Good or Half Good Without Masonite for Soffit Ventilation

[Stringfellow Hawke]

Hello! Why do you have an air gap?
Well, it's not to ventilate away moisture coming from above, but to remove moisture from the indoor environment that always seeps through the vapor barrier via leaks from inside.
It's impossible to make it 100% tight everywhere, plus the plastic itself is not 100% airtight, even if it is of an approved type. If you also don't have mechanical exhaust ventilation that creates a slight negative pressure in the house that draws air through any leaks in the vapor barrier, then the vapor pressure in the house will instead push out warm air (which can carry a lot of moisture) into the insulation and condense in the insulation against the fiberboard, which then breathes out this moisture into the air gap and away/out through the ventilation at the ridge.

Therefore, it's important to have regular and not oil-hardened fiberboard in the gap.
It also obviously does a good job if a leak should occur from outside.

Regards, Pelleulf

The latest findings suggest that roof ventilation will disappear. It has been observed that you get a slightly higher temperature and lower relative humidity if you seal the air gap, i.e., a better and drier climate.

When the ventilation is needed the most, in winter, the outdoor air has an extremely high relative humidity.

This is very controversial, and like most other things in this industry, it will take time to change old truths.

 

[blygarn]

Is there a clear answer to this question? I'm going to build an air gap and the outer roof is concrete tiles and behind it a thick hardened masonite.
Should I have hardened or not? It's quite significant since the material cost is double for hardened!

 

[isolde]

It has been observed that you get slightly higher temperature and lower RH if you seal the air gap, leading to a better and drier climate.

When ventilation is needed the most, in winter, the outdoor air has an extremely high RH.

Exactly. Eave ventilation is a nuisance, but practiced in absurdum because 'that's how we've always done it', which sounds strange when striving for controlled indoor ventilation.

 

[Eddi Thomason]

Exactly. The eave ventilation is a nuisance but is practiced in absurdum because 'that's how we've always done it,' which sounds strange when striving for controlled indoor ventilation.

Is there anything worth reading on the subject? For example, reports from SP or similar?
It makes you curious!

 

[isolde]

Is there anything worth reading on the subject? For example, reports from SP or similar? I'm curious!

There is quite a bit to read on it.

Thermisol writes:
"Full-scale tests show that in an unventilated cold attic it becomes drier and the relative
humidity becomes much more even during periods with significant fluctuations in outdoor climate. For
attics above well-insulated attic floors, an unventilated attic is therefore better than a ventilated one.
A certain air exchange should exist to achieve a drying effect during the summer months. To
ensure this, relatively moderate ventilation from one or a couple of vents per
gable is sufficient."

SP writes:
"It was emphasized that cellulose insulation can buffer moisture and thereby even out moisture variations in the cold attic. Measurements showed that the effect exists but is not particularly large; it was much more effective to limit ventilation. In the unventilated attic space, where no external moisture sources were present, the average climate was drier than in the other attic spaces. This was especially true during periods when the outdoor climate varied between cold and humid nights and warm days."

Then there are those who make a nice profit on very simple products

 

[Fjelie 17]

Well, what should one believe in? We had eave ventilation in our newly built house. Felt good. Then we moved to a farm built in 1899. There is no sign of eave ventilation. The only ventilation that existed was a hole in a chimney. There was hardly any insulation, less than 100mm of sawdust. Naturally, no plastic. Perhaps it should be added that previous owners also made ventilation holes from a couple of bedrooms straight up to the attic... But neither during the inspection nor later have we seen any signs of mold.

I belong to the group that believes limited ventilation is best. Where it is located has less significance.