Warm Roof

[The Natural Philosopher]

I have to agree somewhat. Ventilation on stud walls is not as good as a cold roof.

HOWEVER on stud walls there is also a definite warmn space inside the house, to let the studs breathe backward into the house.

Ther is also far less liklihood of water getting in than in a roof, which dioes get, oper unit area, a lot more wiater on it. In fact with a sensible overhang, and no wing, a wall gets no rain on it at all.

I suspect the issues are to do with thus - the flatter a roof gets the more likely it is to get wet inside the structure. And have that water accumulate.

Condensation is not the only issue.

Also on a stud wall, the insulation properties of te outside callding and airgap are not to be sneezed at. The studs are not that cold, as a result, exceot when there is a lot of air movement behind teh clafdding. Arguably if that is the case its more likely to remove moisture vi ateh breathable membrane anyway.

Damp and condensation control is not an exact science. Its more about being 'good enough' to make sure that timber don't get, and stay, sopping wet.

I think its because BOCO's can get sacked for not doing it. Mine made me punch enormous holes in the eaves, to te extent that it blows a half gale in there sometmes. It puts extreme pressure on the ceiling insulation, and is to my mind complete overkill given that 99% of the ceiling is plated with foil backed plasterboard, to stop moisture getting in.

HOWEVER, when we were biiudling the roof before the final thatching was on, there were leaks. The resulting puddles and wet timber dried out very quickly due to the high levels of ventilation.

Once I get the completion certificate, I may accidentally block some of those vents :-)

[Hugo Nebula]

On Sun, 18 Jan 2004 12:15:11 -0000, a particular chimpanzee named "Peter Taylor" randomly hit the keyboard and produced:

I'm buggered if I'm going to change it just for you (in Scouse the expression is "gerra sensa yuma").

But the sheathing should be vapour permeable, i.e., the vapour will pass through the structure and the VPM and condense on the cavity side. I've not got the software here (nor the inclination) to calculate the dew point, but I'm sure it's outside the timber structure.

But it's not. TRADA's guidelines say "when the sheathing is on the outside, an effective vapour control layer is normally required on the ?warm? side of the insulation to limit the amount of water vapour entering the wall panel". The critical word there is 'limit'. The cavity should also be vented and drained.

It's not acceptable purely by itself for the same reason that it's not acceptable by itself in walls; that is, it can't be relied upon to prevent all moisture from the dwelling passing through to the cold side of the insulation.

WRT ventilating or not ventilating roofs with breather membranes, I defy you to make any sense of the various BBA certificates which need to be picked through with a fine tooth comb, competing manufacturer's details contradicting each other, and the divergent guidance of ADs and BSs. Nevertheless, most BCOs have managed to distill this mush into a series of informal guidelines (not always consistent across areas I'll grant you) that allow for the use of unvented cold roof constructions.

[Hugo Nebula]

On Fri, 16 Jan 2004 11:13:14 -0000, a particular chimpanzee named "IMM" randomly hit the keyboard and produced:

Several loft conversion companies use it. They say it's dearer that Celotex, but you don't have to cut it to fit between the rafters which makes it quicker to install, and there's less waste.

[IMM]

You hear my voice in your sleep.

[IMM]

..a great thinker.

[IMM]

If you have from the outside: plywood, a layer of insulation, then studs, the risk of condensation diminishes greatly and thermal bridging is also diminished greatly.

[Andrew Heggie]

Was this rhetorical and you believe it should be the same?

I have a bit of an interest in dry lining a solid brick wall but my fuel costs are low atm. I also like the idea of a warm loft, so I have considered the celotex between the rafters and 5mm air gap between vented eaves and ridge, but can the ridge end vent into the loft space without large losses to air changes?

AJH

[Peter Taylor]

Hugo Nebula wrote

OK - but you'd get what's coming if you walked up to somebody in Liverpool and called them a Chimpanzee! I have a great sense of humour, but it lives alongside a respect for other people's feelings. I thought about calling you a Baboon, but decided it would be too offensive and immature.

I was trying to get my head round the logic of what I see as an anomaly between the permitted constructions of roofs and timber framed walls in terms of interstitial condensation and ventilation. I am perfectly happy to accept I'm wrong, and if so I was hoping you might be able to explain. We all learn by making mistakes and being corrected (except for IMM that is). But if you're not inclined to understand what I am saying or to give me a logical answer then let's drop it - I will go and ask TRADA. Thanks for trying, anyway.

Peter

[Peter Taylor]

Andrew Heggie wrote

Andrew, Approved Document F, covering control of condensation in roofs, stipulates there is to be at least 50mm air gap above any insulation fitted between the rafters, with ventilation of this gap to outside air at both ridge and eaves equivalent to a continuous gap 5mm wide. The intention is to hasten evaporation of any condensation forming on the roof timbers. This all happens on the cold side of the insulation and does not affect the ventilation of the room on the warm side.

