How do I measure and calculate how much insulation I need in my loft?
It is important to take the time to work out how much insulation you need to ensure you get the best fit and also, so you don’t over or under order. Planning the installation is an important part of the job and correctly laid insulation significantly improves thermal performance.
When insulating using Thermafleece we recommend you should install to a minimum depth of 240/250mm and ideally 300mm particularly for new build properties. You will need to install one layer between the ceiling joists and one or more layers above the level of the joists. The insulation above the joists is cross-laid over the joist meaning it runs perpendicular to the direction of the joist. The joists are the horizontal timbers that support the ceiling below the loft. These are often confused with rafters which are the angled timbers that support the roof.
There are 3 simple steps to measuring for your loft
Step 1. Measure the total loft space.
This will provide a total square metre requirement. Simply multiply the length (in metres) by the width (in metres) of the loft to calculate the area in sq.m. For the layer of insulation that is fitted between joists, deduct 10% of the loft area to account for displacement of the joists. For insulation layers that sit above the level of the joists take the full loft area. If you can’t access the loft or to give yourself a sense check, simply measure the outside of the building for dimensions.
Step 2. Measure the space between your joists.
Measure the width between joist ensuring the tape measure runs perpendicular to the joist. Repeat this to determine the minimum and maximum spacing between joists. This will allow you to determine the width of insulation required to fit between joists. Often and especially in older properties the width does vary but several width options are available, and Thermafleece is a quilt material which is forgiving against the timbers. Thermafleece is available in a variety of widths. This is only required for the layer of insulation fitted between joists, layers cross-laid over joists do not need to fit between anything so we generally recommended our widest widths for this.
Step 3. Measure the depth of the joists.
Typically, if your joists are 100 mm thick, we would recommend you full fill the joists with 100 mm thick. To increase the thickness and eliminate heat loss, we would recommend 100 between and at least 140 mm over the joist. This generally means that you put one layer between the joists then other layers running above the joists across the direction of the joist.
Example Using Thermafleece CosyWool Roll.
Space between joists is 350mm. 370mm insulation width required.
Loft Area – 9 metres x 7 metres = total area 63m2
Joist depth – 100mm
Recommended Insulation depth is 240mm
Between the joists:
100mm thick x 370mm wide. 3 rolls in a pack and total m2 of one pack is 7.22m2
Area required is 63 sq.m less 10% = 56.7 sq.m
56.7 square metres ÷ 7.22 square metres per pack = **7.85 packs
You would need to order 8 packs of 100 x 370 which will provide you with 57.8 sq.m to go between the joists.
Cross-laid over the joists *
140mm deep x 570 wide 2 rolls in a pack and total m2 of one pack is 5.24m2
63 square metres ÷ 5.24 square metres per pack = ** 12.02 packs
You would need to order 12 packs of the 140 mm x 570mm which will provide you with 62.88 square metres to go crossways over the joists.
*Remember: Layers cross-laid over joists don’t need to fit between anything so we generally recommended our widest widths for this
**We don’t split packs so depending on how generous your measurements are, round up or down
Find out more:
Further information is available in our Help and Advice Section: https://naturalinsulations.co.uk/help-advice/
Thermafleece British Sheeps Wool: https://www.thermafleece.com/
Featured Image is from the Cold Roof Application Notes, one of a series available in the Help and Advice Section