Installing Celotex to Rafters.. creative venting questions


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Hi all, I'm planning to install Celotex inside and to underside of rafters to 0.17Uvalue. Currently I have insulation at ceiling level and eaves vented with 5x 90mm circular vents each side of an 8.5m house (15k m2/m). Ideally I would leave a 50mm gap between sarking and celotex and ideally I would upgrade my eaves and ridge to 25kmm2/m and 5kmm2/m respectively as per uk building regs.

However a roofer has told me that I would need scaffolding to install these and this makes the vent upgrade job somewhat expensive (he was saying £800 for scaffolding £250 for parts and (I guess) a couple of days making the total close to £2k. I'm going to get some more quotes but the scaffolding does add a lot.

So, I'm looking for sensible alternatives but I do definitely still want to do the pitch roof insulation if possible (downlighters, wiring, future maintenance, storage etc).

He suggested to only add a load of additional circular vents to underside of the eaves (as they would be easy to add from a ladder) - lets assume 25km2/m and he was suggesting that this would (probably) provide sufficient airflow across to prevent structural issues. He also suggested if I put insulation under rafters (not between) then the gap would be bigger and would allow this airflow more easily.

Also I have 75mm noggins supporting gable straps in between the last 3 rafter each side (12 straps in total). This will reduce the gap to 20mm (rafters 95mm). I could drill a few 25mm holes in the bottom.

My questions if anyone has got this far!
1. Would this eaves-only venting approach likely to cause damp/structural problems (a) with 50mm gap (b) with full 95mm gap between celotex and sarking felt. Any difference in vent effectiveness between the two options? (It would make the insulation easier to put insulation only under the rafters but I only want to do this if it helps and there is a good chance it will eliminate any issues)
2. Would the 20mm gap each side due to strap noggins likely to cause problems? If so would drilling holes in solve them?

Massive thanks for any insight - I just don't have the experience on this to know what is the best alternative.
 
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Can't see how to edit.

One thought I had following this is that I also have 22x100 straps running along and diagonally. Another option is to batten out 22mm below the rafters and then attach the celotex to that. This way there would also be cross rafter ventilation (also helping the noggin situation by providing a natural 45mm gap in total either side).

This means I don't have to cut channels out of the celotex. Lose a bit more headroom but this is the least of my worries right now.

3. So can anyone give any idea whether providing cross rafter venting be the best (or perhaps the worst) solution to solve this vent problem?
Thanks!
 
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Hi zerozero

Perhaps I'm misunderstanding you here, but I have to ask what you intend to achieve. If you are going to increase ventilation to the roof space, what is the Celotex doing. Are you intending to vent the void only, or the whole loft ?
As you mention the word gable, can you not insert vents here.

My apologies if I'm getting you wrong.
 
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Hi JohnKay, sorry if I wasn't very clear I was trying to be concise but perhaps I overdid it! :)

My primary goal here is to get rid of fibreglass as I plan to be spending much more time in the attic and it is nasty stuff. Having considered rigid foam at ceiling level I realised that due to downlighters (holes in ceiling plasterboard that can't be insulated due to heat issues) and other wiring at ceiling level that will be more difficult to get at (and lay more cable in future) better all round to put rigid insulation on the roof pitch in/under rafters.

UK Building regs are saying that between the sarking felt and the celotex there should be a minimum of 50mm vented airgap to prevent damp problems ( I guess due to humidity in the air). As the celotex is below this the insulation works to keep the heat in the loftspace (and through to the rest of the house).

See handy guide on celotex for a diagram (sorry would post link but my IE has crashed)

Gable vents wouldn't work if the celotex is fixed to rafters as there would be no cross-rafter ventilation (each channel would be separate up and down the roof pitch).
Trying to create a false ceiling would be really complex due to long web members fixed to the apex requiring lots of careful cutting and waste and probably lots of squirty foam!

Perhaps gable vents might work if I do my last idea of battening out under the rafters but this would only give 25mm cross-rafter ventilation and I'm not sure this would be enough to vent the middle of the roof. I've been told by glidevale that gable vents wouldn't be able to provide enough venting. Regs say you need 5000m2/m and my roof is 8.5m long which by my calcs means I would just need a vent each side of 14.5 square.. but perhaps it does need to be along the length of the roof.

