Rafter Insulation

tobbslerone

Hi all
I am aiming to achieve a U-value of 0.16 at roof level.The rafters are 150mm deep.I was looking for insulation with a low thermal conductivity to fit between the rafters and a material that is not too thick to go across the rafters. The best product i seen that could go between was Frametherm 32 with thermal conductivity of 0.032,but that would mean using an 82.5mm insulated ploystyrene board to get down to 0.16.I would prefer not to use rigid polystyrene like Kingspan between rafters as it is hard to get right.Anyone got any suggestions as to what to use that is easy fit between and has low thermal conductivity or know of a good product for outside the rafters?

JD6910

im using 150MM metac (0.034) and a 62.5MM insulated slab. im being told that will get me a u-value of 0.15.

tobbslerone

Ya that will give 0.15.What depth of rafters have you? I would do the same but i cant as the rafters are only 150mm in total and i need to leave 50mm for ventilation so can only use 100mm of Metac.

Phirstclass

Tobbslerone, if it is a case that you're not restricted with head height you could consider increasing the depth of your rafter by fixing 50mm battens along your rafters, thus leaving the ventilation gap as required. You could then finish this with a vapour barrier and the 62.5mm insulated plasterboard.
You should however realise that in a cold roof construction like this your U-value will not be as good as calculated as the air movement will reduce the insulation's effectiveness.

tobbslerone

Ya i was thinking of either increasing the depth of the rafters and using 150mm of Metac or else fixing an 82.5mm board instead of a 62.5mm board. Which would you recommend?

Atlantic1

tobbslerone wrote: »

Ya that will give 0.15.What depth of rafters have you? I would do the same but i cant as the rafters are only 150mm in total and i need to leave 50mm for ventilation so can only use 100mm of Metac.

You won't have to leave any air gap for the Metac if you have a breathable membrane.

tobbslerone

I think the felt is breathable. In what instances would you need to leave a ventilation space? Does it depend on the insulation also.Would it not be better to leave a space? Thanks

sydthebeat

Atlantic1 wrote: »

You won't have to leave any air gap for the Metac if you have a breathable membrane.

not correct

tobbslerone

sydthebeat wrote: »

not correct

Why do you say that is not correct?

sydthebeat

tobbslerone wrote: »

Why do you say that is not correct?

http://www.boards.ie/vbulletin/showthread.php?p=68250588

its discussed here.

but as a synopsis, if you want to remove the ventilation gap in a cold roof construction, you need to install a vapour barrier perfectly on the warm side of the insulation with no gaps or chance of loss of performance over teh life of the build. The actual standard that allows this states that doing so is practically impossible anyway.

saying that you can push your metac right up against a breathable felt is simply not correct.

tobbslerone

sydthebeat wrote: »

http://www.boards.ie/vbulletin/showthread.php?p=68250588

its discussed here.

but as a synopsis, if you want to remove the ventilation gap in a cold roof construction, you need to install a vapour barrier perfectly on the warm side of the insulation with no gaps or chance of loss of performance over teh life of the build. The actual standard that allows this states that doing so is practically impossible anyway.

saying that you can push your metac right up against a breathable felt is simply not correct.

Thanks sydthebeat. I was talking to Isover and people in hardware stores and they all said you can put Metac up against felt as long as it is breathable so i am a bit confused on the matter. To achieve 0.16 with a ventilation gap i would need 100mm Metac between rafters,then airtight membrane and an 82.5mm insulated board.Would you have any other suggestion as this is a very thick board and limits living area?

Atlantic1

sydthebeat wrote: »

not correct

Syd, I wouldn't dream of disagreeing with you having got very good advice from you in the past but the Isover Metac site says you don't need to leave any space.

http://www.isover.ie/products-systems/metac-roll/93

http://www.keatinginsulation.com/pdf/metac.pdf

BryanF

Atlantic1 wrote: »

Syd, I wouldn't dream of disagreeing with you having got very good advice from you in the past but the Isover Metac site says you don't need to leave any space.

http://www.isover.ie/products-systems/metac-roll/93

http://www.keatinginsulation.com/pdf/metac.pdf

Can we see an Iab/bba cert showing this

sydthebeat

tobbslerone wrote: »

Thanks sydthebeat. I was talking to Isover and people in hardware stores and they all said you can put Metac up against felt as long as it is breathable so i am a bit confused on the matter. To achieve 0.16 with a ventilation gap i would need 100mm Metac between rafters,then airtight membrane and an 82.5mm insulated board.Would you have any other suggestion as this is a very thick board and limits living area?

