Roof ventilation

[bagstone]

So, moved in about half a year ago, boarding the loft right now. I plan on putting Kingspan in between the rafters, and I wonder... do I need ventilation, and if so which one. Here's a pic of the loft:

Close-up of the eaves:

And a pic from outside to see soffits:

Now to say this straight ahead, AT THE MOMENT we have no problem with condensation at all. All roofers attested that it's a pretty good breathable membrane. There's two Velux windows in there which we can obviously set to ventilate if needed, I have a decent dehumidifier and smart sensors if humidity should crop up. The plan is to turn the loft into mostly storage, maybe "hobby room" where I can put my tools and do some stuff on a rainy day, and that's probably about it. There's proper insulation on the floor (there were typical 1960s tiny 2x4 ceiling joists in there with a 100mm-ish or a bit more rock wool, I've added proper 6x2 joists mounted on wall plates/load-bearing walls, an inch above the ceiling joists, with 170mm rock wool which takes me up to a good 270mm according to regs). The rafters are 100mm so I'll put 50mm of Kingspan in between to leave 50mm air gap between insulation and membrane.

Do I need ventilation, and if so, which? I've contacted several roofers:

- One didn't even come over and just sent a quote for 260 quid to put in soffit vents (which I kind of suggested in my enquiry for the quote, and there's a chance they know the area as most contractors I contact do).
- One came over and basically said "don't bother". He said he wouldn't put in tile vents or anything like that as the air flow wouldn't go sideways, so not between all rafters anyways, it could just make everything more subjective to water ingress, and the most he'd suggest is soffit vents. But he said because my soffits aren't siliconed to the wall there's enough air gaps between/around them to allow, together with the roof's breathable membrane, for some air flow, and if there ever was any condensation it would drip to the bottom and I would see it on the soffits/wall and could put in soffit vents THEN. But for now, don't do anything.
- One came over and said soffit vents won't work because there's not enough space above the eaves for air flow, there would have to be eaves vents, and he suggested 3 tile vents on either side and a ridge vent across the top.

Other non-professional advisors (neighbours, friends, family) were kind of suggesting something similar to #2 - don't bother...

Not sure what to do, if I get more contractors over, if I just wait and see... thoughts?

 

[catlad]

Manthorpe lap vents would be easily at this stage, what tiles do you have?

 

[RandomGrinch]

Ideally, there should be a cross flow of ventilation above the insulation - from the eaves (either soffit, or over fascia vents), to the ridge, and back down the other side.
The minimum gap between PIR insulation slabs and a breathable membrane should be 25mm.
Here's some guidance from a LA regarding ventilation of a loft conversion...

Some of your existing insulation appears to be pushed up tight into the eaves, refurbishment vent trays may help keep the ventilation path clear.

Another issue of slight concern, are your loft windows. Depending on the rafter size, it's usual to double up on the rafters and headers around a new window.
Your rafters may be structurally OK, but it appears that the header is quite slim - probably not a good idea. Were these signed off by BC?

 

[stich17]

Just joined to ask a similar question. We are just in the middle of working a garage conversion and keeping the vaulted ceiling. We have installed ridged insulation and left the 25mm gap above. However after the awful snow we noticed condensation on the top side of the insulation in some places. There is a breeze along the roof trusses from the slither of a gap between the barge board and the wall. There are no soffits the barge boards are very close. Would some vented roof tiles help?

 

[bagstone]

Manthorpe lap vents would be easily at this stage, what tiles do you have?

Just normal concrete tiles I guess. What are lap vents? Something like tile vents? I Googled but the results look strange to me, no clue what that is

Ideally, there should be a cross flow of ventilation above the insulation - from the eaves (either soffit, or over fascia vents), to the ridge, and back down the other side.
The minimum gap between PIR insulation slabs and a breathable membrane should be 25mm.
Here's some guidance from a LA regarding ventilation of a loft conversion...


Some of your existing insulation appears to be pushed up tight into the eaves, refurbishment vent trays may help keep the ventilation path clear.

Another issue of slight concern, are your loft windows. Depending on the rafter size, it's usual to double up on the rafters and headers around a new window.
Your rafters may be structurally OK, but it appears that the header is quite slim - probably not a good idea. Were these signed off by BC?

Thanks for the regulation links! Yeah I know the min gap is 25mm but I don't feel I need to overdo it, I'm fine with 50mm insulation and then having a 50mm gap.

Everything you see was done before we bought the house, and there's no certificates, no documentation, we don't even know how did it. Most of what we can see in and around the house is done to really high standard - the previous previous owner was in the trade themselves (decorator I believe) with good contacts so the plastering and all things in the house are top notch. But we don't know if/where they cut any corners "under the surface". The rafters around the loft windows aren't doubled up, but I don't think that's a DIY fix for me, is it? And generally I don't want to tinker too much with structural stuff. I had a few roofers up to inquire about the vents and no one commented on the loft windows, the only thing they said is the membrane needs to be properly tucked away in some places (is there some "membrane duct tape" or do I just staple it where it's a bit loose or what?)

The insulation that seems to be pushed into the eaves I'm gonna take back. My plan is to take small off cut bits of the Kingspan to put it on the corners of the boards and then push it over the eaves, so that it's guaranteed there's an air flow gap, but no insulation can push into the eaves (as they do right now). Also, there's some random bricks on top of the eaves that I have to remove.

 

[RandomGrinch]

However after the awful snow we noticed condensation on the top side of the insulation in some places.

Also of relevance to the OP, if you want to avoid condensation on top of the insulation, you need to ensure there is a vapour barrier below the insulation.
If you have a foil backed PIR, any gaps should be filled with expanding foam, and joints covered with foil tape.

 

[bagstone]

Yeah I plan to put on foil tape, it was a good hint by one of the roofers.

One plan is to just go ahead for now as is, and then on a really rainy day when humidity is crazy high remove one of the insulation boards to check what it looks like behind, basically a visual inspection to see if we get condensation or not... because of the conflicting advice we get.