ᐅ Moisture in the Attic: Preventing Condensation During Insulation and Renovation

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:

• 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.

Hi,

The condensation is not surprising, even if it usually only appears after everything is already sealed up. The attic is cold, below you bring in warm, moist air, the vapor barrier ends before the ridge, and above there is a cold breathable membrane—this results in dripping condensation. Ventilation openings in the breathable membrane are mostly a cosmetic measure and may completely compromise the membrane’s function, providing little benefit from a building physics perspective. You either insulate consistently all the way to the ridge and extend the vapor barrier airtight up to the ridge with proper seals, or you keep the attic intentionally cold. In the latter case, there must be a defined ventilation layer of about 4cm (1.5 inches) above the insulation up to the ridge, including genuine ridge ventilation. Half warm and half cold never works, regardless of what the roofer said. Using OSB as an airtight layer is also risky—its sd value can quickly be 3m (10 feet) or more, and it does not forgive mistakes. The real question is what the attic should be used for in the long term: storage space or part of the thermal envelope?

The space under the attic roof is intended to become the living room. The attic itself (approximately 1 meter (3 feet) high) is not meant to be used.

If the attic is definitely going to remain unheated and unused, the approach is actually straightforward, even though people tend to overcomplicate it. Warm living space below, cold attic above, with a truly airtight layer in between—anything else will cause problems sooner or later. Your current issue wasn’t so much the construction itself, but the phase in between: heated construction site, building envelope still open, moisture moving around. This happens faster than you might think—a single day with a portable heater can easily release several liters of water into the air.

What’s important now is that the ceiling insulation and the underlayment membrane dry out again, so ventilate thoroughly and heat only the lower living areas, not the attic space. Insulating the attic would be technically possible, but for only 1 meter (3 feet) of height it’s generally unnecessary effort, especially since this would require careful detailing all the way up to the ridge. Ventilation is essential, but should be done in the classic way through the eaves and ridge, no makeshift solutions. After that, you can continue building as usual once everything is dry.

Many thanks for the quick and professional response.

• Should I then open the breather membrane by a few centimeters at the ridge? (I have glued the breather membranes to each other underneath the ridge) This way, any moisture that may appear can also ventilate.
• Leave the breather membranes glued like this?

Best regards

Gladly! – If the attic remains cold, the roofing underlay should be left open at the ridge or routed over a suitable ridge vent; otherwise, moisture will accumulate permanently. The sealing itself is fine, but problematic for an unventilated cold area. It is important that the vapor barrier below is completely airtight; otherwise, you ventilate at the top and add moisture from below, resulting in a zero-sum effect.