Really not my area of expertise as you can see!
 
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I know what you mean about fibreglass !
I think I would go with the soffit vents and the 50mm gap, but with some way of connecting the voids to the other side of the roof, providing cross ventilation. This would bring us to the false ceiling idea that you don't want.
Another thought would be replacement PVC soffit with continuous ventilation along the length,
OK if your existing fascias and soffits are due for attention.
All in all it's a project that's going to take a degree of improvisation, and I wish you the best of luck with it.
There's not much more help I can give you without being there to see the situation, and It's only my opinion. perhaps others may have suggestions.
 
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Thanks for your thoughts John.

There would naturally be connection of the voids of both sides of the roof as the trusses span the full width of the roof and so as long as I mitre the 2 insulation slabs appropriately at the apex, there will be a gap over the top joining the 2 sides of the building. Is this what you meant?.
I guess then I'm just relying largely on air movement from outside to keep the air moving through the voids.

So to clarify I have 3 options
- 50mm gap - voids connecting both sides of building as described above
- 95mm gap - same as 50mm just with a larger gap (not sure if helps ventilation or hinders)
- 120mm gap - same as 50mm + cross rafter ventilation under rafters (not sure if this helps ventilation or hinders)

and you would still prefer the 50mm gap? Can I ask why? Is it because you think it will vent better than the larger cavities?

Replacement fascias and soffits would need scaffolding unfortunately so although it would increase the area of venting if I was going to do that I'd probably do the ridge vents at the same time due to the outlay.

Thanks again.
 
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Yes, that's what I meant, if you mitre in that way, I would expect quite a good air flow from one side of the house to the other. I'm only thinking that a 50mm gap would maybe easier to fix, and 120mm could be OTT, and would take a lot of space. Having said that, I see no reason why you should not go for the 95 if you consider it easier to fix direct to the trusses.

Just thinking that the size sheets required for the 50 gap might just go nicely through a loft hatch.
 
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Thanks John, that's helpful to know you think that would be good enough airflow with no high level vents at 95mm gap with no cross venting below rafters.

I think I had convinced myself that 120 gap was best as:

1. it means that at the 2 outermost voids (at each gable end) where the 75mm noggins are that I still have 45mm of ventilation above and below the noggins rather than 20mm below only.
2. It means that I don't need to cut channels lengthwise into the rigid insulation to go around the rafter straps
3. I know that 110mm of insulation is sufficient (as celotex told me) vs 120mm if I have anything "in" the insulation (straps / rafters)
4. It will mean that there will be more space generally for the air to move more easily to compensate more for no high level ventilation

However it will cost quite a lot in battening as I believe I need 600mm centres to fix the celotex to?

An alternative would be to buy 25mm + 100mm celotex and simply cut the 25mm around the straps.. hmm.. If the airflow at the ends were a problem (would there be a good way to test this somehow?) I guess I could switch round the 25mm and 100mm pieces so the 2 ends of the roof look like they have 25mm thicker insulation from the inside.

Calculating it 25mm + 100mm celotex is actually 10% cheaper than 110mm celotex + 102m of batons.

Typically with this project every time I think I've concluded something, something happens which changes my mind! :)

However unless there is a significant difference, for the sake of losing 10mm more headroom with the battening I still wonder if it is worth the extra screwing cutting and fiddling with 2 pieces rather than 1 and gives better ventilation..

The sheets (even the 45mm thick ones) all seem to come in 1200 x 2400 sheets so no way I could get them through the hatch. What sheets were you thinking of? The only ones I seem to be able to get in smaller sizes are all flat roofing sheets and they are all quite a bit more expensive.

I think I can cut the 2.4x1.2 lengthwise into 600x2400 sheets then get them up there. if I baton the only complex bits are the mitres at the ridge and eaves. I also need some 95mm thick pieces to go in-between the ceiling level joists just at the sides to complete the insulation to the inner wall at the eaves

To cut I will get 2 long bits of timber and 2 short bits and screw together to make a saw guide and clamp them at the correct point to use as a guide to get 90deg and straight. Also I'll mark them with a unique number across the cut so I can connect them once in the attic to get a perfect butt joint where possible
 

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