Atlantic1 wrote: »

Syd, I wouldn't dream of disagreeing with you having got very good advice from you in the past but the Isover Metac site says you don't need to leave any space.

http://www.isover.ie/products-systems/metac-roll/93

http://www.keatinginsulation.com/pdf/metac.pdf

This is a case of half truths.
a few points

ln the documentation shown above, all the details have a Vario membrane on the room side of the insulation. That is the vapour barrier i refer to in my previous post.
Thats the same vapour barrier that the expert testers say

In practice it is extremely difficult to construct a layer which is totally impermeable to water vapour.

So, at an absolute minimum, anyone thinking of doing this has to install a perfectly fitted vapour barrier.
If you try to do this on sloped roof where you already have studs, block walls, chimneys, protrusions etc i can say with a good degree of accuracy that you will not fit a vapour barrier fully sealed.

Secondly, this is my main issue with this detail
The guys in the hardware shops arent going to quote chapter and verse from BS 5250 and describe the risks involved in trying to create a completely vapour proof construction. Saying its ok to go ahead is simply incorrect.

Thirdly, i cannot find any IAB certificate for moy isover. Mainly because this kind of product has been widely used prior to the requirement of testing and has been accepted to work de facto. This is dangerous because new regulations, new product and new systems are being compiled all the time using existing products.

As a professional, i certainly wouldnt be suggesting this construction. You are basically trying to create an umberella under your roof to stop vapour entering the construction.
If you fail to create a fully sealed vapour barrier then

c) Persistently high levels of relative humidity will cause hygroscopic materials, including the rafters and battens, to absorb sufficient water to promote rot in the timbers and the growth of moulds.

JD6910

i understand that metac is ok to fit against the breathable membrane IF you have batten and counter batten on the outside creating the ventilation gap.


what i have done is as follows:

slate
batten and counter batten
top of the range breathable felt (full windtight construction)
150mm rafter with 150MM metac full fill
airtight membrane
62.5MM insulated slab

tobbslerone

JD6910 wrote: »

i understand that metac is ok to fit against the breathable membrane IF you have batten and counter batten on the outside creating the ventilation gap.


what i have done is as follows:

slate
batten and counter batten
top of the range breathable felt (full windtight construction)
150mm rafter with 150MM metac full fill
airtight membrane
62.5MM insulated slab

What you done is very good but you are supposed to keep only 1/3 of insulation to the warm side of the airtight membrane. At that 1/2 is on the warm side. I am not certain on that issue but that is what the manufacturers of the membrane say.

JD6910

tobbslerone wrote: »

What you done is very good but you are supposed to keep only 1/3 of insulation to the warm side of the airtight membrane. At that 1/2 is on the warm side. I am not certain on that issue but that is what the manufacturers of the membrane say.

i would like to think that my HRV system would eliminate this minor risk....

tobbslerone

JD6910 wrote: »

i would like to think that my HRV system would eliminate this minor risk....

Yes i would think so too,I was told that fitting a foil back insulated slab against an airtight membrane can cause condensation too.Isover that make the membrane say that it is a condensation risk but Kingspan say it is of no risk. Who do you believe? Have you heard of this?

JD6910

tobbslerone wrote: »

Yes i would think so too,I was told that fitting a foil back insulated slab against an airtight membrane can cause condensation too.Isover that make the membrane say that it is a condensation risk but Kingspan say it is of no risk. Who do you believe? Have you heard of this?

if you are installing the airtight membrane just put in normal slabs. they are a lot cheaper than the foil back.

i took the difference in cost into account when deciding on airtight membrane or not and it certainly help a little bit in reducing the initial cost.

i hope my HRV and airtight pays dividends!!!!:o

tobbslerone

JD6910 wrote: »

if you are installing the airtight membrane just put in normal slabs. they are a lot cheaper than the foil back.

i took the difference in cost into account when deciding on airtight membrane or not and it certainly help a little bit in reducing the initial cost.

i hope my HRV and airtight pays dividends!!!!:o

Yes you are right,i will use the TW52 slabs for the roof that have no foil back. My make up is a 50mm air space,150mm of Metac and a 50mm insulated slab giving a U-value of 0.154 so i am happy with that.I will do the same on the flat ceiling as i want to floor the attic.
What are you using on the ceiling and the walls?

