warm roof help

[sawdust1]

Hi all, been doing some some work for a couple who live in an old cottage and asked if i could have a look at the loft as condensation
is dripping of the tar backed felt on to items stored underneath. I suggested going for a warm roof and fixing ridged insulation between
the rafters. I was going to use 2x1 battens to keep it in place but on ebay have seen some rafter clips, never herd of these before are they
any good. As 3 inch ridged insulation is expensive is their a cheaper way of doing this ? A previous builder has put 6 vents in the slate roof and
vents each gable end but this has not cured it. Any advice please !

 

[jimmy_s]

I take it the loft insulation thickness has been increased recently?

There must be nowhere near enough vents in the roof, resulting in interstitial condensation. If the loft insulation has been beefed up then you are not going to get a lot of benefit in terms of U value from the extra rigid board. You would be better improving the ventilation of the roofspace by fitting more slate vents I would have thought, ideally to meet the requirement of the Building Regs.

 

[blackrodd]

Is it slating felt or that awful cr*p that Geof Capes advertised years ago, that would sweat like the clappers.
In any event, there's nowhere near enough loft insulation, and it could be the the C/H is set really high and not helping.
I suggest Get the insulation sorted and they will find a big difference,
There are grants for insulation If people care to claim for it too.
Regards Rodders

 

[sawdust1]

jimmy, the loft insulation is old and looks like 4 inch. How many vents would you recommend, the footprint of
the cottage is 10 x 10 Meters?
blackrod, the felt is the old style tared with what looks like a woven string mesh. Upstairs is very hot. Will look into the grants.

 

[Phil Pascoe]

I'd worry about another 6" - 8" of insulation first.

 

[RobinBHM]

Its probably best to ensure gaps are sealed at ceiling level to minimize heat and vapour reaching the loft. Improving insulation at ceiling level leaving the loft as a cold space is the first option.

There should be ventilation at eves level, so its worth making sure the loft insulation does not block this. you may need some ridge vents to encourage ventilation.

The problem with insulation at roof level is that you are trying to insulate a cold space, that is ventilated to the outside.

 

[jimmy_s]

Agh - just lost my message

I would agree with Phil.P - more horizontal insulation will make interstitial condensation worse.

I was expecting you to say you had more insulation installed than 100mm. Interstitial condensation (IC) tends to be more of an issue following insulation levels being increased but with no improvement in ventilation. If nothing has changed in recent years in then I would have thought that if it is IC then there would be visible signs of degradation due to the condensation over the years.

I would check that there are no ducts from extract fans or tumble driers etc that have become disconnected and are introducing moisture laden air into the loft or an open grille from a bathroom. Also worth checking that if there is a water tank that its not leaking or has there been a leaking roof and the roofspace is drying out etc.

If after checking all the above and there is no obvious reason then it sounds like it is an IC problem. You would need to improve the ventilation - it depends on the roof pitch etc and what you can actually do practically. I would read the regs and go from there.

Jimmy

 

[jimmy_s]

Interstitial condensation occurs due to changes in vapour pressures through the structure. All building materials have an ability to slow moisture transfer to some degree. All materials also have thermal insulation properties some much better than others obviously.

You can work out the temperatures through the fabric based on the conductivity of the elements. You can also work out the vapour pressure at each interface based on the permeability of the materials and the indoor and outdoor conditions. Interstitial condensation arises when the vapour pressure at a surface is higher than the saturation pressure for the temperature at that surface. The colder the roof space becomes then the more risk there is of this happening. Ventilation helps as this allows the vapour pressure in the roof space to equalise with outside more readily (ie reduce).

More insulation without proper ventilation normally exacerbates the problem.

Jimmy

 

[sawdust1]

Thanks chaps i will pass on your advice to the home owner and have a look
at how we can improve things.

 

[kostello]

I had one that was the boiler kettling for hours on end.... Steam shot out of the expansion pipe and filled the loft to the point where half of the ceilings in the house had water stains

 

[MMUK]

Surprised no-one has asked this. Are you sure it's condensation and not a roof or other water leak? Is it specific to one place on the roof or all over?

 

[Doug B]

I had one that was the boiler kettling for hours on end.... Steam shot out of the expansion pipe and filled the loft to the point where half of the ceilings in the house had water stains

+1

Really worth checking this if there are header tanks in the roof, there doesn't have to be anything wrong with the system, I worked for an elderly couple who's bungalow was like an oven, the water in the header tank was a lot hotter than you'd want to bath in & as there was no lid on the header tank condensation was forming on a large area of felt in the roof space.

 

[RogerS]

Well, at least that's not as bad as the last place I renovated. The central heating ventpipe vented into the cold water tank and not the ch expansion tank.

'Ere, this tea tastes funny'.