Is internal wall insulation a good idea?

Yes, it's an excellent solution, especially for solid block or stone walls common in Ireland where cavity insulation isn't possible. It dramatically reduces heat loss, making your home warmer and cheaper to heat. However, it involves work inside your home and results in a small loss of room space, typically around 100-150mm. For many, the huge gains in comfort and energy savings far outweigh this drawback. It's crucial that it's installed correctly with a proper vapour control layer to prevent any moisture issues.

What kind of insulation is best for interior walls?

The 'best' insulation depends on your priorities. For maximum thermal performance with minimum space loss, rigid insulation boards like Polyisocyanurate (PIR), often foil-backed (e.g., Kingspan or Celotex), are a popular choice. For better acoustic insulation and breathability, mineral wool (like Rockwool) is excellent, though it requires more thickness for the same thermal performance. Natural options like wood fibre or cork are also available, offering good breathability and sustainability. Your installer can recommend the best option for your specific wall type and budget.

Does internal wall insulation need an air gap?

This is a common misconception. Modern, high-performance internal insulation systems are typically bonded directly to the wall or installed in a way that minimizes air gaps behind the insulation. An unventilated air gap can lead to "thermal looping" which reduces performance, and can also create a space for condensation to form. Instead of an air gap, a critical component is the Vapour Control Layer (VCL), which is installed on the warm side of the insulation (just behind the plasterboard) to stop moisture from your home getting into the wall structure.

What is the healthiest wall insulation?

“Healthiest” can refer to both indoor air quality and material sustainability. Natural insulation materials like wood fibre, cork, and sheep's wool are excellent choices as they are breathable (vapour permeable) and free from synthetic chemicals. They help regulate humidity within the home. Mineral wool (like Rockwool) is also a great option; it's non-combustible, mould-resistant, and doesn't release harmful volatile organic compounds (VOCs). When any insulation is installed correctly with proper ventilation, it contributes to a healthier home by preventing damp and mould growth caused by cold surfaces.

Do internal walls need to breathe?

Yes, it's vital that the entire wall structure can manage moisture – this is what “breathing” means. Walls get wet from rain on the outside and moisture from inside (cooking, breathing, showers). A breathable wall allows this moisture vapour to pass through and escape. When adding internal insulation, it's critical to use a system that either maintains this breathability (using materials like wood fibre) or, more commonly, uses a Vapour Control Layer (VCL) on the warm side to completely stop indoor moisture from entering the wall structure in the first place. This prevents moisture from getting trapped and causing damp.

How much space do you lose with internal wall insulation?

You should expect to lose approximately 100mm to 150mm (4 to 6 inches) of internal room space from each wall that is insulated. This is a combination of the insulation board itself (e.g., 80-100mm of rigid PIR board), the battens or adhesive used to fix it, the plasterboard finish, and a final skim coat. While it is a noticeable reduction, especially in smaller rooms, most homeowners find the significant increase in comfort and reduction in heating bills to be a worthwhile trade-off.

Should internal walls feel cold?

No, they shouldn't. If an internal wall between two heated rooms feels cold, it could indicate a problem like a hidden water leak or significant thermal bridging from an uninsulated external element. If you mean an *internal* surface of an *external* wall, then before insulation, it will definitely feel cold. After insulation, that same surface should feel warm to the touch, close to the room's air temperature. A warm surface is the primary sign that your insulation is working and preventing heat from escaping.

How thick does internal wall insulation need to be?

To meet Ireland's current nZEB (Nearly Zero Energy Building) standards for a major renovation, you typically need around 100-120mm of high-performance rigid PIR insulation. If you use a less thermally efficient material like mineral wool, you would need a greater thickness to achieve the same performance. The final thickness will be specified by your architect or BER assessor to ensure your project complies with Part L of the building regulations and achieves the target U-value.

How to fix cold internal walls?

The most effective way to fix a cold internal surface of an external wall is to insulate it. You have two main options: 1) Internal Insulation (Dry Lining), where insulation is fixed to the inside face of the wall. This is a very effective solution but reduces room size. 2) External Wall Insulation (EWI), where the insulation is applied to the outside of the house. EWI is technically superior as it wraps the entire house without losing internal space, but it is also more expensive and changes the building's exterior appearance.

What is the life expectancy of internal wall insulation?

When installed correctly by a professional, internal wall insulation should last the lifetime of the building. The materials themselves—like rigid PIR boards or mineral wool—do not degrade or lose their thermal performance over time. The key to longevity is correct installation, particularly ensuring the vapour control layer is perfectly sealed. This prevents any moisture from getting into the structure, ensuring the system remains effective for many decades.

