Condensation on Breathable felt

Prior69 · 4 Jan 2010

In October I had the roof replaced on my victorian terraced house (new breathable felt, batons and synthetic slates). I have a small loft conversion set up with dwarf walls with crawl spaces into the attic space at the front and rear of the house. This was in place before the new roof was fitted.

At the rear of the house, I am experiencing massive amounts of condensation forming on the underneath of the felt but at the front of the house there is no problem. On inspection I noticed that Tyvek has been used at the front of the house but at the rear a felt with a green face and white back has been used, I cant find a brand name on it although it looks to be same kind of material as Tyvek. Is it possible this could be a non breathing felt? Are there felts out there that look like they could be breathable felts but are not?

The rear section of the house is above an unused bedroom, the radiator is switched off and the trickle vent on the window is always open, so I dont think any additional moisture/vapour from bleow should be an issue. Although I should mention that this side of the house gets zero sunlight at the moment and is normally covered in frost due to the current weather!

I have pulled insulation back from the eaves in order to try and get some criculation going but my understanding was that ventilation is not much of a concern if you have a breathable membrane??

Please help! Any answers to my questions, thoughts or comments will be gratefully received...I want to cover all angles before I go back to roofer!

Thanks.

noseall · 4 Jan 2010

Breathable membrane is not as 'all singing all dancing' as people make out.
Admittedly, Tyvek is the brand that comes recommended by architects so is likely to be a top performer.

That said, all breathable membranes require other factors of the build to be in place for them to work, i.e. foil backed plasterboard, background ventilation (trickle vents), trap-door seals, roof vents (where applicable), etc, etc.

datarebal · 4 Jan 2010

There are many colours/brands of breatheable underlay, including a couple of green ones . It is likely to be roofshield which is very acceptable.

I think you have hit on the probem being the dark side of the building there fore cold.

how much insulation do you currently have?

Prior69 · 4 Jan 2010

Hi Guys, thanks for the replies. I've got the full 270mm of insulation; 100mm between the rafters and 170mm over lapping at right angles. The room below is really quite warm considering the radiator is never on and window is always ajar.

Just to update you, I've just been in to check the progress and all of the condensation has now frozen on the membrane!...I'm hoping that this is a good sign as it means some air from outside is making its way through and hopefully will help evaporate away the water...when it warms up a bit. What do you reckon?

htgeng · 4 Jan 2010

I too had a re-roof done a couple of years ago with a breathable membrane. I have noticed beaded condensation on the underside of the felt on one of the valleys. I read this is common and the furry surface stops the condensation sliding down. I've set a large fan running in the roof space on a timer during the day so will see how that goes in a couple of days.

weaviestevie · 6 Jan 2010

I have exactly the same problem - my felt is white on one side green on the other with black gridlines marked on it and has millions of pin pricks through it - it is Rubershield Pro and supposed to be fully breathable.

I have no ventilation only breathable felt - and loads of condensation. Do you have any ventilation?

I have a discussion about it here....

http://www.greenbuildingforum.co.uk/forum114/comments.php?DiscussionID=5125&page=1 - Item_12

Prior69 · 6 Jan 2010

weaviestevie - My felt has just a load of white gridlines on it, have been trying to look for images on the web cant find any clue as to what brand it is - does anyone know?

I dont have any specific vent holes but do get a breeze coming through the soffits. When the roof was finished I topped up the insulation and essentially blocked up any passageway for the air to move from the soffits up the rafters. On the Tyvek side, this is fine. On the side with the green felt, a complete covering of condensation and mould growing on the timber in the space of 4 - 6 weeks! I've pulled back the insulation from eaves and sponged off as much condensation as I could but its too early to tell if this has worked.

datarebal · 6 Jan 2010

As I previously said it is likely to be roofshield commonly used an good quality.. http://www.proctorgroup.com/index.asp?lm=25

the underlay used will not be the poblem here.. both are ok but are not better than one another in my opinion.

Prior69 · 6 Jan 2010

datarebal, you seem to know your stuff. Is there any problem with increasing the ventilation in the roof and essentially treating my breathable felt as if it was a non breathable felt?

datarebal · 6 Jan 2010

There are more advantages to venting the loft space than not.
I would like to see an external pic of the roof if possible?

Deluks · 7 Jan 2010

Prior69 said:
I topped up the insulation and essentially blocked up any passageway for the air to move from the soffits up the rafters.

I'd take a guess that the problem is here, the product can breathe but probably needs air movement in order to push the moist air out through it. A supply of fresh cold dry air would reduce the problem and dilute all the moisture coming up through the house.

Is there snow on the roof, and has the condensation coincided with this?

Prior69 · 7 Jan 2010

There has been snow and Ice on the roof - as it is the cold side of the house it doesnt get a chance to melt off at the moment. The condensation has coincided with the bad weather.

I think that ventilation from the soffits should be the cure as although the condensation hasnt gone yet ( it keeps freezing!!) I cant see any new moisture forming and parts of the roof seem to be drier which can only be good news.

htgeng · 9 Jan 2010

Success! I put a couple of fans in the loft on a timer to run for about 5 hours during the daytime. I went up there this evening and all the condensation that I saw before has gone.

Prior69 · 11 Jan 2010

I too have had success! Took a look up there yesterday and it is starting to dry out nicely. Am going to put the insulation back but cut it to size so that the eaves dont get blocked up again.

liena · 17 Jan 2011

Prior69 said:
I too have had success! Took a look up there yesterday and it is starting to dry out nicely. Am going to put the insulation back but cut it to size so that the eaves dont get blocked up again.

Hello, I am responding to your conversatioin about roofing membrames. I don't know how to find out how to answer but if this works then I want to say I am completely panicing - I had my roof redone and it is soaking- I went away for two months and came back just now and it is not just beads of condensation but the roof timbers are soaking in parts - moisture has condensed and the run down them. It used to be slate with this cementy stuff thrown up underneath but now it is this membrame - vent 3 classic - with slates. It is an old house but when it was ventilated before i had no problem with my roof but this membrame seems to trap aLL the moisture. I have left the hatches open to put some heat and air up and have the heating going well. so far not much change. Please encourage me - I want to whip the stuff out altogether. Liena

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Condensation on Breathable felt

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