Cold non-vented roofs - a discussion

sydthebeat

It is becoming more and more common to have people ask questions along the lines of "im using a breather felt, so do i need to leave a ventilation gap or can i full fill the rafters with insulation".

In an era of increased energy awareness, increased regulatory u value requirements and diminishing budgetry constraints, this is a very pertinent question.

Building regulations Part F 2009 ventilations states:

"Adequate provision shall be made to prevent excessive condensation in a roof or in a roof void above an insulated ceiling."

it suggests a method of compliance in cold roofs (i assume at this stage everyone knows the difference between a cold roof and a warm roof) in diagram 11 page 28 of maintaining a 50mm gap between the insulation and the roof deck. paragraph 2.1.16 states:
2.1.16 The void should have a free air space of at least 50 mm between the roof deck and the insulation. Where joists run at right angles to the flow of air, a suitable air space may be formed by using counter battens.

It also states, amongst other issues:
"Pitched roof spaces should have ventilation openings at eaves level to promote cross-ventilation."

It must be understood that the Technical Guidance Documents propose a prima fascia compliance with the regulations. If you offer a sloution as a means of compliance with is not prima fascia, youd better be ready to back up your assertions should you be challanged.

the individual requirements change for different roof types and slopes, but the theory remains the same.
1. limit the transfer of water vapour to cold roof voids
2. ensure adequate and crossed ventilation

It is very interesting to note that in paragraph 2.1.4 it clearly states:
"Use of a vapour control layer at ceiling level, on the warm side of the insulation, will assist in limiting vapour transfer, but cannot be relied on
as an alternative to ventilation"
The relevant to this will become clearer later.

Ok, so the regulation is clear. Cold roofs = ventilation required.

So now lets explore roof felts.
Traditionally bituminous felts were very common in ireland, everyone knows the sandpaper-like black tar felt that was extremely common up to 10 years ago. This is a very competent product in that it is long lasting and very water proof. However it has its draw backs in that it isnt breathable, can cause 'sweating' and is a deritive of crude oil. Most non breathable felts are similar to this in that they are tar or bitumin based.

'Breathable' felts have now become very popular in domestic construction due to a two main reasons, their breathability and their strength. These are mainly a polyethelyne or polypropylene products.

What is 'breathability'?
Breathability in construction materials is the ability of a product to allow vapour through it. It is usually measured in two ways:
water vapour permeability (units = grams per meter square per day [gM2day])
vapour resistance (units = Mega-Newton seconds per gram [MNs/g]

The water vapour permeability is a measure of the rate of a material to transfer moisture.
The vapour resistance of a material is a measure of the material’s reluctance to let water vapour pass through.
There are subtle differences between the two, which i wont get into in case this turns into an acedemic debate, but theyre basically the same thing

The higher the gM2day value, the quicker vapour travels through the material
The higher the MNs/g value the slower vapour travels through the material.

Certification for Breather felts
I have looked at BBA certification for two product, Tyvek Supro and Permavent Eco. Both product have BBA certs for use in cold, non vented roofs. Both products have roughly the same recommendations in that
1. a condensation risk analysis should be done on the total build up
2. the products reduce risk of condensation, but does not negate risk
3. all penetrations should be sealed
4. and for additional protection a vapour control layer, or vapour check plasterboard could be considered.

Here you can see the disparity with the technical guidance documents of Irish Building Regulations.

1. full fill insulation will negate cross ventilation
2. vapour control layers are not considered a superior substitute for ventilation.

So in my opinion, according to irish building regulations, a cross ventilation gap is required in cold roof constructions.

However, the choice of insulation materials is key here too. Insulations with high vapour permeability will evenly distribute this vapour over teh face of the membrane. These insulation include qulited types, sheeps wool and cellulouse. Insulations with low vapour permeability can be problematic. These will not allow vapour through so the vapour will be focused onto the easiest pathway, which will invariably be the rafter or joist. As this timber will also be colder than the insulation, the risk of interstitial condenstaion within the timber increases significantly. If the moisture content rises above 20% in the timbers for a sufficiently long enough time then all sorts of problems occur including mould etc and perhaps eventually strucutral failure. Non breathable insulations include the poly types PUR and PIR ie mainly the foam types. Notably polystyrene lies about halfway between the foams and the quilts.

So again in my opinion, full filling with PUR or PIR is a very bad idea, and not one i would stand over. You are surrounding the timbers in a non breathable material and focusing moisture into it.

