DIY Doctor UK DIY Forums

Hi all

I bought an old bungalow a couple of years ago which we have renovated, including having a completely new roof put on.

I fitted 120mm PIR between the rafters and sealed along the edges with foil tape. During this cold weather I have come to fit a 20mm PIR across those same rafters but have found water dripping through the 120mm boards. I took a piece out and found a lot of condensation behind the board. I have only found it in one gap, it doesn't seem to be leaking anywhere else?

Why is condensation gathering in this particular spot? What can I do?

Thanks for any help

Hi jamgars,
From your description it sounds like you have fully filled the space between the rafters with insulation so there is no gap between the top of the insulation and the underslaters felt. I suspect you will find condensation to varying degrees above all the insulation. If breathable underslaters felt was used when the roof was redone and you fit a damp-proof membrane before installing the 20mm pir overboarding this issue may resolve itself.
Why has it only appeared in one section? Maybe this is above a bathroom or the tape seal is defective.
Regards S

Hi stoneyboy thanks for reply

I have left a gap behind the insulation of about 100mm. I have had a look and I found the insulation beads from between double skin walls have overflowed and filled behind the soffits/fascias and therefore blocked airflow. I've cleared that out and refit the PIR. Does that sound like a likely cause? I'll await to see if I get any more drips.

Hi jamgars,
Lack of ventilation may well be the source of the problem. Perhaps refit the panel where the drips were forming and wait for the next cold spell and see if the problem still exists.
As a matter of interest you appear to have 225 deep rafters - is this correct?
Regards S

stoneyboy wrote: ↑

Tue Jan 14, 2025 10:46 pm

Hi jamgars,
Lack of ventilation may well be the source of the problem. Perhaps refit the panel where the drips were forming and wait for the next cold spell and see if the problem still exists.
As a matter of interest you appear to have 225 deep rafters - is this correct?
Regards S

Hmm, yeah 220 I think