Hi all

We have a 1950s Hipped (Traditional cut) roof bungalow configured in an L shape. So we have no gables anywhere. Currently we are upgrading the loft insulation of a loft conversion that was carried out in the 1980s - We are doing other bits including relocating loft access stairs (Spiral to a proper staircase) and hence Building control are involved.

BCO has asked for a 50mm ventilation gap between the underside of roofing felt and the insulation as we are insulating at rafter level. My understanding is the aim is to have airflow from the eaves vents up through each rafter 'bay' then down the other side of the roof or out of a ventilated ridge.

With this aim I am struggling to see how I can maintain this crossflow air path with the hipped roof construction. We have Hip rafters at each corner, with Jack rafters leading from the eaves up to rest on these Hip rafters. So these effectively are 'dead ends' and airflow would never reach the ridge. Then when it comes to a rafter that does reach from the eaves to the ridge this airflow is blocked by the timber trimmers of Velux windows. So only a handful of the rafter bays will have a true airflow path.

What's the most common way to overcome this issue? I'm not sure drilling holes all along the ends of the rafters does much good to structural strength!

The existing roof membrane is a breathable type, as the bungalow had a new replacement slate roof only 10 years ago. So I'm not overly keen to be stripping any of It off if I can help it.

Any Ideas?

Many thanks in advance

Common problem with hips. The jack rafters and the trimmers round the Velux leave dead end bays, and drilling through rafter ends near the bearing isn't something I'd do without an engineer agreeing it.

First thing I'd do is get the data sheet for the membrane that went on with the slates 10 years ago. If it's a genuine low resistance vapour permeable membrane with third party certification for use without a ventilated void, BS 5250 allows you to insulate without the 50mm cross flow, provided there's a well sealed vapour control layer on the warm side. Plenty of BCOs will accept that if you show them the paperwork, so it's worth asking before you design round a vent path you can't get.

If they still want ventilation, the usual fix is to give each dead end bay its own outlet near the top, either a slate vent fitted from outside in that bay or small vents through the Velux trimmer zone, while keeping the 50mm gap with stops or spacers so the insulation can't creep up to the felt.

Whichever route, the VCL matters most. Tape every joint and seal round the Velux linings and any cables, because that stops the moisture getting there in the first place. Insulating partly between and partly under the rafters also helps you keep the gap and reach the U value.

13 hours ago, JpManning said:

Common problem with hips. The jack rafters and the trimmers round the Velux leave dead end bays, and drilling through rafter ends near the bearing isn't something I'd do without an engineer agreeing it.

First thing I'd do is get the data sheet for the membrane that went on with the slates 10 years ago. If it's a genuine low resistance vapour permeable membrane with third party certification for use without a ventilated void, BS 5250 allows you to insulate without the 50mm cross flow, provided there's a well sealed vapour control layer on the warm side. Plenty of BCOs will accept that if you show them the paperwork, so it's worth asking before you design round a vent path you can't get.

If they still want ventilation, the usual fix is to give each dead end bay its own outlet near the top, either a slate vent fitted from outside in that bay or small vents through the Velux trimmer zone, while keeping the 50mm gap with stops or spacers so the insulation can't creep up to the felt.

Whichever route, the VCL matters most. Tape every joint and seal round the Velux linings and any cables, because that stops the moisture getting there in the first place. Insulating partly between and partly under the rafters also helps you keep the gap and reach the U value.

Thanks for the reply, I did get the date sheet from the manufacturer. Its IKO Rubershield Light - Listed SD value of 0.02m - Which I assume is the vapour permeability figure? I'm not sure if this is low or not?

The internal VCL I was going to use was Proclima Intello plus. It's going to be well sealed as it's going to be my Airtighness layer (Tons of tescon Vana!) as we are going for an MVHR system. Which hopefully I am assuming will also keep internal moisture levels lower and prevent anything migrating and causing issues?

10 hours ago, SteveG40 said:

Thanks for the reply, I did get the date sheet from the manufacturer. Its IKO Rubershield Light - Listed SD value of 0.02m - Which I assume is the vapour permeability figure? I'm not sure if this is low or not?

The internal VCL I was going to use was Proclima Intello plus. It's going to be well sealed as it's going to be my Airtighness layer (Tons of tescon Vana!) as we are going for an MVHR system. Which hopefully I am assuming will also keep internal moisture levels lower and prevent anything migrating and causing issues?

Yes, that's the one. An SD of 0.02m is very vapour open. BS 5250 counts anything around 0.05m or less as low resistance (LR), so you're comfortably in that bracket, which is what lets you leave out the vent gap.

Intello Plus is a good choice for the inside. Being variable, it holds back moisture in winter and lets the roof dry inwards in summer, so you get some forgiveness if anything does get in. The airtightness matters more than the membrane itself, so keep at it with the tape at laps, rafter ends, wall junctions and anything that goes through. A service batten void behind the plasterboard saves you puncturing it for cables and downlights.

MVHR will help by keeping indoor humidity lower, as long as it's balanced and running from the day you move in. Drying out the wet trades with it before you close up is worth doing too.

Last check: make sure the IKO data sheet or certificate says it's suitable for unventilated roofs with your tile type, and leave the batten space under the tiles free to breathe.

On 31/10/2026 at 09:24, JpManning said:

Yes, that's the one. An SD of 0.02m is very vapour open. BS 5250 counts anything around 0.05m or less as low resistance (LR), so you're comfortably in that bracket, which is what lets you leave out the vent gap.

Intello Plus is a good choice for the inside. Being variable, it holds back moisture in winter and lets the roof dry inwards in summer, so you get some forgiveness if anything does get in. The airtightness matters more than the membrane itself, so keep at it with the tape at laps, rafter ends, wall junctions and anything that goes through. A service batten void behind the plasterboard saves you puncturing it for cables and downlights.

MVHR will help by keeping indoor humidity lower, as long as it's balanced and running from the day you move in. Drying out the wet trades with it before you close up is worth doing too.

Last check: make sure the IKO data sheet or certificate says it's suitable for unventilated roofs with your tile type, and leave the batten space under the tiles free to breathe.

Ideal, So seems we are all good on the membrane. We are using Wood fibre insulation so hopefully this will work well with the Intello from a breathable/vapour permeable aspect. Plus the MVHR fingers crossed we keep the timbers dry!

Just had a read of the data sheet that IKO emailed over to me. Top line says this...

High Performance Breather Membrane For Tiled And Slated Pitched Roofs Warm Or Cold Roof Application (Ventilated Or Unventilated Void)

So I guess its ok for this purpose.

Many thanks again.