What is better, Rockwool or Kingspan?

It depends on your priorities. For pure thermal performance in the slimmest profile, Kingspan (which is a brand of PIR/phenolic board) is better. You need less thickness to achieve the same U-value. For fire resistance and acoustic (sound) insulation, Rockwool (a brand of mineral wool) is superior. Rockwool is also more “breathable” (vapour permeable). In internal wall insulation, PIR boards are often preferred to save space, but mineral wool is excellent if soundproofing or breathability is a key concern.

Why are people removing spray foam insulation?

This issue primarily relates to open-cell spray foam applied incorrectly in attics and roofs. When applied directly to the underside of the roof felt, it can trap moisture, lead to timber rot, and make it impossible for surveyors to inspect the roof structure. This has led to many banks refusing mortgages on homes with this type of insulation, forcing homeowners to have it professionally removed. It's less of an issue for closed-cell spray foam applied correctly to walls, but the negative association has made many homeowners and professionals wary. For internal walls, insulated plasterboard or built-up systems are a more common and trusted solution.

What is the cheapest way to insulate walls?

For homes with a cavity, pumping the cavity with insulation beads is by far the cheapest and least disruptive method. For solid walls, the cheapest option is typically installing insulated plasterboard panels yourself. These panels combine insulation and plasterboard into one product. However, this is only suitable for very competent DIYers, as achieving proper airtightness and dealing with details around windows and sockets is crucial. For a professional installation that qualifies for SEAI grants, internal insulation is generally less expensive upfront than external insulation.

What is the downside of Rockwool?

The main downside of Rockwool (mineral wool) compared to rigid boards like PIR (Kingspan) is its lower thermal resistance for a given thickness. This means you need a thicker layer of Rockwool to achieve the same level of insulation, which results in a greater loss of internal room space. It can also be dusty and requires personal protective equipment (PPE) during installation, although it is perfectly safe once enclosed behind the plasterboard.

What happens if you over insulate?

You can't really “over-insulate” a wall in terms of thermal performance; more is generally better for reducing heat loss. However, as you make a house extremely airtight by adding lots of insulation and vapour barriers, you must also install a proper ventilation system. Without adequate ventilation (e.g., Mechanical Ventilation with Heat Recovery - MVHR), indoor air quality can suffer, and you can get problems with condensation, humidity, and stale air. So, “over-insulating” is only a problem if you don't upgrade your ventilation to match.

Can I install internal wall insulation myself?

A competent and experienced DIYer can technically install internal insulation, particularly using insulated plasterboard products. However, it is a challenging job. Achieving a continuous, perfectly sealed Vapour Control Layer (VCL) is critical to prevent moisture problems, and this is where many DIY installations fail. Crucially, to be eligible for any SEAI grants, the work MUST be carried out by a registered SEAI contractor. For this reason, and to ensure performance and avoid long-term issues, professional installation is almost always recommended.

Internal Wall Insulation: Complete Guide for Irish Homeowners

Dry lining is a popular way to insulate solid walls from the inside, making your home warmer and more energy-efficient. Learn about costs, grants, and find trusted installers.

Internal Wall Insulation, commonly known as 'dry lining', is a highly effective method for improving the thermal comfort of homes in Ireland, especially those with solid concrete block or stone walls where cavity insulation isn't an option.

By adding a layer of high-performance insulation to the inside of your external walls, you can dramatically reduce heat loss, lower your energy bills, and create a much cosier living environment. On ISOTHERM, you can connect with verified installers who are registered to carry out work under the SEAI internal insulation grant.

Key Benefits of Internal Insulation

Warmer Home

Eliminates cold wall surfaces for a cosier living space.

Lower Bills

Significantly reduces heat loss, cutting your energy costs.

Better BER Rating

Improves your home’s energy rating, adding to its value.

Grant Eligible

Qualifies for valuable SEAI grants to reduce upfront cost.

Advantages & Disadvantages

Advantages

Generally less expensive than external wall insulation.

Doesn't change the exterior appearance of your home, ideal for brick or stone-faced properties.

Work is not dependent on weather conditions and can be done year-round.

Disadvantages

Reduces internal room size by 100-150mm (4-6 inches).

Disruptive work inside the home; rooms need to be cleared and redecorated.

More difficult to deal with thermal bridges around floor and ceiling junctions.

Verified Internal Wall Insulation Installers

The quality of a dry lining installation is critical. ISOTHERM lists verified professionals who are registered with the SEAI and have a proven track record.

Frequently Asked Questions

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