Ventilation post Velux window install

[Night Owl]

Hello, I recently had a Velux window installed in my roofspace which will be used as an en-suite for the upstairs bedroom.  I will be insulating between the rafters leaving a 50mm gap for ventilation between the roof felt and insulation.  There are two areas (to the left and rightof the window) that will be effectively sealed off (due to double trimmers being fitted) from the ventilation path once plasterboard has been fitted.  Would it be acceptable to drill these trimmers top and bottom to allow some airflow through the void?  If it is, what would be the best quantity / size / location for these holes?

 

Thanks.

 

[Night Owl]

Anyone have any thoughts on the above?

[Wil]

I’m sorry I can’t comment but I’m about to run into the same issue so watching with interest.

 

I can’t see why a couple of holes in the trimmers would make a signifcant difference to the structures, but I’m no expert.

[Night Owl]

Thanks Wil, hopefully someone will be along soon to enlighten us.  In the meantime, if I do find something I will post it here.

[MikeGrahamT21]

Give us some extra detail on the roof, what kind of felt/membrane do you have? Are there any counter battens in play on the tile side?

[Super_Paulie]

not an expert by any means, but if it was my roof i wouldnt have any issues throwing a few 20mm holes along the centre line in advance of the insulation.

[Night Owl]

Thanks for the replies.  Personally I can't see what effect drilling a couple of small holes would have on the overall structure of the roof.  For info, it is approx. 70 years old, made up of concrete tiles, 1x1 lats, bitumen felt, 4x2 rafters plus the 2x2 batons I've recently added to increase depth for insulation.

 

[Nickfromwales]

Whack away, Jim.  

 

The only thing I can fault you on is that the tops of the timbers should have been 'chopped' before installing, so the airflow could be at the top and minimal material was interfering with the membrane.

 

Drill a few holes around 30mm down from the membrane, and 150mm or so wide, and then a couple up each side as close to the membrane (felt?) as you can get WITHOUT damaging it. Then a few careful minutes of chopping out with a nice sharp chisel.

 

IMO it's best here to have an open top to the timbers vs holes in the midrift, so I would cut 'letterboxes' out and have as little against the membrane as is possible. 

[Night Owl]

Thank you Nick, i wouldn't have thought about it that way as I've been more focussed on maintaining the structural integrity of the installation.  From the way you describe it, sounds like it's pretty common practice to 'chop' timbers before installation to address this type of issue?  I'm surprised Velux don't pass this onto their approved installers as my window was actually fitted by one.  Building control never mentioned it either when they looked at it.

[Nickfromwales]

On 08/03/2025 at 08:04, Night Owl said:

Thank you Nick, i wouldn't have thought about it that way as I've been more focussed on maintaining the structural integrity of the installation.  From the way you describe it, sounds like it's pretty common practice to 'chop' timbers before installation to address this type of issue?  I'm surprised Velux don't pass this onto their approved installers as my window was actually fitted by one.  Building control never mentioned it either when they looked at it.

If Velux mention it they then become liable for the success of the suggested 'remedy', so the silence is deafening I'm afraid. The blame game is something everyone avoids like the plague.

 

This one's down to common sense that's all, so the "approved installers" should have been a bit more attentive here IMHO and should have realised their action had interrupted the natural airflow (a significant thing) and then done something about it.

 

There are fitters, and there are fitters, and, from what I have seen to date, being on a list means you haven't yet done something significantly wrong enough to be struck off it.

 

I'd take these matters into my own hands and oversee this being rectified, and move on to the next problem