In conclusion, the acceptance or not of full fill insulation in cold roofs is down to individual certifier. They should base their decision on a risk versus innovation basis and decide whether they are comfortable to stand over the construction.

Any comments most welcome.

heinbloed

Well said!

It is very interesting to note that in paragraph 2.1.4 it clearly states:
"Use of a vapour control layer at ceiling level, on the warm side of the insulation, will assist in limiting vapour transfer, but cannot be relied on
as an alternative to ventilation"
The relevant to this will become clearer later.

but read between the lines:

"..........insulation, will assist in limiting vapour transfer, but cannot be relied on
as an alternative to ventilation ".....

...when used in the traditional build-up including a diffusuion-tight sarking felt.

Mr Gumby

How do you think this will apply in a "conditioned" roof space where a sarking board with high permiability and counter-battening is used( for instance a wood fibre board). Is this technically a cold roof?

sydthebeat

i wouldnt consider that a cold roof mr grumpy.

i suppose a definition of warm or cold need to be clarified, but id suggest a warm roof is a roof where the condensation point is to the external of the structural timbers, whereas a cold roof isnt.

With the wood fibre boards, depending on the thickness, it would probably occur to teh external of teh rafters.

Bitten & Hisses

Syd, I'm not very well-informed on this, so please bear with me. A friend recently moved into his newly-built house, a dormer bungalow. He used a breathable "felt" (something like a TX membrane) and completely filled the space between his rafters with Icynene spray foam insulation. Does this make his method of construction a warm or cold roof? He didn't provide for any form of ventilation, didn't counterbatton and used an ordinary 2 x 2.5" slate lath and natural slate.

Mellor

sydthebeat wrote: »

i wouldnt consider that a cold roof mr grumpy.

i suppose a definition of warm or cold need to be clarified, but id suggest a warm roof is a roof where the condensation point is to the external of the structural timbers, whereas a cold roof isnt.

With the wood fibre boards, depending on the thickness, it would probably occur to teh external of teh rafters.

I've always gone with;
warm: structure inside insulation
cold: structure (partially) outside

But, as you mentioned, as we go higher and higher spec. The line gets blurred.
Take Grumpys example. If it was insulted along the rafters, then I'd consider it warm (for the reason you stated). however, if it was joist insulated with woodfibre board, then its cold. Although, that construction is sort of woodfibre for the sake of woodfibre , although not beyond the realms of possibility.

Bitten & Hisses wrote: »

Syd, I'm not very well-informed on this, so please bear with me. A friend recently moved into his newly-built house, a dormer bungalow. He used a breathable "felt" (something like a TX membrane) and completely filled the space between his rafters with Icynene spray foam insulation. Does this make his method of construction a warm or cold roof? He didn't provide for any form of ventilation, didn't counterbatton and used an ordinary 2 x 2.5" slate lath and natural slate.

This roof should (if it works as expected) be a warm roof. As timbers are fully insulated and vapour will condense outside of the rafters.

Builderfromhell

I read somewhere an American report which showed that breathable felts were not efficient enough to allow vapour to breath through them. My only experience of this has been in new builds where becasue of high humidity (caused by plaster drying out) we had huge amounts of condensation on inside of felt in roof eventhough it was breathable.

I always allow ventilation even if I use a breathable felt. I often put wall vents in gable wall as I worry that eaves vents can get blocked with insulation.

Good thread. With increased insulation, condensation is the enemy.

sydthebeat

i think it must be understood that water droplets ie condensation will NOT be allowed through breather felts. Felts main purpose is as a waterproof layer so it would be a failure of that material if it did allow water through.

Therefore if the dew point does occur to the internal of the felt, in my opinion ventilation is a must!!!

Mr Gumby

Builderfromhell wrote: »

Good thread. With increased insulation, condensation is the enemy.

Condensation and airtightness in my opinion, old houses of the tried and tested type survived to a great extent because of the inadvertent ventilation present. As soon as the draughts and chimneys were sealed the problems started.
I think we have been trying to find a solution for some time.
The ideal situation would appear to be either have the structure inside or out, the problems arise to a great extent where a structual member transits the boundary, for instance a joist that is cold on its top edge as in a cold roof or worse still a joist running into a cold wall.
Thats why the industry has decided on exterior insulation.
However it leaves us with problems like the cold roof, breather membrane and ventilation question.