JD6910

tobbslerone wrote: »

Yes you are right,i will use the TW52 slabs for the roof that have no foil back. My make up is a 50mm air space,150mm of Metac and a 50mm insulated slab giving a U-value of 0.154 so i am happy with that.I will do the same on the flat ceiling as i want to floor the attic.
What are you using on the ceiling and the walls?

i put in circa 500MM earthwool on the flat ceiling with a normal 12MM slab.

i have a 200MM fully pumped cavity and triple glazed windows.

i'm hoping i never need to turn on the heat pump!!!!!!

sydthebeat

JD6910 wrote: »

i put in circa 500MM earthwool on the flat ceiling with a normal 12MM slab.

i have a 200MM fully pumped cavity and triple glazed windows.

i'm hoping i never need to turn on the heat pump!!!!!!

can i ask a slightly off topic question?

if you hope to have your energy demand reduced so much, what attracted you to installing a heat pump as your energy source, seeing as its probably the most expensive 'up front' system going?
did you explore other options? and if so, why did you rule them out?

tobbslerone

JD6910 wrote: »

i put in circa 500MM earthwool on the flat ceiling with a normal 12MM slab.

i have a 200MM fully pumped cavity and triple glazed windows.

i'm hoping i never need to turn on the heat pump!!!!!!

What is the U-value of your walls. You have the house very well insulated fair play.Need no heat at all! Have you the heat pump installed.Im thinkin about different heat systems at the moment.What would you recommend.Do you mind me askin what would the installation cost be roughly?
Yea as Syd the beat was saying did you need a heat pump seeing as your energy demand will be very low. Might be an expensive machine that wont be needed that much.

Atlantic1

tobbslerone wrote: »

Yes you are right,i will use the TW52 slabs for the roof that have no foil back. My make up is a 50mm air space,150mm of Metac and a 50mm insulated slab giving a U-value of 0.154 so i am happy with that.I will do the same on the flat ceiling as i want to floor the attic.
What are you using on the ceiling and the walls?

As far as I know, Kingspan recently started putting foil on TW52 boards. The side that faces away from the plasterboard is still Kraft paper suitable for dod and dab but the reverse side of that paper has foil to act as a vapour barrier.

You can check by contacting Kingspan.

JD6910

sydthebeat wrote: »

can i ask a slightly off topic question?

if you hope to have your energy demand reduced so much, what attracted you to installing a heat pump as your energy source, seeing as its probably the most expensive 'up front' system going?
did you explore other options? and if so, why did you rule them out?

as i see it, the options are:

heat pump
oil or gas boiler and solar

to comply with part L i need a huge amount of solar (6 sq M approx). the cost of that solar with the oil or gas boiler is pretty much the same as the heat pump.

by the way the HP isnt installed or purchased yet so there is still time to change!!! any other options???

also, yes the demand is low but yet there will be some demand and a heat pump is the most efficient way of achieving that demand however low it may be.

furthermore i reckon that for a similar capital outlay you are guaranteed your heat and hot water with the heat pump. with the solar - you are "hoping" to get the weather and ultimately having to turn on the oil to boost the water. oil is only going to go up and up!!!

sydthebeat

JD6910 wrote: »

as i see it, the options are:

heat pump
oil or gas boiler and solar

to comply with part L i need a huge amount of solar (6 sq M approx). the cost of that solar with the oil or gas boiler is pretty much the same as the heat pump.

by the way the HP isnt installed or purchased yet so there is still time to change!!! any other options???

also, yes the demand is low but yet there will be some demand and a heat pump is the most efficient way of achieving that demand however low it may be.

furthermore i reckon that for a similar capital outlay you are guaranteed your heat and hot water with the heat pump. with the solar - you are "hoping" to get the weather and ultimately having to turn on the oil to boost the water. oil is only going to go up and up!!!

ill then assume its an air to water heat pump, because a ground source heat pump system will be much more expensive than a solar / boiler / stove combination.

Your "guaranteed heat" wont be coming at a "guaranteed" cost. The colder winter days when you want most heat will result in higher per unit cost because your pump will be running at lower efficiencies, and requiring electrical boosting.

