If access is easy and your loft joists are regular, you can use rolls of mineral wool insulation. The first layer is laid between the joists – the horizontal beams that make up the floor of the loft – then another layer is laid at right angles to cover the joists and make the insulation up to the required depth.
It's recommended to have between 200 and 300mm of insulation in the loft so if you're installing floor boards you will also need to add another level to the existing joists running in the other direction in a criss cross. In either of the above cases you can add some 150mm timber then add 150mm loft insulation.
Closed cell spray polyurethane foam (SPF) is an excellent choice for insulating attics and rim joists.
Perhaps the most effective heat-retaining insulation on the market, spray foam is a synthetic expanding foam made from polyurethane and boasts some of the highest performance of all insulators on the market. It's typically applied by being sprayed between the rafters from the inner roof of the loft.
One important thing to remember is that insulation should never be pushed right into the eaves because it will interfere with airflow. If you're going for a warm loft then you'll need much more insulation to be able to cover the underside of the roof along with the gable and party walls.
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.
Should I remove old loft insulation before laying new? Older insulation material will typically need to be removed before adding new insulation. However, in some cases the material may be in good condition, meaning you can simply top up the existing insulation material for a fraction of the cost of a full replacement.
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.
If you're concerned about having a positive impact on the environment, fiberglass batt is probably the best insulation option for your attic.
Do you have any plans for furnishing/fitting out your attic in the future? Then you could simply insulate the pitched roof. But the best solution is actually to insulate both the attic floor and the underside of the roof.
Installation from below the floor joists
The insulation boards should be cut to fit snugly between the floor joists. Measure the distance between the joists prior to cutting the boards as the spacings can vary. Push the cut insulation boards between the joists so they are flush with the underside of the floorboards.
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.
The recommended R-value for attic insulation can vary depending on the climate zone in which you live. The U.S. Department of Energy recommends attic insulation levels of R-49 to R-60 for most homes in colder climates, and R-30 to R-49 for homes in warmer climates.
It's important to leave a ventilated air gap between the insulation and the boards to prevent condensation on the underside of the boards. Make sure you don't squash the mineral wool when you fit the boards on top, as this will reduce its insulation value.
Blanket (sheet) insulation
This group of insulation materials - such as mineral wool, sheep wool, and fibreglass - are installed by securing at least 270mm thickness insulation in between and over your roof rafters. A 25mm - 50mm air gap must be left between the material and the roof underside for adequate ventilation.
This is a unit of thermal resistance and rates the insulating properties of the material. The higher the R-value, the greater the insulation. A minimum R-value of 6.1 is recommended for loft floor insulation, but ideally it would be up to 7.0.
While both types provide insulation, R30 is the better option for maximizing energy savings in areas where heat loss or gain is a major concern. For example, R19 may suffice for interior walls, but R30 is more appropriate for exterior-facing attics or floors.
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.
Remember that insulation should never be placed right into the eaves, as this will impact ventilation.
Benefits of Installing Extra Loft Insulation
Energy Saving Trust reports that a detached house can save £590 a year on energy bills if installing 270mm of insulation in the loft where none existed before. If upgrading the thickness from 120mm, this yearly saving drops to £55.
Remember that insulating between the joists will create a cold roof space as the heat is kept underneath the insulation like a blanket. Therefore, if you require a warm space for your storage it's better to insulate between the rafters in the roof rather than the joists.
Leaving Too Many Gaps
One of the biggest problems DIY homeowners face is leaving too many gaps in their insulation. Gaps or insulation that isn't secured correctly can leave the building structure open to air and moisture leaks. Spray foam insulation is best for reaching into small cracks and any areas left open.
Unless it's been damaged, loft insulation should last for around 80 to 100 years. It often depends on the material though. For example, spray foam insulation can last up to 80 years, while foamboard and mineral wool insulation can last a whole century before it needs replacing.