Loft Floor Insulation Using PIR Boards

29 Aug 2017
Reaction score
United Kingdom
Anyone like to comment on my plans for flooring my daughters' loft so it can be safely used for storage. See photo.

The ceiling joists are 4x2in (95x45mm), spaced at approx 450mm intervals. There are 5x2in (125x50mm) binders on top of the joists, 2m apart from each other. Between the joists (hidden by insulation in the photo) are various 50x35mm battens, which I assume are either to maintain the spacing of the joists during construction, and/or used to aid fixing the ceiling below.

The intention is to board out the 2x6m area between the house side walls (it's a semi-detached house) and the binders.

Once the insulation between the binders has been removed and the ceiling vacuumed of any dust/rubbish, I plan to cut, fit and tape 90mm foil faced PIR board between the joists (actually 1200x450mm cavity batts so I can get them through the hatch). Then screw 6 off 3x2in (63x38mm) cross battens, parallel to the binders, giving a spacing of approx 400mm between the cross battens. Between the cross battens I will cut, fit and tape 60mm foil faced PIR boards, giving a total of 90 + 60 = 150mm of PIR board, which gives the equivalent of approx 300mm of fibreglass, slightly exceeding the 270mm recommended depth.

To ensure there is adequate ventilation to guard against interstitial condensation under the floor, I will raise the cross battens up off the joists with 20mm packers. This will give me a nominal 23mm air space between the floor (18mm chipboard panels) and the top of the PIR boards.

I know its not the 50mm gap I've seen mentioned elsewhere on this forum, but it's easy to achieve without using much heavier cross battens, and therefore I think is a good compromise.


  • IMG_20210830_174651.jpg
    504.4 KB · Views: 293
Sponsored Links

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local