Riverside cottage 10 (ping Tim W)

[Tim Lamb]

I am trying to plan the dormer and loft voids insulation. Is there a practical thickness of PIR foam beyond which savings are insignificant?

Sadly the carpenters have fitted some of the side cheek studs flat way in (I was in Madeira at the time:-( making 100mm rafter squeeze hard to fit. Presumably 2 layers of 45mm will do. The architect advises insulation backed plasterboard to reduce cold bridging.

1/3rd of the roof has not been touched so I have the usual problem of 100mm glass fibre insulation lying on the upstairs plasterboard between 100mm ceiling joists and again between the 225mm rafters under the roof felt.

I understand the need to maintain a ventilation path next to the felt but has anyone successfully inserted extra insulation there? Ah! Scrub that. The rafters over the old dormer are only 100mm.

How critical is taping the joints for low humidity areas other than bathrooms?

[Tim Watts]

Ping Me? Or the other Tim W?

But i can certa> I am trying to plan the dormer and loft voids insulation. Is there a

You need to look at the U values of PIR foam, the areas and calculate heat losses (in kW) for some useful temperature difference - eg 21C inside and 0C outside.

As part of the calculation, include, but as a separate figure, any other significant losses, which will probably be your windows.

if you had an airless windowless box, you could usefully keep adding PIR foam to some moderate thickness, perhaps as much as 6-8"

However, there comes a point where the losses through the walls/roof will be dwarfed by the losses through the windows.

I came to the conclusion that going from 4" to 5" made bugger all difference for my setup. Yours may vary depending in areas and windows.

That was comparing against double glazed coated windows to current spec.

The biggest loss apart from that would be having major air leaks where the foam is fitted - so lots of gun foam[1] shoudl be used where taping is not practical.

[1] Recommend fire retardant foam.

Or just 25mm over then plasterboard.

Have you got the headroom to go under? I went 50mm between leaving a

50mm ventilated gap over the PIR and another 50mm under.

In fact I also have a 50mm air void under that, due to mounting the new ceiling on 2x2" battens that are hung off the old rafters with buggeringly long screws and packers. That was because my roof is like a skateboarding park so I came up with that idea to create an adjustable flat level finish.

I *could* have added more PIR therem but I decided a) it would make bugger all difference; b) it was a dead handy cable void for lighting; c) it might offer some acoustic damping.

50/50 I'd say. Do it where you can - but don't panic about odd bits where you can't get at it.

[The Natural Philosopher]

never mind humidity. One 1mm gap can destroy 90% of your insulation on a cold windy winters day.

Aim for airtight.

[Tim Lamb]

OK. You realise this means crawling through spaces smaller than I am!

>

[Tim Lamb]

In message , Tim Watts writes

Yes. You:-)

Digging through Google I have found several district council *guidance for designers* targeted at meeting the revised Part L

We are renewing throughout so will meet current spec.

OK.

Not really. existing is 2.10m but I will lose 25mm by cross battening the floor for heating. This is a chalet bungalow so I can load plenty of Rockwool in the ridge void. I am trying to avoid re-doing the existing plasterboard.

You could have used adjustable wood screws.

OK PIR foam has got to be better than glass wool.

[Tim Watts]

Jolly good :)

Reason being: I fire tested some broken off lumps of old foam I had used. One was OK ish as was the PIR foam - apply flame, black smoke, remove flame, foam goes out.

But one particular foam went "woosh" when lit by a match. I was somewhat unimpressed. OK - it may not have done that when jammed in a joint being cooled by the wood next to it and restricted in the oxygen it could consume. I am not planning to test that either! I wonder if that foam used butane as the propellant (hence the woosh).

But pink fire foam is only about twice the price, which is still not much on a dozen or two cans, and you know (and I proved) it also does not support combustion unaided (ie it goes out).

Still produces shit loads of nasty smoke though.

Handles as well as other foams so no real reason not to use.

Mine is about 2m-2.1m after all the fiddling and I do not find it too low. As long as you use low profile ceiling lights :) Would not like to go much lower though!

Here's mine in various stages of assembly:

By about a factor of 2 on the U value - plus it is a vapour barrier in its own right (the foil layer sees to that)

Good luck :)

[Tim Lamb]

In message , Tim Watts writes

I saw those and others first time round. Excellent, hence the *ping*:-)

So how do you feel about driving up to Hertfordshire?:-)

[Tim Watts]

Well that's very nice of you. But I had help - my excellent mate who knows about stuff helped a lot and we figured it out as we went :)

Can't wait to get a skim coat on it!

Got any photos of you gaff - it might help?

[TimW]

Definitely not for me. Tim W

[John Rumm]

Examples of how to do the calcs here:

When I did my loft I found that even going to 85mm of PIR made a massive difference on the previous arrangement.

did 50 then 30 in my case. (building regs for a loft conversion at the time required 70mm total IIRC)

Its important any time you have enclosed spaces on the outside of the insulation - i.e. where warm wet air could meet cold and condense.

[Tim Lamb]

OK. As has been said, going over 5" may not pay back.

Huh! A chalet bungalow is full of cold voids. I suppose polythene could be tacked up before the inner skin is fitted. The original cold loft had a layer of polythene above the ground floor ceiling. Not mentioned in the building regs. drawings.

I'll put up a few photos for further comment.

[Tim Lamb]

In message , Tim Lamb writes