One possible solution maybe the use of materials which can store moisture for a time before they reach saturation, for example lime mortars. The conditions for condensation rarely last for long and if a buffer is present then liquid water may not result and mould growing conditions improve before growth starts, say the sun comes out and the roof warms and dries?

BL

Very sensible points all round lads,

To take a practical example, I am currently looking at how to insulate an existing flat roofed garage to the side of a 3 bed semi, to be used as a utility/toilet/laundry area. The existing roof construction is exposed 150mm joists with plywood decking above, and some bog standard torch-on felt roofing material above that.

So the simple option is to friction fit 100mm insulation between the joists, stick a vapour barrier underneath and then a plasterboard finish. This leaves a 50mm air gap between the insulation and the plywood deck. However, cross ventilation would be almost impossible to achieve, as the joists span between the gable wall of the house and an identical flat roof garage next door.

The other option is to go with a "warm deck" solution. This would presumably involve removing the existing roofing material, laying new insulation on the plywood and re-roofing above that. Unless there are products that can be laid on top of the roof without re-roofing?

Any pointers would be appreciated, as I'm fairly inexperienced in dealing with the whole insulation retrofit issue. I like the idea of just insulating between the joists, but am concerned at the possibility of trapped condensation and associated damage.

Thanks,

BL

sinnerboy

BL it is possible that the existing felt may be suitable to be left in place and a torch on felt system may be installed over it . The existing felt can act as vapour barrier below an insulation layer which is in turn finished with 2 layers of torch on felt .

foundation10

Excellent post.

Would like to hear some views on what ventilation should be considered at a minimum for following situation.

Natural Slate on
2*1 Battens on
Breather felt on
Sarking Boards on
175mm rafters with rigid rafterloc of 100mm in between rafters
with a wet ridge system.

The roof is an A type roof on a dormer bungalow which is approx.300 sqm

sydthebeat

foundation10 wrote: »

Excellent post.

Would like to hear some views on what ventilation should be considered at a minimum for following situation.

Natural Slate on
2*1 Battens on
Breather felt on
Sarking Boards on
175mm rafters with rigid rafterloc of 100mm in between rafters
with a wet ridge system.

first observation i sthat 100mm rafterloc wouldnt meet regs

second, your leaving a natural 75mm cavity between the insulation and the felt... that would suffice as a vent cavity here.

third, i dont see the requirement of sarking boards here, unless your building right out on the atlantic coastline.

a better build up would be to create a warm roof construction with fibreboard (possibly <50mm) as your sarking layer.. thus negating the need for a vented cavity and allowing for 'full fill' insulation between rafters.

foundation10

sydthebeat wrote: »

first observation is that 100mm rafterloc wouldnt meet regs

second, your leaving a natural 75mm cavity between the insulation and the felt... that would suffice as a vent cavity here.

third, i dont see the requirement of sarking boards here, unless your building right out on the atlantic coastline.

a better build up would be to create a warm roof construction with fibreboard (possibly <50mm) as your sarking layer.. thus negating the need for a vented cavity and allowing for 'full fill' insulation between rafters.

I need to have the sarking board given the exposure as am currently living in a house next door which doesn't have boards and the roof is wet, driving rain is common in winter times here.

I am just worried about the ventilation and what is needed when the ridge will be effectively be sealed, will it be sufficient to leave ventialtion gaps at eaves and the 75mm above the insulation or will there need to be slate vents etc included?

Iwannahurl

I got an attic conversion done recently, and it took a bit of persuasion to get the builder to optimise both insulation and ventilation. The 50mm air gap in Irish guidelines is a bit skimpy. I had QuinnTherm between the trusses and foil-backed insulated board across to get best possible U-value.

While doing a small bit of research prior to the job, I came across mention of the 50% rule in Germany. Here's a reference to it on an Irish website:

Using non breathable rigid sheet insulation in a roof or in a timber frame house is not a good idea because of the 50% rule in Germany which states that structural timbers may not be covered by more than 50% with non breathable materials. In such a structure the only escape route for moisture is through the timbers which quickly become saturated leading to rot.

I worked out that the Irish method was covering 75% of the rafters. Nothing I could do about that in the circumstances, so I had extra soffit vents installed in the hope that this will provide adequate cross-ventilation.

