Ventilated roof

[Alberto Morales]

hi,

What can make a risk for sheathing board to have strong ventilation in a location like london for the general ventilated roof from the guideline "Guideline for the Calculation of Ventilated Pitched Roofs"? moisture accumulation in the sheathing board layer due to air carrying high moisture from air infiltration and cold air temperatures in the layer due to strong ventilation? the input are the same than the example file, I just change location and orientation.

london different ventilation rates.png (33.15 KiB) Viewed 15 times

london different positions.png (33.18 KiB) Viewed 15 times

[Christian Bludau]

Hi Alberto

Strong ventilation of the cavity can indeed increase the moisture risk for the sheathing board, especially in climates such as London.

While increased ventilation is often assumed to improve drying, it can also lead to lower temperatures within the ventilated cavity. A colder sheathing board surface can increase the relative humidity at the board and, under certain conditions, result in higher moisture contents.

This is one of the reasons why the guideline evaluates different ventilation rates rather than assuming that more ventilation is always better.

Looks like the increased ventilation leads to lower sheathing board temperatures, which shifts the balance towards higher relative humidity and moisture content at the board.

Christian