More Attic Questions...

[jungle_jim01]

Thanks for everyone's advice regarding timber treatment. Just a couple more questions now that i've finished removing the old lath and plaster (sorry!...needs must!)...

1. last summer I had the whole roof replaced. Slates + Breathable membrane. (Previously it was in a pretty bad way.) Looking up at the membrane along the gable end, there's a couple of areas with droplets of water on the membrane. I'm not sure if this is a leak as it's been raining heavily and didn't seem to affect it. I'm wondering if there's a possibility of it being condensation?
(i've asked the roofer to return and have a look but would welcome any thoughts).

2. Insulating?
As far as i'm aware this is an 'unvented roof'. I'm thinking of putting kingspan or similar in between the rafters. Then insulated plasterboard screwed to the top of the rafters, leaving the main beams exposed. Having never done this before i'd appreciate any suggestions/tips. ie; should i leave much of an airgap between the kingspan and membrane? Should i batten between the membrane and insulation to help provide a gap? Any alternative recommondations?

Thanks again, I'm learning a lot from here.

 

[chuckey]

There should be a gap of 10mm between the bottom of the "breathable" membrane and the top of the wall or equivalent air vents in your soffits or facia along both sides of your roof. I have this trouble, modern breathable membrane reduces the air cross flow from a tiled roof by a factor of 1000 or so, leading to condensation and damp gable walls. In my case the dampness has spread down and b******d up the bedroom ceiling and wall paint work. I have subsequently installed air vents and am awaiting some results (?)
Frank

 

[Penners]

Looking up at the membrane along the gable end, there's a couple of areas with droplets of water on the membrane. I'm not sure if this is a leak as it's been raining heavily and didn't seem to affect it.

Do you mean the water droplets are on the underside of the membrane? If so, then unless you're able to trace some kind of leak, this would suggest either a small tear in the membrane or condensation.

 

[vicky whitworth]

Hi, I'm not really an expert on this but we have the most horrid condensation in our attic - it drips off the rafters onto the insulation onto the floor of the attic - so I have learnt a few things. We were told we have to decide if we want a 'warm' attic or a 'cold' attic. The difference being whether you use it for living space (warm) or it's just an unused void (cold). If you go with the cold route we have been told to stop as much warm air from the cottage getting into the attic by blocking all gaps, making sure the loft hatches are sealed as tight as possible, and then getting huge amounts of insulation onto the floor of the attic. Then vents are needed at eaves level and at ridge level so that the air flows in low down the floor and up and out - the circulation of the air helping to keep the attic dry. If you had an old roof before you may have had gaps at eaves level if there were no soffits (and these gaps have probably now been blocked if you have a new roof). To get vents at ridge level try LapVents - these won't show from the outside so you don't have to have horrid plasticy things showing on your roof. The breathable membrane breathes but it doesn't help the movement of air.

Apparently it's a common problem when old roofs are replaced with modern roofing methods. Ours was done in the early 1990s by some wag who blocked all the eaves vents with insulation.

Shame about the lath and plaster ceiling - are you sure you wouldn't prefer to live in a more modern house?

 

[middi]

The guaranteed way for a breatheable membrane to do it's job properly, where vented roof tiles are not desirable and vented soffits are not aesthetically correct in a period building, is to use " warm roof construction" technique where "modern" materials is chosen.
ie:- Insulation boards taped and sealed secured by battens on the exterior of the roof rafters using 50x25 battens preferably screwed to the rafters. Breatheable paper above these and counterbattened into position with appropriate battens to accept size of tiles.

The purpose of the paper is there primarely to stop battens to rot prematurely and to offer extra security to "stray" rain. Therefore a warm roof. Moisture from the exterior is totally eliminated and any moisture from withing has to be delt with by correct interior to exterior ventilation.
Having " bretheable" paper in a "open to the element" roof arrangement does not do very much in way of improving the breathing mechaninsm and it is there mainly to stop " stray" rain falling in the roof void. If rain does gain access within the void of the roof the paper fine microns system will actually hinder it's evaporation. A gap of 25mm is recommended between insulation and breatheable felt, as anything smaller than this may result in bridging water traps where the felt sags and touches the insulation board.

Middi

 

[AMc]

2. Insulating?
As far as i'm aware this is an 'unvented roof'. I'm thinking of putting kingspan or similar in between the rafters. Then insulated plasterboard screwed to the top of the rafters, leaving the main beams exposed. Having never done this before i'd appreciate any suggestions/tips. ie; should i leave much of an airgap between the kingspan and membrane? Should i batten between the membrane and insulation to help provide a gap? Any alternative recommondations?

Thanks again, I'm learning a lot from here.

You need to consult the technical information for the breathable membrane and the insulation. Muck up the circulation under the tiles and you could create moisture traps, muck up the sealing of the roof and your precious heat will simply blow around your expensive insulation.
Kingspan (and the other solid board insulation manufacturers) offer technical advice which it is really important to follow.
If you badly fit insulation you can cause more problems than you solve - my sparks told me he'd been quoted a 75% loss of efficiency by a sale rep. if the boards weren't fitted and sealed.
e.g.
http://www.insulation.kingspan.com/uk/literature.htm
Suitable insulation backed plasterboard for example
http://www.insulation.kingspan.com/uk/pdf/k18.pdf

Most of the builders I worked with had a rudimentary understanding of what to do but were unaware or unconcerned about how poor handling, bad fitting and simply not considering the flows of air could compromise the who installation.

You aren't going to do this again so arm yourself with the knowledge you need to get the work done properly.

Sheffield Insulation gave me the best prices and great service for my insulation
http://www.secondsandco.co.uk/ have also been recommended here and on the Green Building Forum.

 

[chuckey]

As Middi says "A gap of 25mm is recommended between insulation and breatheable felt, as anything smaller than this may result in bridging water traps where the felt sags and touches the insulation board." But this must be totally vented to the outside i.e. to the notional 10mm gap. If this 25mm gap disappears at the wall plate or wall, then the void is not vented and any water drips are contained within the building. Or to put it another way, if you have a breathable felt then back up tight to it, impervious insulation board, where is the felt going to breathe to?
Frank

 

[Penners]

You aren't going to do this again so arm yourself with the knowledge you need to get the work done properly.

Excellent advice...