BTW, it was a battle to get the builders to properly seal all gaps that might cause air infiltration beneath the insulation.

beyondpassive

Great topic Syd,

What if your insulation is both outside and inside and there is no dew point?

holdfast

I have 200mm moy between the rafters and 25mm kingspans finest in a room in roof construction. I have a intello between the layer. I felt as i had ventilation from the soffit vents and less than 50% coverage of the members, so i rolled with it. I also put in slate vents, but looking back now I dont think were worth. I still have a niggle though, I have used the TI on both the north side and south of the roof and both are similar which means they are both rotting or both fine. :rolleyes:

Glengormanjay

Hi guys
What do you think about a combination approach of using rafterloc at the cold bridge point where the rafter meets the roofplate and then using cheaper quilt like insulation to pack the rest of the rafet space?

sydthebeat

Glengormanjay wrote: »

Hi guys
What do you think about a combination approach of using rafterloc at the cold bridge point where the rafter meets the roofplate and then using cheaper quilt like insulation to pack the rest of the rafet space?

no its usually the opposite way . the quilted material is used at the awkward spaces at the eaves to cover the wall plate, cavity closer etc
The rafterloc and other rigid insulations then are applied where the plan of the roof is uninterrupted.

YouWantWhat

foundation10 wrote: »

Excellent post.

Would like to hear some views on what ventilation should be considered at a minimum for following situation.

Natural Slate on
2*1 Battens on
Breather felt on
Sarking Boards on
175mm rafters with rigid rafterloc of 100mm in between rafters
with a wet ridge system.

The roof is an A type roof on a dormer bungalow which is approx.300 sqm

The battens are also too small, and you should spec treated battens. If you are going to use sarking boards, then you'll also need counter battens.

JD6910

i have installed a windtight roof on a new built and as it stands the house has no wind blowing in through the joints in the felt or in the facsia and soffit because i have them sealed with a top of the range windtight felt.

the felt has a self adhesive strip at each joint making it windtight. i also completely sealed all wind at the soffit and fascia.

i have HRV and a heatpump

Aside from doing the windows and doors which i plan to seal up with airtight tape, is there a need to install airtight membrane across the ceiling upstairs (i.e) wallplate to wallplate

obviously if money wasnt an option it would be a good job but i am trying to decide if it is overkill with a windtight roof already done.

in reality what air/wind in going to come in through the plasterboard.

sas

You wind tighten on the outside and airtighten on the inside.

You need to keep the warm moist air generated by "living" out of the roof\structure. The airtight layer is positioned as close to the inside as possible so that it's as warm as possible.

If the moist air doesn't touch a surface below the dew point temp, then it won't condense and cause issues such as rotting timber etc.

JD6910

my roof make up is as follows:

slate
battens and counter battens
top of the range breathable felt
150mm deep rafters full filled with metac
possible airtight membrane????
62.5mm insulated slab

if i dont go with the airtight membrane do i need a vapour barrier???
the felt, metac are breathable.

sas

JD6910 wrote: »

my roof make up is as follows:

slate
battens and counter battens
top of the range breathable felt
150mm deep rafters full filled with metac
possible airtight membrane????
62.5mm insulated slab

if i dont go with the airtight membrane do i need a vapour barrier???
the felt, metac are breathable.

Vapour barrier and airtight membrane are pretty much the same thing. The terms get interchanged frequently. Bottom line is that you need to keep moisture laden air out of the roof structure.

I can't comment on the positioning of your membrane because you've some insulation inside it and some outside it. There is a general rule of thumb that you must have 2/3 of your insulation on the cold side of the barrier. However, don't make the mistake of assuming that means 2/3 of the thickness. You are mixing materials and hence they offer different insulating values.

JD6910

sas wrote: »

Vapour barrier and airtight membrane are pretty much the same thing. The terms get interchanged frequently. Bottom line is that you need to keep moisture laden air out of the roof structure.

I can't comment on the positioning of your membrane because you've some insulation inside it and some outside it. There is a general rule of thumb that you must have 2/3 of your insulation on the cold side of the barrier. However, don't make the mistake of assuming that means 2/3 of the thickness. You are mixing materials and hence they offer different insulating values.

so if i dont go with the airtight membrane do i need a vapour barrier (500G plastic or equivalent) or perhaps the foilbacked insulated slabs will keep any vapour from travelling???

i am of the understanding that i can install the insulated slabs directly onto the rafters.

sas

JD6910 wrote: »

so if i dont go with the airtight membrane do i need a vapour barrier (500G plastic or equivalent) or perhaps the foilbacked insulated slabs will keep any vapour from travelling???

i am of the understanding that i can install the insulated slabs directly onto the rafters.

