Ceiling removed, roof insulation discoloured. Worry?

[autopoiesis]

We've removed the ceiling panels/wainscotting(?) from our bedroom ceiling, in advance of Monday visit by plasterer to fit an additional layer of insulation over the top (or beneath, if you will), vapour barrier, and steel chassis + fermacell, which he'll joint and smooth ready for us to paint.

I'm concerned by the discolouration of the backing paper of the original insulation, which looks (to me) like damp spreading outwards from the rafters (some of the centre areas are un-discoloured).

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It's fairly even across the entire surface, suggesting no single point of water ingress; and (in 4.5 years here) we've never noted any dampness, dripping, or smell. That said, the wood panelling was impregnated with 22 years of tobacco smoke and wouldn't easily show a stain(!).

Exposing a rafter at one of the darker points, it's dry to the touch and resists an awl-prod with a nice dry woody sound. No smell, insulation paper dry to touch. However, the pavaroof does feel slightly damp from the inside.

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Exterior view of the roof looks AOK (concrete tiles and pavaroof topside of rafters). (Bit mossy - must get up there and sort)

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The house dates from 1987. Is this discolouration normal, or am I right to be worried?

Hope an expert eye can help.

 

[roofmonkey]

Hi Autopoiesis,

It looks like a case of roof sweat.

Here is an explanation of cold roofs versus warm roofs. It looks like the loft was converted (?), the house previously having a cold roof - and the builders didn't insulate enough to make a "warm roof" or vent the roof when converting the loft.

 

[autopoiesis]

Hi roofmonkey,

Your first post? Welcome to the forum (if a total non-expert can say that), and thanks for your reply and link.

No conversion - it's the original roof, and if I lift a tile I can see the topside of the pavaroof which is seen from below in the second pic. There's about 4" space between the underside of the tiles and the top of the pavaroof, which (if I understood it correctly) provides the ventilation referred to in your link, from which:

"A cold pitched roof is where the insulation is placed either ‘between’ or ‘between and under’ the rafters or at ceiling joist level. This type of insulation system can be ventilated or not ventilated. However, if the roof is not ventilated then it will require a breathable membrane between the insulation and structure."

With the addition next week of the internal framing, insulation and fermacell, it'll go from 'between' to 'between and under' and - I hope - with adequate ventilation (pavaroof is listed as breathable: http://www.fourdimensional.co.uk/clients/nbt/systems/new-build/pavaroof-newbuild).

Back to the roof sweat - is that a problem, or just unsightly?

Thanks again,
AP

 

[roofmonkey]

Thanks for the welcome AP

If the roof is sweating, then cold bridging is taking place. If that's the case, the pavaroof insulation boards aren't doing their job properly.

This is obviously all theoretical and should be inspected externally to confirm the size of the insulation complies with current regs. If the boards aren't big enough, you can increase the U-value by adding thermal plasterboards - do this before you get the room plastered.

Back to the roof sweat - is that a problem, or just unsightly?

I've seen a case of wet rot as well as wall plates disintegrating because of it. It all depends on how much time you actually live in your house or if you have your central heating on for extended periods as well as how many people live in the house.

So, from the above explanation, the roof sweat stains may be an old symptom of the previous occupants?

Hope this helps

 

[tony1851]

The problem is caused by the lack of a vapour barrier on the warm side of the insulation.

 

[autopoiesis]

The problem is caused by the lack of a vapour barrier on the warm side of the insulation.

Thank you for your response Tony,

If I've understood you correctly
- the inclusion of a VB with new ceiling is correct (just behind the fermacell, I believe)
- I shouldn't worry about the discolouration of the old stuff
- any humidity in existing insulation will evaporate through the roof once it's no longer being added to from within the room

Right?