What is the best thickness of loft insulation?

Author: Terry Roberts  |  Last update: Saturday, May 10, 2025

Since 2003, current building regulations recommend a depth of at least 270mm (mineral or glass wool) in the loft – use this as a starting point rather than a finishing line to see larger savings. In 1995, the recommended depth was 200mm. Prior to that it was less than 100mm.

Is 400mm loft insulation too much?

We'd recommend everything in moderation. While some people argue that an over-insulated house could cause damp, mould and other issues. Typically, upping your loft insulation from 270 mm to 400 mm isn't really going to make a big difference, and so it's probably worth sticking with the lesser amount.

Is a 100mm of insulation enough in a loft?

What's the best loft insulation thickness? The UK government recommend a minimum insulation thickness of 270mm although we tell people that 100mm should be the absolute minimum.

Is 50mm loft insulation enough?

The 50mm thickness is a common standard for loft insulation and provides a good level of thermal resistance to prevent heat from escaping through the roof. By installing this type of insulation, homeowners can improve the energy efficiency of their homes, reduce their carbon footprint, and save money on heating bills.

What thickness is best for attic insulation?

The higher the R-Value, the better the thermal performance of the insulation. The recommended level for most attics is to insulate to R-38 or about 10 to 14 inches, depending on insulation type.

The the wrong and the right way to insulate a loft .

Can loft insulation be too thick?

Can Loft Insulation Be Too Thick? Although it can be tempting to believe more is always better, exceeding recommended loft insulation thicknesses can actually have a number of negative consequences. The most common of these is poor ventilation, which can lead to issues such as damp, condensation, and mould.

Can you over insulate an attic?

While loft insulation is an effective way to improve energy efficiency and reduce heating costs, over-insulating can lead to several potential issues. Firstly, over-insulation can lead to poor ventilation in the attic, which can cause moisture buildup and increase the risk of mould growth.

Can you put too much insulation in a loft?

Excessively thick insulation can block most ventilation sources, reducing air quality and restricting air exchange within the loft. It can also negatively impact air circulation, which causes respiratory problems and other related health issues.

Is 25mm insulation worth it?

25mm Mannok PIR Insulation Board is an excellent choice for anyone looking for a high-performance insulation solution that provides superior thermal efficiency. It is ideal for use in any construction project, such as commercial and residential, in wall applications as well as roofing and flooring applications.

Where not to put loft insulation?

Don't block any vents such as soffit vents with insulation. Don't squash the insulation as this can reduce its thermal performance by over 50%. Don't install loft boards directly to joists or trusses.

How thick should loft insulation be in 2024?

In 2024, UK loft insulation rules require a minimum insulation thickness of 270mm to meet a U-value of 0.16 W/m²K, along with proper ventilation and fire safety compliance.

Can insulation be too thick?

It is possible to over-insulate your house so much that it can't breathe. The whole point of home insulation is to tightly seal your home's interior. But if it becomes too tightly sealed with too many layers of insulation, moisture can get trapped inside those layers.

What is the difference between 50mm and 100mm insulation?

100mm Insulation: The Ultimate Thermal Efficiency

With double the thickness of 50mm insulation and quadruple the thickness of 25mm insulation, it provides an excellent barrier against heat transfer.

Does 300mm of loft insulation make a difference?

It is recommended to have at least 270mm of loft insulation material in place, especially when using insulation materials such as mineral wool or sheep wool. Increasing this thickness to 300mm may provide slightly better insulation, but 270mm is considered more than adequate for the job.

Is fibreglass loft insulation illegal in the UK?

Fibreglass insulation is not illegal in the UK and is quite common in loft upgrades, however, it's important to handle it carefully to avoid health risks.

How much gap do you need to leave loft insulation?

You'll need to start laying with a minimum gap of 25mm between the eaves and the insulation to ensure a suitable air gap. Maintain this gap along two opposite sides of the loft.

How many mm should loft insulation be?

Current government recommendations are for loft insulation thickness to achieve a depth of 270mm, but some new properties are increasing their level of loft insulation to 300mm.

How do I choose insulation thickness?

Choosing the right thickness for your insulation depends on various factors such as climate, location within your home, and type of material used. It's crucial to do thorough research, consult with professionals, and ensure proper installation for maximum efficiency.

Is 25mm insulation enough for a shed?

25mm boards provide a good blend of price and effectiveness for insulating a shed on a budget.

How do I know if my loft insulation is OK?

Check your loft for insulation
  1. The thickness of the existing insulation: Any insulation is better than no insulation but the recommended thickness for maximum performance is 400mm.
  2. The state of the insulation: Is it mouldy? Is it falling to pieces? ...
  3. How the insulation is laid: Are there gaps where air could pass?

Is it okay to cover attic insulation with plywood?

Can I put plywood over insulation in the attic? Yes you can install plywood subflooring directly over the floor joists and insulation on your floor. You need to install rigid insulation boards on top of the attic flooring joists and then install plywood over the top.

What happens if you over insulate an attic?

Too much insulation will leave humidity and moisture to accumulate in your attic space. This can lead to mold growth on your walls and insulation. There might also be water damage to the support beams in your home's roof.

Where should you not put insulation in the attic?

Also, when laying down additional insulation, work from the perimeter toward the attic opening. Never lay insulation over recessed light fixtures or soffit vents. Keep all insulation at least 3 inches away from recessed "can" lights, unless they are rated IC (Insulated Ceiling).

Is it OK to board over loft insulation?

Boards for loft flooring can be installed on top of insulation using plywood, wood, or plastics. Installing loft flooring and boarding over insulation helps save space, prevent heat loss, and increase home value.

Should I remove old attic insulation before adding new?

Do I Have To Remove Old Insulation? Unless your insulation is wet, moldy or infested with pests, you don't need to remove it before adding insulation on top of it. ENERGY STAR® recommends that you inspect your insulation for signs of mold, mildew, moisture and pest infiltration before making this determination.

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