Im not critizing your decision, im just wondering why you are going down that route.

whats your heat pumps efficiency when it comes to domestic hot water production in DEAP?

tobbslerone

Atlantic1 wrote: »

As far as I know, Kingspan recently started putting foil on TW52 boards. The side that faces away from the plasterboard is still Kraft paper suitable for dod and dab but the reverse side of that paper has foil to act as a vapour barrier.

You can check by contacting Kingspan.

Yes you are correct,I contacted Kingspan and they said they have just started making them with the foil.They advised me to get the ones from a supplier that had the old slabs in stock without the foil.
Makes you wonder why are they making them with foils now so!

kieranhr

JD6910 wrote: »

as i see it, the options are:

heat pump
oil or gas boiler and solar

There's a third option, which would be my first preference, which is timber pellet. Knocks the renewables contribution out of the park, and gives you Syd's "guaranteed heat at a guaranteed cost".

JD6910

kieranhr wrote: »

There's a third option, which would be my first preference, which is timber pellet. Knocks the renewables contribution out of the park, and gives you Syd's "guaranteed heat at a guaranteed cost".

timber pellet has its draw backs:

• you are the mercy of a third party picking the price
• storage and trying to keep moisture content correct etc...

i acknowledge that electricity prices can vary but the price of electricity effects everyone everyday in everything we do therefore i reckon the price of electricity can only go so high as everyone is affected by it - even us using this laptop to type this!!!

also we can do something about electricity - install night rate and or PV when the capital price gets a bit more reasonable which im sure it will in years to come.

kieranhr

89% of our electricity is generated by non-renewable sources, so if the cost of the underlying fossil fuel rises (which it is doing, at an alarming rate), then the cost of electricity also rises. Timber pellet on the other hand grows on trees!

muffler

Lads, we're way over the ditch into the next field at this stage. Can we get back to the topic please? Thanks

kieranhr

To be fair, it was your fellow moderator Syd that opened the gate to the next field. We just happened to wander out there after him.

antiskeptic

sydthebeat wrote: »

but as a synopsis, if you want to remove the ventilation gap in a cold roof construction, you need to install a vapour barrier perfectly on the warm side of the insulation with no gaps or chance of loss of performance over teh life of the build. The actual standard that allows this states that doing so is practically impossible anyway.

saying that you can push your metac right up against a breathable felt is simply not correct.

Edit

Forget I asked - I clicked on the link Syd put up. Sorry

sydthebeat

antiskeptic wrote: »

I'm not doubting what Syd says (especially not the impossibility of constructing a perfect vapour barrier). But could someone explain this to me?

If there's a breathable felt under the slate and on top of the Metac, what's the use of the air gap. Does not the vapour go through the Metac and through the breathable membrane?.

No, because at some point close to the top of the metac the "vapour" turns into "droplets" of condensation.... this point is called the dew point.

breathable felt allows vapour through it.. but it certainly doesnt let water droplets through it.

the air continuous gaps "dries out" the formed water on teh metac / roof timbers... like a washing line allows clothes to dry out.

antiskeptic

sydthebeat wrote: »

No, because at some point close to the top of the metac the "vapour" turns into "droplets" of condensation.... this point is called the dew point.

breathable felt allows vapour through it.. but it certainly doesnt let water droplets through it.

the air continuous gaps "dries out" the formed water on teh metac / roof timbers... like a washing line allows clothes to dry out.

Gotcha!

BoardsMember

Hope it's ok to jump on this thread, if not tell me and I'll set up another one.

I saw a renovated house recently use spray foam insulation. I'd be keen to look at something similar ourselves. My concern is about breathability and avoiding doing the wrong thing. Our roof is old, but secure. There is no membrane under the tiles, there was a layer of lime of something plastered on donkeys years ago, this is disintegrating and mostly gone. A couple of yeas back I put in floor insulation in the attic and it has make some improvement, I used rockwool, two layers perpendicular to each other.

The wind whistles through it so I thought with foam insulation I could get the whole attic area fully sealed. The wind gets in under the tiles but also between the top of the wall and the roof. What I dont know is can one put in this foam type stuff and seal off the attic completely. I have spoken to a few suppliers and some say they staple a breathable membrane onto the rafters near/against the tile, and spray onto that, and also seal at the top of the walls. I'm just wondering if I am letting myself in for long term risks of rotting etc if I do this?