Vapour Barrier question for specific insulation spec

eire1977

Hi all,
I've had a look around various websites looking for the answer to my question and no luck. Hopefully someone here will be able to help.

I just about to start phase two of insulation our timber frame house.
Over the Christmas was put in 150mm of Rockwool in the external wall cavity. I now intend to put 25mm of Kingspan across the stud and then slab. I've decided not to use insulated slabs as I think I'll be able to achieve a better insulation finish with the separate Kingspan - i.e. no (or little) gaps between the Kingspan compared to the insulated slab boards.

My question: Do I use a vapour barrier and if so where in the insulation/slab sandwich does it go?
Should it go over my Rockwool but under my Kingspan?
Should it go over my Kingspan but under my slab?
Or do I need it at all??
should I just tape my Kingspan joint?

All / any help would be great
Thanks in advance.

sydthebeat

eire1977 wrote: »

Hi all,
I've had a look around various websites looking for the answer to my question and no luck. Hopefully someone here will be able to help.

I just about to start phase two of insulation our timber frame house.
Over the Christmas was put in 150mm of Rockwool in the external wall cavity. I now intend to put 25mm of Kingspan across the stud and then slab. I've decided not to use insulated slabs as I think I'll be able to achieve a better insulation finish with the separate Kingspan - i.e. no (or little) gaps between the Kingspan compared to the insulated slab boards.

My question: Do I use a vapour barrier and if so where in the insulation/slab sandwich does it go?
Should it go over my Rockwool but under my Kingspan?
Should it go over my Kingspan but under my slab?
Or do I need it at all??
should I just tape my Kingspan joint?

All / any help would be great
Thanks in advance.

you have a perfectly good breathable timber frame...
but now you want to cover it with a non-breathable insulation... not a good idea in my opinion.

If i was doing this i would use rigid batts of rockwool instead of the kingspan...

yes there is a loss of performance (on paper!!) but the gain is so much more.... a fully breathable construction.... and a construction that has a higher thermal mass to aid in both thermal storage and to aid in prevention of summer overheating...... also the associate health benefits of a breathable construction.

RKQ

eire1977 wrote: »

My question: Do I use a vapour barrier and if so where in the insulation/slab sandwich does it go?
Should it go over my Rockwool but under my Kingspan?
Should it go over my Kingspan but under my slab?
Or do I need it at all??
should I just tape my Kingspan joint?

To answer your questions - if you want to have an airtight structure you must install a vapour barrier. (look at Siga, ProClima or Moy)

Ideally install vp on the warm side of insulation, over the kingspan. However, in reality 25mm kingspan isn't going to be a problem, so from a practical point of view, you could put vp over studs behind kingspan. Tape and seal all overlapped joint in vp.

Sparky78

would taping the joints on the foil facing of the insulation not create a suitable vapour barrier?

RKQ

Sparky78 wrote: »

would taping the joints on the foil facing of the insulation not create a suitable vapour barrier?

No! In reality these boards are hard to cut perfectly straight ( even with a circular saw), the foil can tear and "pressing" on the boards can dent them. So, in practice its difficult to tape the joints.

Airtightness is good because it slows the movement of air thru your wall. This keeps the warm air in your building for longer, which means the same room temp for less heat proiduction or for a londger period of time. (Airtightness will not turn your house in to a submarine!)

Each gap or draught will "suck" that warm air from your house like a leak in a tyre. So its important to have a good seal. Taping between boards will help but won't be as good as a proper membrane.

Invest in 500 guage plastic vp or better still use Siga, Moy or Proclima. Check out their site for full info.

topcatcbr

eire1977 wrote: »

Hi all,
I've had a look around various websites looking for the answer to my question and no luck. Hopefully someone here will be able to help.

I just about to start phase two of insulation our timber frame house.
Over the Christmas was put in 150mm of Rockwool in the external wall cavity. I now intend to put 25mm of Kingspan across the stud and then slab. I've decided not to use insulated slabs as I think I'll be able to achieve a better insulation finish with the separate Kingspan - i.e. no (or little) gaps between the Kingspan compared to the insulated slab boards.

My question: Do I use a vapour barrier and if so where in the insulation/slab sandwich does it go?
Should it go over my Rockwool but under my Kingspan?
Should it go over my Kingspan but under my slab?
Or do I need it at all??
should I just tape my Kingspan joint?

All / any help would be great
Thanks in advance.

The vapor barrier goes on the warm side of the insulation as already advised.

It is good practice to then provide a service gap (batten out) so that mech and elec dont have to cut the vapor barrier to install wireing or piping. these holes can be difficult to seal up after.

Sparky78

RKQ wrote: »

No! In reality these boards are hard to cut perfectly straight ( even with a circular saw), the foil can tear and "pressing" on the boards can dent them. So, in practice its difficult to tape the joints.

Airtightness is good because it slows the movement of air thru your wall. This keeps the warm air in your building for longer, which means the same room temp for less heat proiduction or for a londger period of time. (Airtightness will not turn your house in to a submarine!)