You don't have a choice based on my understanding. You need to keep warm air out of the roof structure.

Airtight membranes and vapour control layers are by and large the exact same products based on my experience.

Plastic sheeting as I understand it is not suitable. It doesn't take stapling well because it will stretch at the staples. Also it will perish over time rendering it useless.

Your vapour layer needs to be continuous. Given that the foil is on the back of the slabs, how would you propose on taping them to each other once they are fixed in position so that they form a continuous layer.

I'm not an expert here, I've offered as much as I can at this point.

Good luck with your project.

sydthebeat

JD6910 wrote: »

my roof make up is as follows:

slate
battens and counter battens
top of the range breathable felt
150mm deep rafters full filled with metac
possible airtight membrane????
62.5mm insulated slab

if i dont go with the airtight membrane do i need a vapour barrier???
the felt, metac are breathable.

That construction is not a warm roof construction therefore you need to maintain a continuous 50mm ventilation gap between the insulation and the felt.

If you insulated OVER the rafters then its a warm roof construction and that gap that i referred to isnt required.

JD6910

sydthebeat wrote: »

That construction is not a warm roof construction therefore you need to maintain a continuous 50mm ventilation gap between the insulation and the felt.

If you insulated OVER the rafters then its a warm roof construction and that gap that i referred to isnt required.

im pretty sure you are incorrect. with breathable felt you can full fill the depth of the rafter

sydthebeat

JD6910 wrote: »

im pretty sure you are incorrect. with breathable felt you can full fill the depth of the rafter

im not sure why you think this.

breathable felt allows vapour through, not moisture. If it allowed moisture it would be a pretty crap felt.

Now, as your felt is cold, and your insulation is warm, the condensation point is going to be within your metac insulation. You have no way of exhausting this condensation.

JD6910

http://www2.dupont.com/Tyvek_Construction/en_IE/assets/downloads/iab_04_0157_tyvek_supro.pdf


see page 3 among others

sydthebeat

JD6910 wrote: »

http://www2.dupont.com/Tyvek_Construction/en_IE/assets/downloads/iab_04_0157_tyvek_supro.pdf


see page 3 among others

the idea of the "sealed roof" construction detail there is that vapour control layer will stop any vapour entering the construction.

if you read what it says about part f ventilation in non ventilated roofs in that certification you will see it refers to BS5250.

Notwithstanding the fact that British standard requires a condensation risk analysis to be carried out, section 8.2 on vapour control layers states:

In practice it is extremely difficult to construct a layer which is totally impermeable to water vapour. The performance of a vapour control layer depends upon the vapour resistance of the material selected, the practicability of the design and the standard of workmanship involved in its installation. Any unsealed holes, fixings, pipes, electrical fittings, etc. which pass through the vapour control layer, will downgrade performance; methods of avoiding such penetrations should be considered in the design stage.

Personally, i wouldnt be taking that risk in my own house.

Interesting that in section 8.4.3.2 of the BS it states that in pitched roofs with a small void or no void

8.4.2.3.2.2 High moisture levels and interstitial condensation can cause problems in this type of roof.
a) Condensate might form on the underside of the underlay overnight at any time of the year; problems arise if conditions allow an accumulation of condensate sufficient to run and drip onto the insulation and the roof timbers.
b) Condensate can accumulate on the underside of the outer covering of the roof in sufficient amount to lead to increased moisture content of the tiling battens.
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.

again, its not something id risk myself.

If i was building a roof, it would either be fully cold, or fully warm. The hybrid you describe is problematic.

JD6910

can i not do the following makeup starting from the inside out:

foil backed slabs
150mm metac breathable insulation in between 150mm rafters
tyvek windtight breathable felt
batten
counterbatten
slate

sydthebeat

JD6910 wrote: »

can i not do the following makeup starting from the inside out:

foil backed slabs
150mm metac breathable insulation in between 150mm rafters
tyvek windtight breathable felt
batten
counterbatten
slate

probably doubthful the 150 metac will meet regs.

why dont you just apply figure 4 from the tyvek cert?
a proper warm roof construction.

