Roof ventilation help.

[Liz Fry]

We are hoping to insulate the roof in our 400yr old cottage. The roof has an old non-breathable felt on it. We have no immediate plans to replace the membrane. The roof covers the main bedroom and is vaulted so no attic area. There is currently no insulation on the rafters.

Naturally as new owners we would like to insulate the roof however have struck a problem..... how to ventilate to avoid damp timbers. The rafters are 3 by 2s and as after 50mm air gap we have virtually nothing to work with for insulation we plan to insulate underneath them and therefore leave the whole 3inches free. the problem i`m tring to work through is the airflow bit. a horizontal beam sits across the top of the wall and the rafters finish on top flush with the wall.. hence no soffit area to work with. I don`t think changing the roof tiles to the venting sort is an option at this point due to the location of the roof... we would need to get the scaffolders out and £1000 to do that I`d rather not!

I`ve considered putting adjustable vents at the bottom of the airgap back into the house and nr the verge to generate some sort of airflow and them open them periodically from time .to time............. however am not sure if this would actually help or not.

HELP

 

[Liz Fry]

thought this might help

(NB the horizontal beam is being replaced this week ........ :lol: )

 

[MdB]

A possible solution for the ventilation is http://www.lapvent.co.uk/. No idea how good they are but they have been mentioned in the past though nobody has yet found out pricing.

 

[Jon Maine]

I presume that you will be insulating at ceiling level, That being the case you could try the following.

Buy polythene on a roll fit snugly between all joists and tape all joins, try to make a continuous membrane lay your 300mm of insulation on top of this. The loft hatch will need to be sealed well too.

The principle is that if you can stop the moist warm air perrculating through the insulation it should not condense on the underside of your roofing felt. As it remains warm in the insulated zone the water is not given up from the air.

It is wise of course to ventilate any void within a building structure. However this is something you could try and monitor easily. You may find that you do not have to worry about the extra ventilation until the next time you have to put scaffolding up for another maintenance issue.

 

[caliwag]

What insulation are you going to add, given that there is no attic?
Are you applying insulation to the inside of the plaster?

 

[Schoolmarm]

Bryan is a very talented member of the old forum. As he invented the lapvent, it will work.

 

[Jon Maine]

It would appear I did not read your first post thoroughly I concentrated on the pictures , You are saying a bedroom is open to the underside of this roof?

You may be able to use an auger and drill diagonally through the wallplate to gain ventilation.

 

[Liz Fry]

kingspan Insulation boards will be applied to underside of rafters. Plasterboard to underneath that fix with super long screws to rafters through the insulation.

Yes the bedroom is currently open to the underside of this roof. There was originally a dodgy false ceiling fitted above the bedroom made of chipboard with lots of polystyrene tiles fitted to it. The insulation was the old yellow nasty stuff and was filled with hundreds of dead composting mice. The false ceiling framework was rather ropey so it came down too. We like the open vaulted effect so much we thought we would try to keep it.

Damp was everywhere in the property (which we have addressed now). THe cottage is now nice and dry and as you can see we are starting now to "build" back up having stripped it all back to the bare shell.

 

[Liz Fry]

that lapvent looks interesting.....................off now to have a thorough read of the site

 

[MdB]

that lapvent looks interesting.....................off now to have a thorough read of the site

If you find out a price, please can you let me know. Thanks.

 

[Liz Fry]

have sent an email and will ring tomorrow. A quick google search suggests that someone was quoted £90 + VAt for two in 2006.

Seems just the sort of thing we are after. Has anyone else any experience of LapVents?????

many thanks

Liz

 

[MattJ]

Yes, I fitted Lapvents to a loft conversion. I had a thick roofing felt and the lapvents fitted neatly into the lap between the sheets of felt. Only problem was my roof is Cotswold stone slate, i.e. very heavy, so the rafters are close together, and I had to nibble a little from each rafter to get the lapvents between them. I also needed quite a few of them, so the £90 for two can quickly add up. Couldn't see any other solution though.

I also fitted one of the lapvents that allows 4" ducting to be vented through the lap, so I could fit extractor fans to bathrooms and not have drill holes in walls.

Matt

 

[MdB]

I also fitted one of the lapvents that allows 4" ducting to be vented through the lap, so I could fit extractor fans to bathrooms and not have drill holes in walls.

:shock: - goes off to have a look again at lapvents as that may cure my bathroom extractor fan problem...

 

[Liz Fry]

£38 + VAT each

reduced to £36 + VAt if you order more than 10

Think I will need about 6 so around £260 cost to me (can`t reclaim VAT)