insulation type, whether any insulation is already present and the depth of insulation required to achieve the desired U-Value. While selecting a cheaper insulation option might seem like a good decision up front, it may also reduce the amount you can save on your heating bills.
The average cost of Sheep’s Wool/Mineral Wool/Fibreglass per m² is €10 – €30. The full cost per m² will also include the installer’s labour costs, so insulating the attic floor can vary anywhere from €600 – €2,400 depending on the size of your attic and the material used. These are only rough estimates and are subject to change based on the cost of materials and each individual property.
The SEAI (Sustainable Energy Authority of Ireland) offers grants to those who wish to install or upgrade the levels of attic insulation in their home. To qualify for this grant you must:
- Be the owner of a property built and occupied before 2011. (This is defined as the date your electricity meter was installed).
- Use an insulation contractor from the SEAI’s registered list.
- Use newly fitted materials and products.
- Comply with building regulation standards for any attic insulation works.
- Get a BER assessment done after the works are completed using a BER assessor from the SEAI’s National Register.
Please note: If your home has already received a particular type of insulation grant through any government scheme, it cannot receive additional support for further insulation works of this type. (However, if a grant was previously received for cavity wall insulation for example, the homeowner is still able to claim for an attic insulation grant).
For more detailed information, visit the SEAI insulation grants page here.
The value of the 2025 SEAI attic insulation grant varies according to your type of home.*
- Apartment** – €800
- Mid-Terrace – €1,200
- Semi-Detached or End-of-Terrace – €1,300
- Detached – €1,500
* Where actual expenditure is lower than the grant value, only the lower amount will be paid.
** The attic insulation grant for apartments is only available to those located on the top-floor. An apartment which has other separate apartments located directly above it is not eligible for this grant.