We're being told by an architect that there is no point putting more than 200mm of rockwool in our loft space as the roof is not sealed. (The insulation will go on the floor between the joists).

Apart from being less than the building regs requirement, I'd always understood the rule here was as much as possible (we won't use the space for storage).

Can anyone confirm whether or not the architect's view is reasonable, and if not, point we towards some authoritative source to contradict him?

Many thanks.

200mm does not even meet current building regs!

I would like to see 350mm or more and the bare minimum to meet regs is insufficient in my opinion.

Your architect may be right. If the roof is not sealed and there is a fair amount of air movement then the effectiveness of the insulation is significantly reduced...so adding more is kind of pointless.

Excellent post Colin

<blockquote><cite>Posted By: CWatters</cite> For example lets assume that the ventilation reduces the effectivness of the top 50mm of the insulation.</blockquote>

Lets assume that the insulation is not tight fitting, and in several layers, so the ventilation caused circulation in and around the insulation medium making the insulation largely ineffective regardless of thickness. Lets add to our wonderful assumption that the light fitting recesses are not sealed either with the resultant air flow through the insulation material further reducing the effectiveness of the insulation.

Of course we could assume lots of things...

Posted By: trule

Posted By: CWattersFor example lets assume that the ventilation reduces the effectivness of the top 50mm of the insulation.

Lets assume that the insulation is not tight fitting, and in several layers, so the ventilation caused circulation in and around the insulation medium making the insulation largely ineffective regardless of thickness. Lets add to our wonderful assumption that the light fitting recesses are not sealed either with the resultant air flow through the insulation material further reducing the effectiveness of the insulation.

Of course we could assume lots of things...

It's why I think we should consider banning so called "cold roof" designs and mandating "warm roof" with the insulation above the rafters or perhaps SIPs.

but why should existing and new build be any different?

I think that's a good point Sinnerboy- we do need more informatio to be sure. In some instances, 200mm may achieve compliance, such as renovation of thermal elements, but if we are talking about new build [as you say] or an extension, then the requirement is 0.16W/m2K -so 200mm not enough
Edit: should have said 200mm mineral wool or similar

Sinnerboy, the point Tony made about putting in 350mm was, as he said, an opinion. One point seems to be that really we should be looking at how much could be put in, rather than how little. The regulations (whether for new build or existing) are for a minimum not an optimum amount.

Thanks for all that interesting comment and debate. The house is old, and we are upgrading the insulation in the roof space at the same time as re-roofing. We have discussed this further with the architect and he has backtracked from the reasoning around non-sealed roof, and has said

"in my experience once you get over 200mm Rockwool, which gives you .22 on its own, the savings are so minimal as the thickness increases you are far better spending the money on other things like windows which let out most of the heat as this will make the greatest saving in the reduction of energy loss". [We can't replace the windows as the building is Grade II listed].

However, he's now also said that as we have about 70mm of existing Rockwool we will have 270mm altogether.

In light of my own researches and comments above, we are specifying 300mm of new Rockwook which will give us nearly 400mm altogether.

Good for you crt.

Nick - what if during the coming years the gutters are not cleared ? The slates are damaged and not spotted ? If for some reason the vents are either missing or , if present , compromised by insects or vegetation ? If through neglect the local fabric ( pointing ) admits water ?

The application of treatment now to joist / rafter feet and wallplates could see off the risk of future rot

<blockquote><cite>Posted By: tony</cite> Construction timber should not need to be treated as it should never be getting wet or even damp.</blockquote>

But it does happen Tony . Through neglect as I outlined above - also
rads leak
rwps crack and leak
carefully placed insulations and vapour barriers do get displaced by uncaring or ignorant follow on workers / users

For the sake of the judicious application of preservative - the risks can and should be mitigated

Posted By: tonyDry wood is of no interest to woodworm or fungal spores. Construction timber should not need to be treated as it should never be getting wet or even damp.

.....and even if it does get damp it will dry out again if adequate ventilation is present

Wet rot in roof timbers is rare.. dry rot even rarer

Edit. obviously serious roof leaks are different - i'm referring here to dampness, such as that caused by a slipped slate or poor flashings for example

Do you have a thriving dry rot industry in Canada Paul ? Because we do here . I have seen enough perished wall plates in historic structures and the consequential loss of decorative cornicing during structural repair works to urge the op with his/her listed property- treat the timbers