Over The Purlins

How to install insulation ?

Why install insulation over roof purlins?

Insulation over roof purlins is one of the most effective ways to cut radiant heat gain through a steel or timber roof. The reflective layer sits directly beneath the roof covering, so it intercepts heat from the sun before it can warm the space below. Alumix reflective foil insulation is made for this job, and the photos above show the installation stage by stage.

Fitting it over the purlins also gives you one continuous layer across the whole roof, with no gaps where the framing would otherwise act as a thermal bridge.

How insulation over the purlins works

Reflective foil insulation, often called a radiant barrier, reflects radiant heat instead of absorbing it. Because most of the heat that enters through a sun-heated roof is radiant, a reflective layer under the roof sheets keeps the building cooler in summer and helps limit condensation on the underside of the roof.

This method suits new roofs on steel and timber purlins, and it is the simplest approach when the roof sheets have not yet been laid.

Step-by-step: installing insulation over roof purlins

1. Measure the roof and plan the roll layout so seams run in a neat, consistent direction.
2. Clear the purlins of debris, sharp edges and loose fixings that could tear the foil.
3. Unroll the insulation across the purlins with the reflective face pointing up toward the roof covering.
4. Pull the sheet taut and fix it temporarily at the edges so it does not sag between purlins.
5. Overlap neighbouring rolls and seal every joint with foil tape for a continuous barrier.
6. Lay the roof sheets or tiles over the insulation and fasten them through it.
7. Check the finished layer for tears and patch any damage with foil tape.

Choosing the right Alumix product

Alumix offers several reflective insulation rolls for roof applications, including:

• Bronze FR 4mm Fire Rated Reflective Foil Insulation
• Alumix Silver 6mm Basic/FR Insulation
• Multi+ 10mm Fire Rated Multi-Layer IXPE Reflective Insulation
• RADWRAP T180 REF Breathable Reflective House Wrap

Visit each product page for full details, or contact the Alumix team for help choosing.

Installation tips

• Tape every seam. Sealed joints keep humid air away from cold roof sheets.
• Keep the foil clean. Dust and dirt reduce how well the surface reflects heat.
• Leave an air gap where you can. Reflective foil performs best facing open air.
• Work safely at height. Use proper access equipment and follow local safety rules.

Frequently asked questions

Can I install insulation over timber and steel purlins?

Yes. Reflective foil insulation can be laid over both timber and steel purlins, provided the surface is clear of sharp edges and the sheet is fixed securely.

Which way should the reflective side face?

Face the reflective side toward the heat source, which on a roof is the roof covering. Follow the instructions supplied with your roll.

Do I need to seal the joints?

Yes. Taping every overlap creates a continuous layer and helps control moisture and condensation.

Related installation guides

Compare this method with installing under the batten, see how to insulate a metal building, or browse the full application gallery.