ᐅ Moisture in the Attic: Preventing Condensation During Insulation and Renovation
Created on: 27 Dec 2025 17:35
W
worf1985
Hello respected experts,
We are currently working on converting our attic. This is being done from the inside, and following the advice of a friend who is a roofer, we have implemented the following steps:
Now we have noticed by chance that there is considerable condensation forming on the roofing membrane in the attic space. This moisture is dripping onto the ceiling insulation and running down at the sides of the insulation. It basically dawned on me when I heated the construction site briefly with a small fan heater. It seems that warm air is being drawn through the not yet completed ceiling into the attic, where it condenses. This was obviously a foolish mistake on my part.
What do you recommend?
I would greatly appreciate helpful tips.
Best regards and many thanks in advance.
We are currently working on converting our attic. This is being done from the inside, and following the advice of a friend who is a roofer, we have implemented the following steps:
- Breathable roofing membrane installed between the rafters up to the roof ridge
- Insulation between the rafters up to the ceiling level, but not reaching the ridge
- Vapor barrier applied fully and airtight on top of the insulation
- Additional insulation beneath the rafters
- OSB boards installed
Now we have noticed by chance that there is considerable condensation forming on the roofing membrane in the attic space. This moisture is dripping onto the ceiling insulation and running down at the sides of the insulation. It basically dawned on me when I heated the construction site briefly with a small fan heater. It seems that warm air is being drawn through the not yet completed ceiling into the attic, where it condenses. This was obviously a foolish mistake on my part.
What do you recommend?
- There is still no heating installed in the attic / we have installed the vapor barrier very carefully / but about 30% of the conversion work is still missing.
- Should the attic also be additionally insulated all the way to the ridge?
- Should the attic be ventilated?
- Can we simply continue the conversion after airing out the self-generated moisture?
I would greatly appreciate helpful tips.
Best regards and many thanks in advance.
Many thanks,
I have now taken a closer look and noticed the following. On one side, I kept the old roofing underlay based on the advice of several "experts." I have just felt behind the insulation between the rafters.
On the side with the old roofing underlay (which seems more like a vapor barrier to me), everything is damp.
On the side with the correct new roofing underlay (everything is dry).
This means for me starting over from scratch and replacing the old roofing underlay with the new, properly vapor-permeable one??
If you already rely on your experts, I was convinced from the beginning that I needed to install the new one, but everyone said I could save myself the work.
I have now taken a closer look and noticed the following. On one side, I kept the old roofing underlay based on the advice of several "experts." I have just felt behind the insulation between the rafters.
On the side with the old roofing underlay (which seems more like a vapor barrier to me), everything is damp.
On the side with the correct new roofing underlay (everything is dry).
This means for me starting over from scratch and replacing the old roofing underlay with the new, properly vapor-permeable one??
If you already rely on your experts, I was convinced from the beginning that I needed to install the new one, but everyone said I could save myself the work.
The old roofing membrane has proven to be a moisture trap, just as expected. If you leave the attic space unheated, you definitely need a new vapor-permeable underlayment membrane; otherwise, moisture will continue to accumulate, and the insulation will remain wet permanently. Seals, ridge connections, eaves – everything must be done properly, or else the attic will only theoretically ventilate, but not practically. Half-heartedly leaving the old membrane in place was a classic mistake, as you could have avoided the condensation forming directly on the underlayment membrane. The question now is whether you want to rebuild completely or just partially replace it, since any area where the old membrane remains poses a potential condensation risk.
So, I’m going to completely rebuild. Everything else is old news. On one side of the roof, I’ve already installed the new underlay membrane, which means OSB on one side and down to the ceiling again. Vapor barrier removed. Insulation below and between the rafters taken out. Then the old membrane removed and replaced with a new vapor-permeable one, and then rebuilt. This way, moisture can diffuse outward and the problem is solved.
It’s frustrating on one hand, but better to discover this now than after years.
A roofing company raised the collar beams in the attic and advised us to do exactly what we did, including leaving the old membrane in place. Sometimes I wonder.
Tomorrow is the plan for the dismantling. I will definitely keep you updated.
It’s frustrating on one hand, but better to discover this now than after years.
A roofing company raised the collar beams in the attic and advised us to do exactly what we did, including leaving the old membrane in place. Sometimes I wonder.
Tomorrow is the plan for the dismantling. I will definitely keep you updated.
@Nauer
So, we have now replaced the old membrane with a new breathable membrane. The insulation has been reinstalled dry.
One question still needs to be answered for now 😉
Does the roof underlay really need to be open by about 5cm (2 inches) at the ridge? Is a non-sealed overlap at the ridge sufficient? Does this opening need to be present at every rafter space?
Kind regards
So, we have now replaced the old membrane with a new breathable membrane. The insulation has been reinstalled dry.
One question still needs to be answered for now 😉
Does the roof underlay really need to be open by about 5cm (2 inches) at the ridge? Is a non-sealed overlap at the ridge sufficient? Does this opening need to be present at every rafter space?
Kind regards
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