Good or Half Good Without Masonite for Soffit Ventilation

[wretan]

Hello,

I am in the process of building on our roof trusses and adding insulation to the slanted roof of our 50s house, which currently has only 100mm.

In the midst of tearing things down, the magazine "Do it Yourself" lands in the mailbox, and they happen to have a version of just this But instead of the usual way of placing masonite with spacers against the roof boards for the eaves ventilation, they stretched stainless steel wire between the trusses about 50mm from the roof boards and used this as an air gap.

Very convenient and cheaper than masonite, but what do you think, does it compromise the protection for the insulation?

Best regards,
Johan

 

[JanneL]

I think it sounds strange. What I have learned is that the air should not be able to blow into the insulation. Did you complement with some form of wind barrier? In the air gap, there will be a significant draft. I created an air gap this summer on a house in connection with a roof replacement. We used hardboard, mounted it 38 mm down on the rafter and placed the board. You have to be careful not to make it too tight between the rafters, so it doesn't warp and gap down at the eaves.

 

[injonil]

wretan, with that construction you should have insulation with wind paper on top! Paroc has that type, I know, and Isover probably does too.

 

[wretan]

Hi!

I agree with you, it feels dangerous to ignore some form of wind protection... I don't understand how they can in good conscience exclude it from the article

for me, it will probably be the tried and tested version with masonite thanks JanneL for the tip about the boards, one has seen quite a few warped boards in one's days

 

[pelleulf]

hi!
Masonite it should be!
However, do not use hardened masonite as it does not breathe, which is the point of the air gap...
Pelleulf

 

[Mikael_L]

hello!
Masonite is the way to go!
However, do not use hardened masonite because it does not breathe, which is the whole point of the air gap...
Pelleulf

Now, it's not that it "breathes" through the masonite in such an air gap, but rather the air blows or convects from the low end of the air gap to the high. So how dense the masonite is doesn't matter for the function. Many choose hardened for moisture protection (condensation moisture dripping down), I think it should work quite well with the regular one too.

 

[andersmc]

But instead of the usual method of placing hardboard with spacers against the sheathing for the eave ventilation, they stretched stainless steel wire between the rafters about 50mm from the sheathing and used this as an air gap.

Very convenient and cheaper than hardboard but what do you think, does it compromise the protection for the insulation?

That sounds very uncertain. The hardboard is not just to create a distance but it's also supposed to handle the water that inevitably reaches the insulation. It's condensation, drifting snow that blows in, water from small leaks in the roof covering... All this would otherwise go straight into the insulation.

 

[Mikael_L]

Exactly, the masonite has three functions.
Keep the insulation away from the sub-roof.
Protect the insulation from dripping water (condensation, melting drifting snow, etc.)
Provide wind protection for the insulation.

All three functions are important.

 

[gadu^]

But does regular masonite withstand the mentioned load or should one use oil-hardened?

 

[Mikael_L]

I would have gone with oil-hardened, so you don't have to worry.

 

[Immobil]

Common is good enough.

 

[JanneL]

"It sounded very uncertain. The hardboard is not just to create a gap, but it should also handle the water that inevitably will reach the insulation. It's condensation, drifting snow that blows in, water from small leaks in the roofing...All this would otherwise go directly into the insulation."

I would argue that regular hardboard is quite sufficient. How on earth have they built it if drifting snow gets in, and why would water from small leaks in the roofing only come in where the air gap has been made? When you replace the roof, it tends to be tight for a long time afterwards.

 

[Mikael_L]

I think Immobil is right. If there's so much moisture that standard masonite can't handle it, then there's another, more serious problem somewhere.

But at the same time, the price difference isn't that significant...

 

[andersmc]

"It sounded very uncertain. The masonite is not just to create a distance, but it also has to handle the water that inevitably reaches the insulation. That's condensation, drift snow blowing in, water from small leaks in the roofing... All of this would otherwise go straight into the insulation."

I would argue that ordinary masonite is quite sufficient. How on earth has one constructed it if drift snow gets in, and why would the water from small leaks in the roofing only come in where the air gap has been made? When you change the roof, it usually remains tight for a long time ahead.

What are you opposing? Yes, one should have masonite, that's what I'm writing!

Drift snow gets in, yes. You might not have that type of snow in Norrland, but the Skåne snow does at least blow in through gaps and ventilation grills. Naturally, the leak water doesn't only come where the air gap is. Has anyone claimed that? But it doesn't become worse if it doesn't get wet right there, does it? And sure, the roof is "tight for a long time." But that's not the same as "tight forever."

 

[pelleulf]

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

Therefore, it is important to use regular and not oil-hardened masonite in the gap.
It is, of course, also beneficial if a leak should occur from the outside.

Best regards, Pelleulf