Advice on attic ventilation

raimiska

Hi.

I've got an attic ventilation issue I'm trying to figure out. It's a concrete tile roof with some sort of a membrane underneath(No idea if its breathable, but it definitely lets air through the gaps). The rafters have thin sheets of ply fixed on the inside and a PVC vapour barrier laid over them. I can see that there's been some condensation or moisture trapped behind the PVC before. I have no idea if the ply and the VP were there originally or if somebody put it up there sometime later.

But my question is if it should be there at all, or if it's better to remove the ply to let the attic ventilate? There are no soffit vents and no vents on the roof either. Just a single soil pipe size hole in the chimney, which is the only way to the outside world. The house is pre-2000s, somewhere around 1970-1990. I was planning on installing some vents, so maybe just leave all that there and get some vents instead? Or get rid of the ply and PVC, and also get some vents?

Thanks

10-10-20

https://www.boards.ie/discussion/2058422203/advice-on-attic-ventilation

A photo of the ply and PVC will help here.

What about the position and depth of insulation, where is that in the makeup of the roof?

raimiska

https://www.boards.ie/discussion/comment/123734120#Comment_123734120

I might have a photo somewhere, but if not, it might take a day or two until I can take one, but essentially its bare minimum thin ply sheets across the rafters and about 1000 gauge vapour barrier fixed with some pins to the plywood across the whole area. As for insulation, there is none in the rafters. The ceiling, aka the floor in the attic, is 4x2's, so it's only 100mm of insulation, which is below what houses would have nowadays. I was also planning to install stilts up there so I could add more insulation.

raimiska

https://www.boards.ie/discussion/comment/123734120#Comment_123734120

20250505_114040.jpg

20250505_114050.jpg

20250505_114055.jpg

20250505_114104.jpg

20250505_114136.jpg

20250505_114208.jpg

20250505_114217.jpg

20250505_114236.jpg

Attached some photos. The blurry one is where I saw the condensation at the top of the rafters. That's also where the open cold storage and f&e tank is located below, so maybe that could also promote condensation forming there. Also, the vapour barrier is somewhat poorly jammed there.

10-10-20

Interesting job that. So the condensation was on the external side of the vapour barrier and not on the internal side?

If that's right then I think it means that the warm sun drove the moisture to the coolest surface, away from the tiles and membrane, where it condensed on the top-side of the barrier.

Regardless, I think that the plastic barrier and the top and bottom rows of ply need to be stripped off to encourage air movement and prevent the rafters from rotting at the black membrane.

Let's see what others say.

The Continental Op

If you want to increase airflow then felt vents might be of some use. Just check the clips are big enough to go over that old style of felt.

That really heavy felt will attract some condensation in cold weather its the nature of the material, but its nothing to worry about.

As for the plastic I don't think whoever put it up had a clue what they were doing. They might have seen condensation of the felt on a cold day and panicked?

raimiska

https://www.boards.ie/discussion/comment/123734505#Comment_123734505

Thanks for the advice. I'll go ahead and rip it out so and hopefully no surprises hide behind the ply. No idea how long its been there.

raimiska

An older couple used to live there, so maybe for them, "cold breeze in the attic" = Bad. And yeah + possible condensation might of been the reason why they put these up.

I'll have a look at the felt vents and order one to see how it fits. Might just skip the soffit vent idea if these feel sufficient.

Thanks a mil.

whizbang

It was probably an attempt to reduce the draughts in the attic, which would be sucking the heat out of the fibre insulation.

You chould look at ways to prevent this in your upgrade.

I would be a bit concerned about warm air rising up into the attic thro those gaps. If you can stop the warm damp air from living space getting into attic, you won't have any condensation worries.

GreeBo

It does look like a naive attempts to reduce drafts alright!

99% of the time that plastic isn't going to cause an issue as condensation will form on the inside of it rather than the rafters, the exceptionally hot weather coupled with some morning dew is probably causing that condensation that you do see.

Flooring the attic will reduce air washing over the insulation, but if you are concerned about lack of airflow, then washing isnt a problem :)
Once your insulation doesnt go right into the tiles you will most likely get enough through ventilation, you can add vent cards to ensure the gap.

Removing the ply and plastic is an obvious first step, you can replace the plastic with a breathable house wrap type fabric, if you feel the need, but I personally wouldnt unless/until I had a good reason to!

I didnt see it in the pictures, but I would be far more concerned about a hole into the attic space from the chimney, thats the first thing I would be addressing as its asking to burn the house down if someone lights a fire.