Roof insulated with plastic

[marsaday]

i am doing some work on my slate roof and it seems the previous people have reroofed. however they havent used breathable membrane, but plastic sheeting.

in the attic space their is a roof void which is insulated. it seems that it is sweating a lot in here.

do i add 2 roof tile vents AND pierce the plastic membrane or do i leave this intact ?

 

[Trevor2]

The plastic sheeting you mentioned will probably be a felt membrane called monofol and it will cause condensation in your roof space. I suggest you fit 8 nr slate vents,. If you place 2 at the bottom of the roof and 2 at the top on each elevation it will enable an air flow on either side.

ps make sure you cut the felt under each vent.

i am doing some work on my slate roof and it ses the previous people have reroofed. however they havent used breathable membrane, but plastic sheeting.

in the attic space their is a roof void which is insulated. it seems that it is sweating a lot in here.

do i add 2 roof tile vents AND pierce the plastic membrane or do i leave this intact ?

 

[marsaday]

thanks for help

 

[CloakedVerger09]

provided an adequate amount of ventilation is build into the roof it won't matter if the underlay is plastic, bituminous (1F) or high performance VPU.

BS recommends (assuming insulation is at ceiling level) 10,000 mm2/m at low level on either side of the roof and (if roof pitch is > 35 degrees) 5,000mm2/m at high level.

If your insulation is between rafters then increase the low level to 25,000mm2/m and ensure a 50mm gap between top of insulation and underside of underlay. you must also include for high level in this instance.

If you're going to install vent tiles to achieve this make sure you install them approriately. Check the manufacturers advice and follow it.

 

[sparkykink]

Hi,
It looks like I have the same problem, but I already have roof vents and the 50mm circular soffiit vents both front and back.
I have only been in the bungalow for 1 winter but it was dripping heavily from the plastic monofol during the cold weather.
Only the back of the bungalow has been re-roofed using this sheeting, so ultimately I'm going to have to re-roof the front.
Question is, should I cut my losses and take the tiles off the back and replace the monofol with breatable material or try further ventilation methods?

PS : at the moment the loft insulation is only about 100mm deep, so is the problem partly down to the loft getting too warm, and does it make a difference it being a bungalow?

Any help would be welcome.

Thanks,
Dave

 

[lorreal]

Hi,
To keep attic cool, the most traditional method is used blankets or rolls that can be spread out and stapled to your rafters. The most common materials are fiberglass, mineral wool, plastic and natural fibers. Blankets and rolls are a solid choice for insulation against heat conduction but do not have an appreciable effect on blocking heat radiation.
So the best choice is to keep your existing plasic insulation in place and then add a layer of radiant barrier foil. This will keep out 97% of radiant heat while also insulating against conductive heat transfer.

 

[sparkykink]

Hi,
I increased the insulation by adding a 200mm blanket on top of the original, but still have the problem.
I'll try the barrier next, but the snag there is that I'll not be able to get it fitted at low level as the roof slopes down to the wall plate and there is a whacking great wooden purlin stopping me from getting in.
If I can only fit the barrier two thirds of the way down, will it make any significant difference?

Cheers,
Dave

 

[alittlerespect]

Hi

If your using a fibreglass quilt it has negligable vapour resistance and you should take the insulation out, fit a vapour proof barrier over the ceiling and joists and then replace the insulation.

With regard to the sarking felt, some years ago there was a felt on the market (forgotten its name) but it was advertised as being virtually indestructible and to all intents and purposes it lived up to its name, the only problem its not a breatheable membrane and signficant problems can be encountered with condensation where internal moisture laden air comes into contact with the cold underside of the felt.

The problem being worsened by the introduction of the fibreglass insulation which allows the temperature in the roof void above the insulation to reduce and brings the air near to its saturation point and then easily deposits as condensation on the underside of the felt.

Regards

 

[sparkykink]

Hi,
Thanks for the reply.
I am sure we are talking about the same plastic felt.
As a way forward, are you saying tht the loft is too cold?
I could easily put a tubular "shed heater" up there if that would help.

Cheers.

 

[sparkykink]

Hello again,
Thought I would post an update if it can be of help to others.
I had the whole roof re-tiled, re-felted with the breathable type (not sure of the brand) and battened and in the summer.

All the old monorfol sheeting was removed and up to now there has been no condensation and all is well.

I suppose it's basics really, in as much that if there is a potential for condensation you can rely on plastic to attract it.

Cheers,
Dave

 

[sparkykink]

Hi,
Bad news I'm afraid.
The roof is still condensating although nowhere near as bad as it was with the plastic stuff.
It will probably be OK again in a month or so because it's only ever condensated in the very cold winter months.
It's freezing cold in the loft so I don't think hot air rising from the rooms below is an issue, just lack of airflow.
In the summer I'll put air bricks in the gables and see how that goes next winter.
So the truth is, the breathable felt isn't a cure all.

Cheers,
Dave