PIR in porch roof, vents?

[Super_Paulie]

Quick question for the masses.

My porch has a vaulted roof with joists so I can have a flat ceiling in there. It's brick with an insulated cavity.

 

I've installed 75mm PIR between the joists and made it air tight, more to stop drafts than anything but I have found the roof void above the PIR is not vented.

Should I add a vent into the soffit or not bother due to the small footprint, it's like 1.8m x 1.8m. I could also potentially "knock through" the dividing OSB board into the canopy which is open to the elements.

 

Cheers gents

[Redbeard]

Any lean-to roof has a problem in that, ventilation-wise, you have an 'in' and no 'out'. Good cross-ventilation is not ensured, and sometimes cannot be. I am not sure I understand your reference to the 'canopy'. Do you mean the roof to the bay windows. I cannot see from the pic provided whether there is going to be a continuous roof void above the bays from extreme L to extreme R. If there is, and the porch roof connects to it, then there is your 'out'. 'In' via soffit vents (more vents than gut-feeling might suggest) and out via the vents which either already are, or will be provided in the bay windows' roof.

[Super_Paulie]

1 hour ago, Redbeard said:

Any lean-to roof has a problem in that, ventilation-wise, you have an 'in' and no 'out'. Good cross-ventilation is not ensured, and sometimes cannot be. I am not sure I understand your reference to the 'canopy'. Do you mean the roof to the bay windows. I cannot see from the pic provided whether there is going to be a continuous roof void above the bays from extreme L to extreme R. If there is, and the porch roof connects to it, then there is your 'out'. 'In' via soffit vents (more vents than gut-feeling might suggest) and out via the vents which either already are, or will be provided in the bay windows' roof.

 

sorry, yeah the canopy above the bay window. This is currently "open" underneath as i havent added any soffits yet. There is an osb divider that separates the porch and the bay window roofs, so i could open that up so the airflow can pass all the way through as you say extreme L to extreme R. As it stands there are no vents in the porch roof and it is a separate entity to the bay roof.

 

As there are no vents on the porch side, i could add some quite easily. Im not sure what the builder intended by not putting vents in he did on the rear pitched roof. I would obviously have had to add a ceiling into the porch so what he intended i have no idea.

[Redbeard]

Switching on my brain I can correct myself here! I must only have had half an eye open not to notice that the porch roof is a hipped roof. So you do have 'ins' and 'out', on the LH and RH sides. Good-sized soffit vents and you are done! I stress'more vents than gut-feeling might suggest' as many builders seem to put c70mm round vents at 1000-1200mm centres. Compare the cross-sectional area of those vents to that of  the ideal  'slot' over the entire lgth of eaves and you may be unpleasantly surprised.

[Super_Paulie]

2 hours ago, Redbeard said:

Switching on my brain I can correct myself here! I must only have had half an eye open not to notice that the porch roof is a hipped roof. So you do have 'ins' and 'out', on the LH and RH sides. Good-sized soffit vents and you are done! I stress'more vents than gut-feeling might suggest' as many builders seem to put c70mm round vents at 1000-1200mm centres. Compare the cross-sectional area of those vents to that of  the ideal  'slot' over the entire lgth of eaves and you may be unpleasantly surprised.

 

yeah sorry the photo doesnt show that it is hipped very clearly.

So a few soffit vents on the left and right of the porch will surrfice? Wont be too difficult id imagine, cheers!

[Super_Paulie]

Job done, thanks for the input ✌️