How many soffit vents do I need?

Mossy

The house was built in the early 90s, semi-detached. The attic is used for storage, area is something like 8m x 5m. I've not seen any sign of moisture problems up there, so I'm not trying to solve any particular problem, just taking steps to prevent them.

The few houses arounf that have soffit vents have two in the front of the house, though one neighbour did a DIY job with about five times as many using circular vents (see picture).

How do I figure out many vents I should have?

muffler

This takes me back a few years and if I could only remember the formula in more recent years it would be great. I don't believe the requirement for soffit ventilation ever changed so it used to an area equivalent of a 10mm wide strip for the full length of the soffitt.

bfclancy2

https://www.boards.ie/discussion/2058363645/how-many-soffit-vents-do-i-need

well it shouldn't look like swiss cheese anyway

Mossy

https://www.boards.ie/discussion/comment/122483945#Comment_122483945

Thanks, that makes sense of what I've seen. The house with the circular vents, pictured above, is using 70mm wide vents, whereas the other houses have a lower number of bigger vents.

I don't have ridge vents. Does that make a difference?

DOCARCH

https://www.boards.ie/discussion/comment/122484170#Comment_122484170

It can do if you need equivalent of 25mm continuous ventilation!

muffler

https://www.boards.ie/discussion/comment/122484184#Comment_122484184

Unless you have a (developed) attic space in which case you will need ridge ventilation and then the soffitt venting increases to the equivalent of a 25 mms strip.….I think. I'd need to check that or perhaps someone else might confirm.

DOCARCH

https://www.boards.ie/discussion/comment/122484314#Comment_122484314

Yes, that is the case, where roofspace developed/insulation along line of rafters (full or partilally) equivalent of 25mm continuous strip ventilation at eaves, and equivalent of 55mm continuous strip ventilation at ridge.

muffler

https://www.boards.ie/discussion/comment/122484361#Comment_122484361

I knew I had back-up here. Thanks DOCARCH.

It is amazing how easy it is to forget the simple things when you've been out of the saddle for a while!