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

[Oxbow16]

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

[saveasteading]

So many questions. Yes it is complex.

 

If an existing building has no dampness then you can leave it alone.

If you are stripping tiles from the roof then fit a vcl before retiling. The cost is low and it can do no harm.

Old roofs in england usually have nothing under thr tiles. You get wind and driven snow in the attic and so insulate the ceilng. What little wster comes u der the tiles dries out again...usually.

More modern roofs have felt. The same applies except the ventlation is only around the tiles.

In scotland (and posher houses in E&W) roofs have sarking. 4 inch boards with gaps between, so it ventilates.

 

Since breathable membranes were invented all reroofs benefit from them, whether new of retrofit.

Osb sarking doesnt have many gaps so needs a space under it to allow ore ventilation. The traditional gaps are also left between sarking boards but they also now get a space beneath, because why not?

 

Summary. Yes roofs should breathe. 

 

[Mr Punter]

A few layers of paint on a plasterboard ceiling is fairly vapour impermeable.  Most of the vapour will be coming through light fittings, boxings, ill-fitting loft hatches etc.

[Oxbow16]

@saveasteading - thanks for all the info.  TBH, I'm thinking more the VCL at ceiling level rather than at the roof.  IE - between the joist insulation and plasterboard.  Or in a warm roof, the VCL between the plasterboard and insulation.  I've seen many cases in which people are using PIR and then going to great length to tape up all joins and gaps.  You're info about roofs is also very interesting (and we do actually have sarking boards in one area of the house), so I'll file that for future reference when work on the roof is being considered.  But for now it's more about the ceilings...   

 

@Mr Punter - that's a good point actually.  And now that you come to say that, with solid walls there's an argument that you shouldn't use normal emulsions because they trap moisture and don't allow the walls to breathe.  Knowing that, I should have figures out the effect of paint on the ceilings for myself!  But with that in mind, again, why do folk go to great lengths to tape up all joins etc with foil tape, and/or recommend and put in dedicated VCLs at ceiling level?  

 

Thank you both very much

[saveasteading]

There is a design principle that we first design for draught proofing, and then ventilate as necessary. That is for energy efficiency. Control.

The pir is taped to stop draughts or other air circulation.

Ventilation of rooms is then controlled by windows, vents or fans.

 

The vapour control is about protecting the timber structure from damp and rot, and so is a separate issue. The membrane allows humid air to vent out, but is waterproof against rain coming in. Thus the structure inside the vcl is always dry, while outside it the dampness gets through but is stopped and the ventilation lets it dry again.

 

If you get an offcut of vcl, play with it. Fold into a cup shape to see if it holds water (it does). Then blow or suck air through it. Even when you know this, it helps to demonstrate it.

It also avoids any thought of using leftovers as filter membrane, or vice versa.

[Oxbow16]

Many thanks for all the replies.  

 

On 09/03/2023 at 14:26, saveasteading said:

If an existing building has no dampness then you can leave it alone.

 

 

No, 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).  

 

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, 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.

 

Does that all sound like a good plan?  

 

Many thanks

Edited March 16, 2023 by Oxbow16

[Dave Jones]

in a nutshell, cold roof doesnt need a VCL a warm roof does.

[saveasteading]

Sounds like you have it. Plastic skin inside the insulation, so that humid air does not get from a room into the insulation. Breathable outside it, to let any humidity escape.

[Oxbow16]

Thanks for the replies  

 

[Coyne]

Can I ask a follow-on question to this (very informative) thread: When looking at hybrid roof build ups online - where there is insulation both between the rafters and above the roof decking - it seems the recommendation is for the VCL to go above the decking, as it would in a warm roof. So the VCL is essentially sandwiched between the 2 layers of insulation, with more above than below. 

 

Is that correct? I can't understand why with a hybrid roof, the VCL would not be below the rafters at ceiling level? 

 

I am looking at using rockwool insulation between the rafters, as it is breathable, and PIR above the decking.

 

Thanks