Each gap or draught will "suck" that warm air from your house like a leak in a tyre. So its important to have a good seal. Taping between boards will help but won't be as good as a proper membrane.

Invest in 500 guage plastic vp or better still use Siga, Moy or Proclima. Check out their site for full info.

But the boards would only need to be cut where they meet the ceiling as they are going over the studs of a timber frame not between from what I gathered from the original post.
Would taping not be sufficient in this circumstance ?

RKQ

No! Sparky
Like I said above..... airtightness is too important. You only get one chance!

Are you suggesting a 50mm wide tape is sufficient to tape the board to the ceiling or tape the board to the floor?

Sparky78

are 50mm tapes not used in the systems you are recommending?

RKQ

Sparky78 wrote: »

are 50mm tapes not used in the systems you are recommending?

Yes, 50mm (extremely sticky) tape is used to seal the overlapped vapour barrier. The overlap is at least 150mm. The foil insulating boards do not overlap!

As said above, look at the manufacturers web sites for details. Taping the boards will help but it is no substitute for an sealed and overlapped vapour barrier.

Sparky78

I believe that as the original poster is not building a passive house and has not mentioned the use of a HRV and will be putting holes in the walls for ventilation that it would not be cost effective to spend the 1000's on 1 of these systems.
I think that if he/she tapes the joints on the foil between the boards and uses a sealant and tape at the ceiling and floor, and then plasters, the vapour control layer will be suffient for his purposes.

If I was putting in a HRV I would consider it then.

Poor Uncle Tom

Why prevent someone providing a HRV system in the future, just to compromise detail at this stage..?

RKQ

Sparky78 wrote: »

I believe that as the original poster is not building a passive house and has not mentioned the use of a HRV and will be putting holes in the walls for ventilation

The OP is specifically asking about Vapour Barriers. Why assume there will be wall vents?

Sparky78 wrote: »

I think that if he/she tapes the joints on the foil between the boards and uses a sealant and tape at the ceiling and floor, and then plasters, the vapour control layer will be suffient for his purposes.

I think you have never done this on site. What tape would you use? (Guarenteed to stay on foil?)

It would be hard to do and very time consuming. The boards are rarely plumb. Crush easily under thumb etc.

I believe in vapour barriers, air tightness, good practice and offering the self-builder the oppertunity to install a MHRV at a later date.

topcatcbr

Sparky78 wrote: »

I believe that as the original poster is not building a passive house and has not mentioned the use of a HRV and will be putting holes in the walls for ventilation that it would not be cost effective to spend the 1000's on 1 of these systems.
I think that if he/she tapes the joints on the foil between the boards and uses a sealant and tape at the ceiling and floor, and then plasters, the vapour control layer will be suffient for his purposes.

If I was putting in a HRV I would consider it then.

What happens when the Sparkey comes in and cuts through the plaster and foil to install switch and socket boxes then chases up the wall for runs of conduit.

What use is taping the joints then. Like i said earlier it is better to provide a vapor control layer with a service cavity .

Slig

Sparky78 wrote: »

I believe that as the original poster is not building a passive house and has not mentioned the use of a HRV and will be putting holes in the walls for ventilation that it would not be cost effective to spend the 1000's on 1 of these systems.
I think that if he/she tapes the joints on the foil between the boards and uses a sealant and tape at the ceiling and floor, and then plasters, the vapour control layer will be suffient for his purposes.

If I was putting in a HRV I would consider it then.

The use of a vapour barrier is to stop moisture but allow the wall to breath. If the joints are taped they create a completely sealed environment. this is grand but if there is one hole all your moisture rich air will leave through this and result in localised mould growth.

Personally I would go with Topcacbr or sydthebeats solution

RKQ

Slig wrote: »

Personally I would go with Topcacbr or sydthebeats solution

Which Slig???
Topcatcbr advocates a vapour barrier with service duct ( I'd agree) while Sydthebeat questioned the use of non-breathable foil insulation.

Not using a vapour barrier can result in interstitial condensation in the insulation!

I wonder what the OP thinks? What did his/her timber frame manufacturer recommend?

ardara1

Slig wrote: »

The use of a vapour barrier is to stop moisture but allow the wall to breath. If the joints are taped they create a completely sealed environment. this is grand but if there is one hole all your moisture rich air will leave through this and result in localised mould growth.

Personally I would go with Topcacbr or sydthebeats solution

A lot of chat about these tape - and I've asked a number of times about the resultant air permeability achieved - how much better than good practice wet trade?

Next question is do you want the house to be that air tight that you actually require mechanical ventilation? Can you rely on the user to operate the system effectively/safely?

Why not just go with reasonable air tightness and use natural ventilation?

As for breathable walls - putting a full PLASTIC barrier to the inside creates 2different environments - one inside one outside - if it gets wet form the outside it'll dry (breath?) to the outside - or is this too simple?

