kingspan or celotex

GClyde

OOPS!!....the BS should be BS5250, not BS5220.

Anyway, In order to work out how many vents you need, you find out the free flow vent area of the vent and divide that into the area required. For example, say that your ridge measures 10m in length; well, you need 5mm x 10000mm = 50000sqmm vent area. Likewise, with the eaves, you need 25mm x 10000 = 250000sqmm vent area.

Therefore, to work out the number of vents needed, you obtain the free flow vent area of a vent - say 7500sqmm per ridge vent, and 20000sqmm tile vents.

7500/50000= 6.66 (say 7 number ridge vents), 20000/250000= 12.5 (say 13 tile vents)

The idea is to create air flow to remove moisture. There are breathable roofing membranes out there that can be used in conjunction with timber sarking boards - thus removing the need to install the vents. But that would mean that you'd need to strip the roof back and remove the felt and replace it with a breathable membrane. The use of OSB (plywood) and a breathable membrane does not work, so stay away from that build up.

Simple....well, I hope so.

Regards

Graham

typeractive

GClyde wrote: »

In order to work out how many vents you need, you find out the free flow vent area of the vent and divide that into the area required. For example, say that your ridge measures 10m in length; well, you need 5mm x 10000mm = 50000sqmm vent area. Likewise, with the eaves, you need 25mm x 10000 = 250000sqmm vent area.

Ok - this is pretty simple. I can calculate that from my plans. Basically it is the length of the building.

Therefore, to work out the number of vents needed, you obtain the free flow vent area of a vent - say 7500sqmm per ridge vent, and 20000sqmm tile vents.

Is that 7500sqmm and 20000sqmm a standard size?

7500/50000= 6.66 (say 7 number ridge vents), 20000/250000= 12.5 (say 13 tile vents)

Check.

The idea is to create air flow to remove moisture. There are breathable roofing membranes out there that can be used in conjunction with timber sarking boards - thus removing the need to install the vents. But that would mean that you'd need to strip the roof back and remove the felt and replace it with a breathable membrane. The use of OSB (plywood) and a breathable membrane does not work, so stay away from that build up.

Trust me I ain't stripping the roof back, I've been working my bum off scraping off the expanding foam - very hard work. Just about done though! I've managed to find a few nice leaks too! :mad: Still....better to find them now than when the attic is all finished off!

Simple....well, I hope so.

Regards

Graham

Thanks Graham, may I ask what you do for a living or do you just know about all this stuff? You sound like you're in the roofing / building business!

Cheers
:beer:

GClyde

Hi, you need to watch, as some proprietary vents give varying amounts of free flow air.

I do know a lot about building - I am an ex building control officer, and now practice as a building & energy consultant

Good luck with your roof:j

Graham

ds1980

Just seen the post finally took off! I actually got some seconds 25mm boards from secondsandco came in at about £10 a 1200x2400 sheet.

They were very easy to deal with. Have gone the battening, then insulation and plasterboard on top route. Same will apply for the ceiling. Forget the laminate stuff with integral plasterboard twice as expensive.

All their products are ultimately the same just badged up differently.

Graham, can you confirm i am ok to batten roof etc same as walls? I may well have electric, ch and water through the space.

thanks,

GClyde

Hi ds1980, it would be great if you had a sketch of the build up, then I can appreciate the build up you have in mind.

In the meantime, service voids are good, they are even encourged on the Energy Saving Trust's web site. An emissivity quality benefit could be achieved with an air gap too, although there needs to be a reflective membrane/foil on the other side of the gap for this to work.

Take care and look forward to seeing your sketch.

Regards

graham

ds1980

graham,

Don't know how to add attachment.

Basically out house front and rear are single brick. One side is attached to next doors utility area however only to perhaps to 2 metres height remaining 50cm is exposed. Not sure whether it is single brick separating us?? assume so though. Other side is cavity wall attached to the house.

Flat roof that as it appears doesnt leak although there are some water marks on roof. plan was to batten everything with 2"x1" probably. then attach the 25mm kingspan (foil backed on both sides) and 12.5mm plasterboard on top or aqua panel or similar in wet areas.

this will leave only 25mm gap behind insulation but the utility area as it is now doesnt leak nor have condensation so im pretty sure it will be ok. Can do remedial work to flat roof if needed also. Not sure if that helps??

GClyde

Right, so you've got a half brick thick wall, and an exisitng flat roof. The intention is to insulate both from the inside - applying thermal insulation on battens.

Well, lets look at each separately:

Half brick wall -
The existing 100mm thick wall, on it's own, would achieve a U value of around 3.1. Current regulations has it that new walls need to be around 0.25. So, you can appreciate that there is a big difference here.

Anyway, adding 25mm PUR (poly urethane rigid board) with a thermal conductivity of 0.23 to the wall would give you a U value of around 0.64 - which is a remarkable difference.

U value calcs for the wall:
Layer .............. d (mm) ..................l layer .................l bridge ................Fraction ...................R layer ...............R bridge ................Description
.......................................................................................................................................0.130 Rsi
1 .....................12.5 .................... 0.160 ...........................................................................0.078 ............................................Knauf vapourshield
2 .....................25 .......................R-value ................0.130 ................. 0.150 .......................0.180 ................. 0.192 .................Cavity unventilated
3 .....................25 ....................... 0.023 ...........................................................................1.087 ............................................polyurethane
4 .....................100 ......................0.770 ........................................................................... 0.130 ........................................... Brick
........................................................................................................................................0.040 ........................................... Rse
163 mm (total wall thickness) 1.645

Total resistance: Upper limit: 1.647 Lower limit: 1.647 Ratio: 1.000 Average: 1.647 m²K/W

U-value (uncorrected) 0.6073

U-value corrections
Air gaps in layer 3 DU = 0.0044 (Level 1)
Fixings in layer 3 DU = 0.0279 (4.00 per m², 10.0 mm² cross-section, l = 50.0)

Total DU 0.0323

U-value (corrected) 0.640
U-value (rounded) 0.64 W/m²K

I'd recommend that a foil faced plasterboard be used or a plastic vapour control layer inserted directly behind the standard plasterboard.

next post will be for the flat roof.......

GClyde

Flat roof;
Up here in Scotland, we do not endorse the addition of insulation between or under the flat roof joists. This would create a cold roof build up, which may cause condensation.

However, as I am led to believe, in England, and due to the slightly different climate, you can place thermal insulation between the flat roof joists. Although, the normal position of thermal insualtion is on top of the flat roof decking - creating a warm roof. A cold roof build up would also benefit from having roof void ventilation .

As you've got an exisitng flat roof, and intend insulating it from the underside, then a more intense study would be necessary. This would take the form of a condensation risk analysis beign carried out.

In other words, it's not as straight forward as you may think and you may be making things worse.

Regards

Graham

ds1980

great no idea what the first post means but the kingspan is foil backed so thats fine. As for the roof!!!!!!!:eek:

Sure it will be fine!!!

GClyde

Basically, the higher the U value, the poorer the thermal performance.

With regards to the flat roof, i'd check things out thoroughly if I were you or you could end up looking like :eek:

Why not bounce the idea off your local building control officer, I am sure that he'd only be too happy to advise. He will know the climate conditions for your area too.

Good luck with the upgrade.

Graham