JD6910

sydthebeat wrote: »

probably doubthful the 150 metac will meet regs.

why dont you just apply figure 4 from the tyvek cert?
a proper warm roof construction.

because i have built figure 3 from the tyvek cert!!!!!!

sydthebeat

JD6910 wrote: »

because i have built figure 3 from the tyvek cert!!!!!!

plaster slabs on
50 service cavity filled with 50mm rigid rockwool
proclima vapour control layer on
150mm metac breathable insulation in between 150mm rafters
tyvek windtight breathable felt
batten
counterbatten
slate

that spec
1. protects your air tight layer allowing full installation before studs.
2. includes only breathable materials

JD6910

sydthebeat wrote: »

plaster slabs on
50 service cavity filled with 50mm rigid rockwool
proclima vapour control layer on
150mm metac breathable insulation in between 150mm rafters
tyvek windtight breathable felt
batten
counterbatten
slate

that spec
1. protects your air tight layer allowing full installation before studs.
2. includes only breathable materials

thats an option i am considering. the question i am asking is:
if i do not install the proclima layer will foil backed slabs be sufficient?
apparently foil back slabs have a vapour barrier built in??

sydthebeat

JD6910 wrote: »

thats an option i am considering. the question i am asking is:
if i do not install the proclima layer will foil backed slabs be sufficient?
apparently foil back slabs have a vapour barrier built in??

as was said already, the continuity of your vapour control layer is affected if you make your foil back slabs your VCL. I wouldnt advise it. Youd be better off using regular 9mm slabs and install your VCL as a continuous layer.

BryanF

JD6910 wrote: »

thats an option i am considering. the question i am asking is:
if i do not install the proclima layer will foil backed slabs be sufficient?
apparently foil back slabs have a vapour barrier built in??

are the joints in foil backed slabs Vapour impermeable? I've yet to see such a system. as others have said you need a vapour barrier (which doubles as an air-tightness barrier) on the warm side of your insulation.

sydthebeat

Ive merged from post #21 to here from another thread as i think its more suited to being included here.

Id appreciate other AT input on this issue, as to how they view this 'full fill' method which is becoming more and more popular.

MicktheMan

sydthebeat wrote: »

Id appreciate other AT input on this issue, as to how they view this 'full fill' method which is becoming more and more popular.

As said earlier, either go fully cold or fully warm. Risks are just too high otherwise.
Partial pressure differences between the warm and cold side of a vapour barrier will force a lot of vapour laden air through even the smallest of punctures or gaps.
Also, it is one thing to build a vapour impermeable roof structure, it is another thing entirely to keep it vapour impermeable over the lifetime of the build.

JD6910

folks,

in reality how much water vapour is going to penetrate the plasterboard with foil back - especially if the house has a heat recovery system continuously venting and supplying fresh air. the high risk area is the shower area, which will have a moisture slab as well as a PVC "ceiling" over the shower area. also the HRV will be extracting the moisture laden air immediately.

my build up is as follows:

slate on
batten on
counter batten (providing 55mm ventilation outside the felt) on
tyvek supro plus breathable felt (full windtight construction) on
150MM rafter with 150mm full fill breathable metac with
POSSIBLE AIRTIGHT LAYER???? - i am still debating on whether i should put in an airtight layer
62.5mm foil backed insulated slab

if i dont put in the airtight layer, do i need a vapour barrier or will the foil backed insulated slabs be sufficient IN REALITY along with my HRV system ventilating the house 24/7 and removing stale (moisture laden) air and replacing it with fresh filtered air.

sydthebeat

Its all about the risk and how you can reduce the risk...

remember that an adult human can loose 2-3 litres of water a day alone from their bodies.
Add on top of that cooking, drying, etc

Yes, a HRV system will definitely reduce the amount of vapour in the air, but most certainly wont extract all of it. People need a certain level of relative humidity to live comfortably and you should have RH meters installed.

As to your question, personally i dont think foil back slabs are worth a curse as a vapour barrier.
see figure 2 here
any gap in your vapour layer actually focuses moisture into one area... which is exactly what you dont want to happen.

Also, if you go without an air tight membrane, it will be much harder to achieve, and maintain, a good air tightness level which you require for efficient running of a HRV system.
Its also worth noting that insulated foil back slabs arent at all "breathable" so again all you are doing is focusing moisture into vulnerable areas.

Risk versus reward.

