Attic Condensation - how to prioritise issues

kildarejohn

Attic Condensation has been discussed several times before in this forum and I have checked out previous posts. Based on this and talking to some guys in building trade, my condensation has at least 6 causes!
My question is how to prioritise solutions?

Structure is; dormer roof with cut joists, slates laid on non-breathable felt, ceiling of tongue-groove timber, fibreglass blanket between joists, ventilation at eaves.
Issues/causes -
1. No vapour barrier in ceiling; moist air from bathroom and bedrooms passes straight through T&G ceiling
2. Non-breathable felt below slates, lots of drops of condensation visible on underside of felt.
3. Air flow from eaves vents is blocked by fibreglass insulation
4. No high level ventilation in roof space
5. No extractor fan in bathroom, only permanently open 100mm sq wall vent.
6. Up to 2 years ago attic ventilation was helped by the fact that the wall cavity was unsealed at the top and draughts through the cavity could be felt in attic; but we got cavity pumped with bonded beads - stops draughts in attic.

Solutions
Easiest/cheapest solutions would be (in ascending cost order)-
1. Slash felt with a knife to make it breathable
2. Drill 1 vent ope c.100mm dia in each of 2 end gable walls near apex
3. Take down ceiling in bathroom, tack PVC vapour barrier to underside of joists and replace ceiling.

Would those 3 things be enough to solve problem?
Looking for advice based on practical experience of similar.

hesker

kildarejohn wrote: »

3. Air flow from eaves vents is blocked by fibreglass insulation

Tackle this first. Pull the insulation back so that the vent is not blocked

kildarejohn wrote: »

Solutions
1. Slash felt with a knife to make it breathable

Don't do that.

kildarejohn

hesker wrote: »

Tackle this first. Pull the insulation back so that the vent is not blocked .

Thanks hesker.
I know it sounds easy, but this is a dormer type roof. c.1m sloping section to ceilings in bedrooms. So attic space is just the space above the collar ties, to reach the eaves you have to reach down 1m (long arms?) into the gaps between the joists and pull out insulation, cut off some, then stuff it back down again. Might be better if doing this to pull out the fibreglass and replace it with Kingspan board which would achieve better insulation with less thickness, so leaving more room for air - big/expensive job.

sydthebeat

order of priority

1. install extract fan on wet rooms (bathroom, utility, anywhere steam is developed) with min 30 litres per second with an over run facility
2. ensure all habitable rooms have proper unobstructed permanent background ventilation
3. ensure min 50mm air gap between insulation and felt at eaves, also ensure min 50 mm air gap on sloped section of dormer ceiling. it might be necessary here to install cut offs of 50mm dia. piping here
4. drill min 2 no 150 dia permanent vents into the high level gables on both ends (ie 4 vents in total. thats roughly equal to a 5mm continuous vent on a 12 m long house) If there are more available gables, put vents in them

Calahonda52

As above re advice, am a bit puzzled by the descriptions so OP can you discuss your setup in the context of the two attachments.
I get the impression that there is zero crawl space...
Thank you

kildarejohn

Calahonda52 wrote: »

As above re advice, am a bit puzzled by the descriptions so OP can you discuss your setup in the context of the two attachments.
I get the impression that there is zero crawl space...
Thank you

Thanks Calahonda, I attach sketch to clarify, hope you can advise.
My sketch is not to scale, attic is more than a crawl space, its c.3m wide and c.1.5m high

Calahonda52

Nice work Picasso, you get the gig!

Now, i would describe that as a storey and a half construction but I will let the experts confirm. Its not what I know as a dormer, hence my confusion, as I live in one....
So:
where is the cold water storage tank?
How have you become aware of the condensation issue in the attic?
what brings you up there?

re the rooms, what floor to being height do you have and I presume it passes the habitable room test as per the building regs?

kildarejohn

Calahonda52 wrote: »

Nice work Picasso, you get the gig!

Now, i would describe that as a storey and a half construction but I will let the experts confirm. Its not what I know as a dormer, hence my confusion, as I live in one....
So:
where is the cold water storage tank?
How have you become aware of the condensation issue in the attic?
what brings you up there?

re the rooms, what floor to being height do you have and I presume it passes the habitable room test as per the building regs?

answers to above-
1 + 1/2 storey - correct
cold tank - above hot press. no dampness near, don't think this is an issue
how aware - when getting Xmas decns
what brings me up there - need for solitude??:rolleyes:
heights - all per regs

Don't know if above answers help

Calahonda52

Answers are fine, now we know what it is.

The options are basically budget dependent.
The reason I asked about the heights was if you took off the T&G could you add some insulation across the roof rafters.

If the T&G is as porous as you say then you have a serious heat loss issue?

If it were my gaff, I would work from outside, maybe look at external roof insulation.
will pm u a link

kildarejohn

Calahonda52 wrote: »

If the T&G is as porous as you say then you have a serious heat loss issue?

Had not been thinking about heat, only moisture.
But you made me do some calcs. - T&G boards are narrow, so big no. boards across width, total length of joints must be over 300m. if joints are 0.5mm wide, then area of gaps is 300000X0.5 = 150,000 mm2.
That's equivalent to a 500mmX300mm hole - the same as if the trapdoor to attic was half open.
So we must be losing a lot of heat all right.

Calahonda52

At the risk of offending you, are you certain there is nothing behind it? Am puzzled from a fire rating perspective. Not my area but am just wondering out loud

kildarejohn

Calahonda52 wrote: »

At the risk of offending you, are you certain there is nothing behind it? Am puzzled from a fire rating perspective. Not my area but am just wondering out loud

No offence!
Ceiling is only 12mm boards with loose fibreglass over them.
I know fire requirements are in Tech Guid Doc B on DoE website, but its complicated doc - any experts out there know if this ceiling meets reqs?