There is a helpful diagram of this on page 15 of Approved Document F, which you can find at

may have to repair this link where it wraps. It's a big 12.8MB download, so if you haven't got Broadband let me know and I'll scan it and send you a pic.

No, the question was not rhetorical. The Building Regulations cover interstitial condensation in roofs but not in walls, which I find illogical. Still, there ya go!

Peter

[IMM]

You have?

I'm right 99.99% of the time.

[IMM]

Couldn't the internal plaster and insulation form a flat section at the top of the loft, leaving a triangular section inside the loft at the apex. Then, if two gable walls, install a vent in each gable at each end of the triangular section. Then air from the eaves rises up the between the rafters, enters the triangular section out via the ends through the gable vents.

[Hugo Nebula]

...And nothing to do with my big red arse.

I apologize if you were offended. It certainly wasn't meant to be offensive, and no-one who I've ever replied to with that introduction has taken it so.

AFAIK a timber framed wall construction and a cold roof above a vaulted ceiling utilising a breather membrane in lieu of ventilation of the void between the rafters are the same. I'm sorry, but I don't understand where you think the difference is, and I don't know how to make my explanation any clearer.

It's not been my intention to have a go at anyone, and I'm sorry if you think I have been. I've only been trying to answer your questions to the best of my knowledge.

[G&M]

In middle of winter the roof may have snow, ice or frost on it for long periods of time leading to the underside of the felt becoming much colder than would the inside of the wall. Thus the condensation problem is much worse.

[Andrew Heggie]

Thanks, Imm made what seemed to be a workable solution rather than relaying the ridge.

Oddly enough I was working at a site today where a renovation was being done. The stable block was being revamped as an accommodation for the owners polo team when they visit. The single story ridges building was being converted with open space to the ridge and all rafters visible. It had been arranged as a warm roof with pasteboards on top of the rafters, then actis trisco super 9 ( a 9 layer loose sheaf of aluminised plastic separated by thin foam polyethylene looking sheets and one which looked like the lining from a cheap fleece jacket) This is

[Peter Taylor]

Andrew Heggie wrote

I was just quoting the AD. If you can provide a permanent vents in gable walls the total effective veentilation area would need to be at least the length of the ridge x 5mm.

Excellent. I designed some alterations and refurb to a 14th Century Manor House in 2001 - did the warm roof thing as you describe but had to use rigid board. Raising up the tiling by 75mm made it very difficult to emulate the original eaves detail, and the lead flashings on the main wall at one end had to be removed and refixed higher up. All this would have been unnecessary using this Actis stuff. Sounds a really good idea.

There's no need for ventilation or air gaps with a warm roof. The insulation is only crushed over the rafters where it doesn't have much function. :)

So are they kicking out the ponies to make space for the riders?

[IMM]

Strange. I though the Actis needed a 25mm unvented gap on the warm side of the roof to reflect heat back in. the 200mm thickness adds some conventional insulation, although not much. Then it would be: rafters, plasterboard on top showing the rafters, 25mm batons, Actis, 25mm batons, roof membrane (Tyvek which breathes), counter batons to hold the tiles, tiles.

[timegoesby]

That to all who replied.

What I wanted was what I have found to be a warm loft rather than warm roof. I don't want to get on the roof and do all from inside. Mr's Heggie and IMM probably got there. Will this vent at the ends of the gables work? It appears a good way to go. I did not expect Mr Taylor to be so offensive. Gentlemen please refrain.

thanks

[Andrew Heggie]

Mmm quite a few airbricks then!

None at all? I did wonder about the batten between the actis ant the tyvek, it seemed bound to be compromised by the tyvek sagging. Imm seems to suggest there should be an airgap between the plasterboard and actis, I cannot see the reasoning even though internal reflection in the actis seems to be a feature (its a bit like a laminate of space blanket and closed cell transparent plastic foam).

OK but how do you calculate the u value for the ares that is crushed?

I find it very hard to believe it is as good as 400mm of glass fiber, unfortunately I did not look at the wrapper properly to see any performance data.

Ponies have moved into the cowshed (former dairy farm), the stable block is very picturesque and the mixture of dressed local stone and flintknap with brick features is the same as the main residence.

AJH

>

[Andrew Heggie]

^^^^20mm

I cannot see the need for 25mm or indeed any gap for radiant effects. I am sure they would not add extra height unnecessarily. I'm a bit confuses now as Peter has said the air gap is unnecessary above the actis, unfortunately I shall not have the chance to look more closely.

AJH

[IMM]

Reflective foils require a 25mm air gap on the house side to "reflect heat back. that is how they work.