Sparky78

I'm all for airtightness and hrv as i'm going down this route myself. But if the op is using natural ventilation either by air vents in wall or window, then i dont think spending the money on an airtight system is money well spent. I would spend it on increasing the 25mm insulation, which would give a better ber.

sydthebeat

ardara1 wrote: »

As for breathable walls - putting a full PLASTIC barrier to the inside creates 2different environments - one inside one outside - if it gets wet form the outside it'll dry (breath?) to the outside - or is this too simple?

yes, too simple

unless the OP is planning on using a full airtightness membrane, and hoping to achieve passive standard air tightness results... then there will be locations in this barrier that are not fully air tight. This will lead to moisture being drawn to these locations and an unsafe moisture content forming at these points. Similar to a pin hole in a dam.......

If the OP already has a very good breathable construction, i do not see the logic behind actually building another 'environment' within this construction that HAS TO act independant of the main stucture.

Again, my advise to the OP would be to substitute the PU insulation for rigid batts of either rockwool or fibreboard, even increase the 25mm to 50mm to make up for the shortfall in the (theoretical) performance. This rigid insulation area can also act as the service cavity. The reason for this specification is as outlined in my first post.

To the OP, perhaps have a read of this, if you get the time:
http://www.naturalinsulation.co.uk/cms_items/20060607164406.pdf

have a look at the vapour resistance factors...
lime plaster 1.5 MNs/g
Bricks 5 MNs/g

Polyiso with foilback 2150 MNs/g
woodfibre insul 2.5 MNs/g
mineral wool 0.6 MNs/g

ardara1

How does a breathable membrane and tape cope with 60 years of emulsion, gloss, wallpaper and grease build up on the plasterboard?


In the past when a lot of timber framed construction were built using traditional methods - when will this problem manifest its self in them?

Why is it now that there are 4 or 5 manufacturers of breathable tapes that this has become such an issue.

I contend that the enrgy savings accrued with air permeability levels below 5 - probably aren't worth the effort.

RKQ

This problem hasn't manifested itself in Germany, where breathable vapour barriers and tapes, blow door tests are standard. These standards have been used for years in Germany.

The German ProClima system breathes - fibres open in summer to release any trapped moisture in the wall or roof. Their roof felt is IMO the best on the Irish market - I have used many breathable felts but theirs is the strongest, layered felt.

Slig

ardara1 wrote: »

How does a breathable membrane and tape cope with 60 years of emulsion, gloss, wallpaper and grease build up on the plasterboard?


In the past when a lot of timber framed construction were built using traditional methods - when will this problem manifest its self in them?

Why is it now that there are 4 or 5 manufacturers of breathable tapes that this has become such an issue.

I contend that the enrgy savings accrued with air permeability levels below 5 - probably aren't worth the effort.

Our standards of insulation were so far behind everywhere else until recently that we are only really seeing these problems manifesting themselves recently.

Up until 2 years ago it was perfectly acceptable to put in enough insulation into a wall to give you a U-value of 0.27 and then drill a 100mm dia hole through it.

muffler

Slig wrote: »

Our standards of insulation were so far behind everywhere else until recently that we are only really seeing these problems manifesting themselves recently.

Up until 2 years ago it was perfectly acceptable to put in enough insulation into a wall to give you a U-value of 0.27 and then drill a 100mm dia hole through it.

Nicely summed up.

ardara1

Slig wrote: »

Our standards of insulation were so far behind everywhere else until recently that we are only really seeing these problems manifesting themselves recently.

Up until 2 years ago it was perfectly acceptable to put in enough insulation into a wall to give you a U-value of 0.27 and then drill a 100mm dia hole through it.

So going forward - where's the balance between insulated fabic & air tightness?

There seems to be a lot more emphasis on air tightness.

sydthebeat

ardara1 wrote: »

So going forward - where's the balance between insulated fabic & air tightness?

There seems to be a lot more emphasis on air tightness.

50 - 50 in my opinion....
The success of the insulation layer depends on good airtightness, the success of the air tightness is useless without good insulation.

Slig

and the more insulated your structure the more important air tightness is to prevent mould growth

Sparky78

Do these moisture problems occur when you use hole's in the wall or windows for ventilation. I cant see moisture being forced through small holes in the wall when theres already a big hole there already. Would the moisture not escape through the path of least resistance.

ardara1

Slig wrote: »

and the more insulated your structure the more important air tightness is to prevent mould growth

Surley the more air tight (not insulated) your house the more critical the ventilation is?

Slig

ardara1 wrote: »

Surley the more air tight (not insulated) your house the more critical the ventilation is?

Sorry ardara my head is spinning as this goes round. I'm trying to remember the way it was explained to me.

Does hot air expand?

I remember that the moisture rich air will mostly travel through this hole and create damp but I cant remember why.

sydthebeat

ardara1 wrote: »

Surley the more air tight (not insulated) your house the more critical the ventilation is?

see Draft new ventilation regs.

Table 1 note C refers .

Where air tightness Q50 test result of 7m3hr/m2 at P50 is achieved, background ventilation to be increased by 50% .

:eek:

topcatcbr

Sparky78 wrote: »

Do these moisture problems occur when you use hole's in the wall or windows for ventilation.

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

I cant see moisture being forced through small holes in the wall when theres already a big hole there already

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)

Would the moisture not escape through the path of least resistance.

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.