Greg M Posted June 11, 2023 Share Posted June 11, 2023 Hi everybody. I have a quick question which hopefully somebody will be able to help me with. I bought some lap vents for the inside of my roof as we get condensation during the winter, however when I went to install them I noticed that the felt on the inside of the roof does not overlap, but instead is one continuous sheet of felt that is attached to the roof. My question is, would it be possible to make a slit in the single layer of felt to be able to place the lap vents in or would that compromise the roof and allow water to enter? Thanks in advance, G Link to comment Share on other sites More sharing options...
nod Posted June 12, 2023 Share Posted June 12, 2023 If it’s one piece (tarp) That’s probably why you have condensation issues Probably needs redoing Link to comment Share on other sites More sharing options...
Greg M Posted June 12, 2023 Author Share Posted June 12, 2023 Thanks for the reply Nod. Yeah I was given a quote for £1200 to instal new felt and roof vents however I can't afford this amount at the moment hence trying the lap vents. Link to comment Share on other sites More sharing options...
makie Posted June 12, 2023 Share Posted June 12, 2023 Can you take a picture of the felt? Just so we can clearly see what we are looking at. Link to comment Share on other sites More sharing options...
Temp Posted June 12, 2023 Share Posted June 12, 2023 Have you checked you have sufficient ventilation at the eaves. You typically need the equivalent of a 10 - 25 mm wide slot the full length. That's a lot of 80mm round vents, perhaps 4 - 5 per rafter bay. Sometimes you have to have two staggered rows to meet recommendations a one line won't fit. https://nhbc-standards.co.uk/7-roofs/7-2-pitched-roofs/7-2-15-ventilation-vapour-control-and-insulation/ Link to comment Share on other sites More sharing options...
ETC Posted June 12, 2023 Share Posted June 12, 2023 21 hours ago, Greg M said: Hi everybody. I have a quick question which hopefully somebody will be able to help me with. I bought some lap vents for the inside of my roof as we get condensation during the winter, however when I went to install them I noticed that the felt on the inside of the roof does not overlap, but instead is one continuous sheet of felt that is attached to the roof. My question is, would it be possible to make a slit in the single layer of felt to be able to place the lap vents in or would that compromise the roof and allow water to enter? Thanks in advance, G If you’re getting condensation in the roof space you should try and increase the insulation in the roof space making sure that you overlap insulation at the eaves and provide adequate ventilation. However lap vents aren’t great. Where the insulation is at ceiling level you should be providing 10,000mm2/m ventilation at the eaves and - depending on the span and pitch - 5,000mm2/m ridge ventilation. For insulation between the rafters the requirement is higher. Take a look at Glidevale’s website for products and take a quick look at the White Paper which explains a little bit about ventilation in buildings. Link to comment Share on other sites More sharing options...
Greg M Posted June 13, 2023 Author Share Posted June 13, 2023 20 hours ago, makie said: Can you take a picture of the felt? Just so we can clearly see what we are looking at. I will take a picture when I finish work and post it onto the forum 👍 Link to comment Share on other sites More sharing options...
Greg M Posted July 2, 2023 Author Share Posted July 2, 2023 On 12/06/2023 at 17:57, makie said: Can you take a picture of the felt? Just so we can clearly see what we are looking at. Link to comment Share on other sites More sharing options...
bassanclan Posted July 2, 2023 Share Posted July 2, 2023 Looks like old type bitumen felt rather than modern breathable membrane, never the less it is on a roll, so there are laps where one roll overlaps the next 1 Link to comment Share on other sites More sharing options...
Greg M Posted July 7, 2023 Author Share Posted July 7, 2023 On 02/07/2023 at 23:07, bassanclan said: Looks like old type bitumen felt rather than modern breathable membrane, never the less it is on a roll, so there are laps where one roll overlaps the next Thanks very much. I really appreciate it 👍 On 12/06/2023 at 17:57, makie said: Can you take a picture of the felt? Just so we can clearly see what we are looking at. Link to comment Share on other sites More sharing options...
makie Posted July 7, 2023 Share Posted July 7, 2023 19 minutes ago, Greg M said: Thanks very much. I really appreciate it 👍 bassandclan already gave you the answer, it's got laps. Link to comment Share on other sites More sharing options...
Dave Jones Posted July 7, 2023 Share Posted July 7, 2023 what sort of tiles are on the roof ? Probably quicker and cheaper to just add a bunch of tile vents both sides of the roof. Link to comment Share on other sites More sharing options...
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