When is a Ceiling VCL Needed/Not Needed? When is roof ventilation alone enough? And other ponderings!

[Black Timber]

Hi

I've got some work to do to a ceiling (and the insulation above), which I might have more specific questions about later. For now, I've been left a bit confused as to when - more generally - a VCL is or isn't needed, and the potential problems with not having one.

Would I be right in saying that the majority of houses don't have a VCL? Victorian terraces, 1950s/60s bungalows. LA and ex-La houses, etc. etc. - do they have VCLs?

If they don't, then I'm guessing that's why it's so important that roof spaces are adequately ventilated. But by that reckoning, if an adequately ventilated roof is capable of dealing with problems from household moisture, and resulting condensation, why when I read info about doing work to ceilings and roofs (including loft conversions) are VCLs almost always recommended?

Taking that premise a step further - if an adequately ventilated loft can deal with household moisture without a VCL, then could adding a VCL be detrimental by trapping more moisture into the habitable parts of the house rather than letting it escape to the roof space where it can be dealt with?

Which brings me nicely onto my last confudlement. We live in an old solid wall house (albeit with more modern extensions), and so have been learning about the need for letting the walls breath by using lime, etc. Is the same not true of ceilings and roofs? Are there merits to having breathable ceilings/insulation/roof space in old houses? If so, what further confuses me is that while I can see the need for different approaches to different types of wall (modern materials with a cavity VS stone walls without a cavity = different approaches needed) are there any major differences between old and new roofs? Putting thatched roofs and the like aside, are old and new both principally the same insomuch as they have a hard roof covering and timber supports?

It's all left me scratching my head, so I wondered if anyone could help explain please?

Many thanks

 

[Cubist]

It's all left me scratching my head, so I wondered if anyone could help explain please?

And you're far from being the only one!

That aside, Welcome to this motley crew.

When I came to consider the issue of breathability I quickly realised that the ceilings in most houses were the largest single areas through which moisture generated indoors could, in effect, most easily radiate/migrate to other parts of the building. Ceilings are also much more efficient than other elements given their relative thickness and composition. The various voids under upper storey floors and in loft spaces assist the process of moving moisture out of the building via the provision of effective channels of ventilation. For me, the key is to make sure that there are no traps along the routes to the exterior that can capture moisture or allow it to condense and be absorbed into the fabric of the building. To my mind then a VCL can only work by allowing moisture captured by the ceiling materials to radiate back out into the rooms from whence it came.

The only logic I've perceived in the use of VCLs in houses that have cavity walls is that any moisture captured in air above a ceiling will condense out when the warmer air flow passes above the cold air in the cavity. As your walls are solid I would therefore suggest that a VCL has little merit.

 

[noj]

My thoughts:

A lot of houses do have a 'VCL' installed when insulated. Simply a sheet of plastic put down before the insulation.

These aren't sealed at the edges where it's coldest and seem pretty pointless. A modern plasterboard ceiling with a few coats of vinyl emulsion is probably a more effective barrier although it's need is questionable.

I think the argument is that you could get interstitial condensation in the insulation as moist air travels up through it to the cold roof above. Maybe the current recommended depth of the stuff increases that risk?

A well ventilated 'cold' roof should cope anyway - and when there's any sun the space gets far from cold.

I'm generally wary of introducing completely impermeable layers into a breathable structure. Always picture collecting water in a desert using a sheet of plastic over a hole in the ground...

I do think there's an argument for covering the top of roll form insulation with a breathable membrane to prevent it clogging with dust and protect it from any drips of condensation which could occur from above in the wrong conditions. This probably isn't really worth the expense unless you have spare membrane from another job...

 

[Black Timber]

Sorry for the late return. And thanks a million for the helpful replies.

I should have added in my first post: We don't have damp problems in the house. But we do often have very high humidity. That might be in part due to where we live (Wales, lots of trees are around and water), and also might be due to the house having previously had cement render and modern plaster on the walls (has recently been removed and replaced with lime).

We live a somewhat odd fashioned lifestyle. Not much hot water usage. A wood burner for most the heating (oil CH radiators for back up). Opening windows in the morning, Etc.

I think what I am going to do is add PIR to the skeilings between rafters, with another layer perpendicular. That's because space is limited in and beneath the skeilings. For the loft proper, I'll stick to normal insulation, which will be a combination of vermiculite between joists (because that's what is already there), and perpendicular mineral wool.

VCL... In the bathroom, which will ALL be insulated with PIR, I'll use tape on the PIR to create a VCL, seeing as it is a moist environment. TBH, it won't get used for showers/baths more than once or twice a week if that, but I'll still aim to do it in case things ever change. For the rest of the rooms upstairs I'll tape up the PIR in the skeilings, but more to keep it as airtight as possible. Those rooms are all part skeiling, part ceiling, and as I'm not doing work to the ceilings (apart from adding the insulation in the loft) the opportunity to put in a VCL isn't there. Plus, I think I'd rather some moisture get into the (well ventilated) loft space than have it all trapped in the house, as kindly described and explained above by you guys.

The only logic I've perceived in the use of VCLs in houses that have cavity walls is that any moisture captured in air above a ceiling will condense out when the warmer air flow passes above the cold air in the cavity. As your walls are solid I would therefore suggest that a VCL has little merit.

The part of the house the bathroom is in is a 1970s extension to the old part of the house, and so does indeed have cavity walls.


Does that all sound like a good plan?

Many thanks