Condensation inside new roof

aislingw

Hi

Wonder if anyone can help. I'm not 100% au-fait with what has happened, but this is what himself asked me to post, hope it makes sense....

New roof on an extension - 3/4 height ceiling, ie, higher than your average dormer but insulation on the slope rather than in an attic, if you know what I mean.

Between the rafters there is 4" (100mm) rigid insulation, leaving 1" (25mm) ventilation gap. Usually the recommended ventilation gap is 50 mm but he was told that 25mm was enought since the roof was done with a breathable membrane rather than felt as such, and just to tape the joints. Then it's clad in insulated plasterboard - 25mm insulation on 13mm. Now it turns out there are massive amounts of condensation on the felt. Only discovered as some dripped through and we thought the roof was leaking. Thankfully the rooms have not been plastered yet - waiting on plasterers, a blessing now I suppose!!

Anyway, any ideas as to what our options are now? Drill holes in the soffit (fascia? I never know which is which) and put vents in? Rip out all the insulation and put in some other sort of membrane? Trim an inch or so off all the insulation (foil faced on each side) to increase the ventilation gap? Thanks for any help!

Aisling

davidoco

A certain level of condensation would be normal inside a new roof, also the weather on a particular day can have a big impact.

You should have vents in your soffit (the underside) if not you should get the person who fitted them back in to fit vents.

aislingw

Just looked up the breathable membrane - it's kloeber permo forte which it says on the kloeber site is:

Very strong and robust vapour permeable underlay.
Suitable for non ventilated cold and warm roof applications.
No ventilation of the roof void required.

So if no ventilation is required, what is the problem?! And these are rooms that aren't even heated yet, only what they get from the rest of the house, which shouldn't be much as doorways blocked off - how bad would the condensation be if they were?!

aislingw

Thanks davidco - must have been posting at the same time - would a "certain level" be equal to "more or less completely covered in beads of water"? The guy who fitted them is our builder neighbour up the road, he was down last night drilling a vent to see if it makes any difference - will see later on today when himself gets home. Should we have a vent in each "bay" or whatever you call it - ie, in the soffit between each rafter?

davidoco

I personally would fit small vents every two to three metres (not very scientific) even if they say it doesn't need it or just loads of 5mm holes.

Yeah loads of beads like on a newly washed car wouldn't be uncommon. How are you seeing them if the facia and soffit are on?

I think in my own house it took at least the first year for the beads/drops to go away. It's all the water in the contruction concrete, blocks etc.

aislingw

Well it's a poroton-build - ie, terracotta blocks so shouldn't have the same moisture levels as a concrete build - although the upstairs floor is concrete so I suppose there is a big mass of concrete there. HOpefully that's it, and that venting will sort it out.

How we can see it - we spotted a drip of water on an internal beam and on the floor so thought the roof was leaking and took off a bit of plaster board and insulation to investigate. Plus it's not a dormer as such - the head height at the walls is 5 foot and there is a normal ceiling in the middle of one of the rooms - there's a small attic and you can go up there and look down the slope into the eaves and in the ventilation gap you can see the beads.

Aargh - it's an up-down up-down roof, ie, like 2 houses stuck together with a valley in between so in that valley we don't have soffits, so will that mean we need to put vents in those "internal" (used to be the external) walls?! So much for our breathable blocks and plaster!!

davidoco

up down up down - hope it's not up down snow down down at some stage

it's probably a bit late now but you could have put in vent tiles or vent slates to take care of the unvented area.

I bet the designer/architect put in somewhere about the need for venting in the soffit and i also bet he mentioned vent tiles or slates somewhere even if you do have breathable felt. You have to go on what every body is doing even with the breathable felt in putting in ventilation.

rooferPete

Hi aislingw,

Products like vapour permeable membranes have one fault, they are often fitted upside down.

You can fit vents in the soffit but unless it's a continous vent (not likely at this stage) there are 50 mm vents available that fit into the soffitt.

As your roof is a double apex shape you can fit tile or slate vents on the valley side of the roof which should work as it is very rare that there would no ventilation at the valley.

It's never too late to install slate vents all it needs is the right tools and the know how.

A bit O/T but Tyvek were claiming their vapour permeable membrane didn't need any vents, they had to withdraw that claim when the BBA refused them a certificate unless they reverted to venting the roofs.

The builder could make the argument that he has fitted the insulation 50 mm below the roof covering when the space between the felt and the roof finish (slate or tiles) is taken into account.

There are still thousands of roofs all around the country without vents or underfelt and the timbers are solid.

The cause may be the heat from the house meeting the cold underlay.

.

aislingw

Just wanted to say thanks for the responses, David and Pete - we put a few holes in the fascia to check and it is definitely dryign out there. So looks like it just needed more ventilation.