JD6910

am i correct in saying that, if i install the airtight membrane then i dont need foil backed slabs and that "normal" 12mm slabs will be sufficient????

sydthebeat

JD6910 wrote: »

am i correct in saying that, if i install the airtight membrane then i dont need foil backed slabs and that "normal" 12mm slabs will be sufficient????

you would be correct. That would be a much better solution.

tobbslerone

sydthebeat wrote: »

the idea of the "sealed roof" construction detail there is that vapour control layer will stop any vapour entering the construction.

if you read what it says about part f ventilation in non ventilated roofs in that certification you will see it refers to BS5250.

Notwithstanding the fact that British standard requires a condensation risk analysis to be carried out, section 8.2 on vapour control layers states:




Personally, i wouldnt be taking that risk in my own house.

Interesting that in section 8.4.3.2 of the BS it states that in pitched roofs with a small void or no void


again, its not something id risk myself.

If i was building a roof, it would either be fully cold, or fully warm. The hybrid you describe is problematic.

I understand what you are saying.I contacted Isover technical team and they said that there would be no problem as long as the membrane was installed correctly like you said.The guy said if there was only slight leakage that it wouldnt cause a problem though.He also said that if a foil backed slab was put up against an airtight membrane then there can be a clash and condensation can form between the two so he would recommend only using one or the other.Do anybody know of an issue with this?

sydthebeat

tobbslerone wrote: »

...Do anybody know of an issue with this?

have a read of the previous 6 posts.

are you installing a HRV system?

tobbslerone

sydthebeat wrote: »

have a read of the previous 6 posts.

are you installing a HRV system?

I am installing a HRV system yes.I was talking to a guy in Ecological Building Systems.He said that the gap would have to be maintained if the roof was not counter battened and the best thing for me to do was increase the size of the rafters from 150mm to 200mm,put in 150mm of Metac and then a lighter insulated slab.He said that no more than 1/3 of the insulation should be on the room side of the airtight membrane.

sydthebeat

tobbslerone wrote: »

I am installing a HRV system yes.I was talking to a guy in Ecological Building Systems.He said that the gap would have to be maintained if the roof was not counter battened and the best thing for me to do was increase the size of the rafters from 150mm to 200mm,put in 150mm of Metac and then a lighter insulated slab.He said that no more than 1/3 of the insulation should be on the room side of the airtight membrane.

sounds like a much better solution in my opinion.

as said previous, either build completely warm roof where you have insulation over the structural rafters... or a completely cold roof, where you maintain the 50mm gap continuously.

Im just wondering if you actually have any professional input in this build tobbslerone? who carried out your provisional DEAP assessment?

tobbslerone

sydthebeat wrote: »

sounds like a much better solution in my opinion.

as said previous, either build completely warm roof where you have insulation over the structural rafters... or a completely cold roof, where you maintain the 50mm gap continuously.

Im just wondering if you actually have any professional input in this build tobbslerone? who carried out your provisional DEAP assessment?

It is a relations house.Didnt get an assessment carried out yet.The architect hasnt specified what insulation to use.You are correct though in everything you say as regards ventilation so thanks for your help.
How would you recommend getting 0.16 in the sloped roof with 150mm rafters?

sydthebeat

tobbslerone wrote: »

It is a relations house.Didnt get an assessment carried out yet.The architect hasnt specified what insulation to use.You are correct though in everything you say as regards ventilation so thanks for your help.
How would you recommend getting 0.16 in the sloped roof with 150mm rafters?

id stop right now and get an assessment carried out to ensure that what is being done complies with regulations. there is NO WAY of knowing what is bing build complys without an assessment of teh plans and specifications.
The whole idea is NOT to be making it up as you go along and end up with a mish mash of different solutions to problems which shouldnt have occured in teh beginning.... for example your roof specification build up.

Your architect, assuming s/he is certifying the build, is really playing a dangerous game.

tobbslerone

sydthebeat wrote: »

id stop right now and get an assessment carried out to ensure that what is being done complies with regulations. there is NO WAY of knowing what is bing build complys without an assessment of teh plans and specifications.
The whole idea is NOT to be making it up as you go along and end up with a mish mash of different solutions to problems which shouldnt have occured in teh beginning.... for example your roof specification build up.

Your architect, assuming s/he is certifying the build, is really playing a dangerous game.

I agree with you completely. There is a huge problem in this country with the way buildings are certified.The architect done the drawings but is narky if you ask for anymore detail.