Vapour Barrier question for specific insulation spec

Slig · 22-01-2009 06:26PM

topcatcbr wrote: »

Not realy a problem if condensation does occur there is enough air circulation to remove the potential for mold growth.

Moisture is present in air naturally and the higher the temp the more moisture it can hold.
If air travels through these small holes (it has been proven to) then moisture travels with it. This would be fine except as it travels through the insulation the air cools. It then looses the potential to hold the moisture this then condensated within the insulation creating ideal breathing ground for mold.

There are two main ways to prevent this
1) as syd suggests using natural insulation products which can absorb moisture during the winter and release it during the summer through its inherant breathability.

2) Preventing moisture from getting into the insulation in the first place through air and vapor impermiable barriers on the warm side of the insulation (eg 1200 gauge Polythene. ( a better idea than impermiable layer is a smart membrane There are many brands)

Most would but it will also travel through the other gaps and holes.

Think of a barrel full of water with holes in it. The water will not only travel through the largest hole but through all of them at different rates depending on pressure and size.

First a Duvet and now a barrel. Any time I try and think of comparisons all I can come up with is cars. Well explained though

Sparky78 · 22-01-2009 07:15PM

topcatcbr wrote: »

Not realy a problem if condensation does occur there is enough air circulation to remove the potential for mold growth.

Moisture is present in air naturally and the higher the temp the more moisture it can hold.
If air travels through these small holes (it has been proven to) then moisture travels with it. This would be fine except as it travels through the insulation the air cools. It then looses the potential to hold the moisture this then condensated within the insulation creating ideal breathing ground for mold.

There are two main ways to prevent this
1) as syd suggests using natural insulation products which can absorb moisture during the winter and release it during the summer through its inherant breathability.

2) Preventing moisture from getting into the insulation in the first place through air and vapor impermiable barriers on the warm side of the insulation (eg 1200 gauge Polythene. ( a better idea than impermiable layer is a smart membrane There are many brands)

Most would but it will also travel through the other gaps and holes.

Think of a barrel full of water with holes in it. The water will not only travel through the largest hole but through all of them at different rates depending on pressure and size.

Thanks.Great explanation.

But playing devils advocate again.
Would this not mean that most attic conversions in the country that used pu between rafters or over rafters will be suffering from mould growth as most only have foil backed plaster board as the vapour control layer?
Is there evidence of this or is airtightness in a well ventilated room over kill?

topcatcbr · 22-01-2009 08:28PM

Sparky78 wrote: »

Thanks.Great explanation.

But playing devils advocate again.
Would this not mean that most attic conversions in the country that used pu between rafters or over rafters will be suffering from mould growth as most only have foil backed plaster board as the vapour control layer?
Is there evidence of this or is airtightness in a well ventilated room over kill?

There is evedence of this but not alot from ireland as we havnt been insulating to a high standard until very recently.

We have and still use a ventilated the attic space to avoid this. the 50mm gap in attic conversions was the solution to this problem but this created another in that with the ventilation went our heat and we are slowly moving away from this type of construction in attic room design. Also Fibre insulation was the most common material used here and this is naturaly mold resistant. It is only when dust and debris got into the fibres that mold would grow and develop.

Id advise doing one of the Siga or proclima Airtightness courses to find out more.
I know it is brand specific but the concept remains the same

Most evidence is from schemes in mainland europe

Sparky78 · 22-01-2009 09:21PM

I will have 220 rafters @600 centres full filled with metac insulation (thermal conductivity .034) in my dormer roof(counter batten for tiles over breather membrane to provide ventilation).
I will then place the vapour control layer (siga or pro clima) and then a composite board with 30mm pu insulation and plaster board.

Would it be ok to place the services(ie electric cables, have concrete floors so heating pipes will go in screed) between the vc layer and the composite board without a service cavity?

topcatcbr · 22-01-2009 10:50PM

Sparky78 wrote: »

I will have 220 rafters @600 centres full filled with metac insulation (thermal conductivity .034) in my dormer roof(counter batten for tiles over breather membrane to provide ventilation).
I will then place the vapour control layer (siga or pro clima) and then a composite board with 30mm pu insulation and plaster board.

Would it be ok to place the services(ie electric cables, have concrete floors so heating pipes will go in screed) between the vc layer and the composite board without a service cavity?

I think it would but youd better check that cables wont over heat (Fire risk) as the insulation is there to do just that (Insulate). it wont differentiate between wanted and unwanted heat.

I dont know about this but it is something id ask.

eire1977 · 22-01-2009 11:01PM

Thanks for all the replies.
I am not trying to build a fully air tight home. We have air vents in our windows. The first fix electrics is all ready done - so no need for a service cavity.
I really don't get the idea of spending large sums of money on an air tightness barrier, putting a slab against it and then putting a screw through both of them into the timber stud.
I'm leaning towards Syd's first suggestion - using a breathable alternative to the Kingspan but if I stick with my Kingspan plan, will putting a vapour barrier (just good standard Polythene, with taped joints) keep my timber frame/Rockwool from rotting?

Also meant to mention, on all slopped ceilings and in the attic - we are putting 150mm Metac between stud (battened to leaving air cavity between insulation and breathable felt) and again 25mm Kingspan over stud. I presume whatever vapour solution I arrive at, I should continue this onto all sloped ceilings and attic space (attic space will only ever be used for storage due to reduced head height)

Thanks again for all your replies but I not sure if I'm any closer to a solution.

BTW, my engineer was very little help - said I didn't need one - - - is he right??

Slig · 23-01-2009 10:50AM

eire1977 wrote: »

Thanks for all the replies.
I am not trying to build a fully air tight home. We have air vents in our windows. The first fix electrics is all ready done - so no need for a service cavity.
I really don't get the idea of spending large sums of money on an air tightness barrier, putting a slab against it and then putting a screw through both of them into the timber stud.
I'm leaning towards Syd's first suggestion - using a breathable alternative to the Kingspan but if I stick with my Kingspan plan, will putting a vapour barrier (just good standard Polythene, with taped joints) keep my timber frame/Rockwool from rotting?

Also meant to mention, on all slopped ceilings and in the attic - we are putting 150mm Metac between stud (battened to leaving air cavity between insulation and breathable felt) and again 25mm Kingspan over stud. I presume whatever vapour solution I arrive at, I should continue this onto all sloped ceilings and attic space (attic space will only ever be used for storage due to reduced head height)

Thanks again for all your replies but I not sure if I'm any closer to a solution.

BTW, my engineer was very little help - said I didn't need one - - - is he right??

As I said earlier, This phenomenon of super insulating and air proofing a house is fairly new. It takes a lot of time, effort and research to keep up with all the new technologies, building systems and procedures.

There are alot of people (both professional and not) that either dont have the time or inclination to do this and so will continue to work the way they always have.

Alot of these systems are so new here that we dont know what will happen, we are basing our solutions on what has been done elsewhere in the world. Of course Ireland has a different climate than most places (very high humidity, no consistency in the weather or outside temperatures) and because of this I know at several professionals that see this whole energy effecency movement as just a fad, something that wont last.

Vapour Barrier question for specific insulation spec

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