Hi ONQ,
I have seen some of your posts before and you seem to know a bit about this area.
Given that both internal and external insulation have their drawbacks,have you any comments on the idea of mounting internal insulation on battens with a 25mm ventilated cavity between the wall and the insulation.
Of course every effort should also be made to vapour check as well and there is a loss of performance due the fact that you would have to ignore the existing wall in terms of insulation.
Would be interesed in your thoughts.
Thanks
3CC
Hi 3CC,
<bows>
Thank you for your vote of confidence.
Don't take any one person's word for it - including mine - check different opinions.
That having been said, lets review some options first.
Yes you can do this, the problem is the venting.
Use of vented cavities:
Vented cavities are best located on the cold side [outside] of the insulation.
This is the basis for the timber frame cavity detail.
A one way breather paper allows water vapour to pass through.
Moisture condensing on the outer face cannot pass back in.
The cavity is vented to aid evaporation and carry it away.
It is vented top and bottom to encourage ventilation.
The cavity runs from below Gd Floor DPM to Eaves.
All ope and penetration passing through it are firesealed.
In relation to what I understand your suggestion to be:
You cannot ventilate an internal cavity between the masonry wall and the insulation unless you; -
A) vent it back into the room which defeats the purpose.
[Venting like this removes any vapour checking action but see Note: 1 below.]
B) vent it with supply air naturally
[if cold, this will reduce the effectiveness of ordinary quilted insulation]
C vent it with MVHR supply air
[pointless exercise as venting the room would be better]
Note: 1
I've seen Option A) used to deal with dry rot in older buildings - by preventing a build up of humidity within the structure.
That is a special case and the insulation may be located differently, i.e. there may be none, just plaster board on battens
A variation exists for new construction where variable vapour membranes can allow interstitial moisture that's built up back into the room to be vented away in certain conditions. Joseph Little Archtiects has written about this on Construct Ireland.
I don'trecommend internal cavities as you describe - they can become routes for cold smoke and gases to reach a sleeping occupancy and such require to be carefully sealed at floors, ceilings, penetrations and around opes [doors, windows, room vents] etc.
Battening out and filling with fibreglass doesn't help - filling with Rockwool may add to the fire resistance of a stud, but it still has to be sealed as noted above.
This can be achieved using timber stud members or continuous dabs of plasterboard in the locations noted, but the junction of the ceiling and wall needs special attention regardless.
Seal that properly [and any ceiling penetrations] and smoke will find it hard find its way up to the bedrooms.
In an ordinary house it will still get out the room door, but that will pass by and activate the fire alarm.
The important thing is that people don't die in their sleep from toxic fumes and smoke before this.
Kingspan do a reasonably simple mechanical fixed warmboard with integral vapour check - flat to the wall, no cavity.
The danger with the warmboard is that there are no frame members or plaster dabs-
This means that the junction between block and ceiling plaster must be carefully sealed.
In all proposed solutions You have to think through the detail in section - floor, walls, roof.
This will identify any discontinunity in the vapour check, insulation or vented cavity.
It will help identify any lack of fire sealing at walls, floor ceilings and penetrations.
A note on condensation also; -
Thermal and humidity gradients across internal spaces and external envelope are what gives rise to condensation.
Warm air absorbs moisture in the kitchen [say] and wanders through the house until it finds a surface that is RELATIVELY cold enough for that heavily laden air to deposit its moisture on.
Consider this if
not using your extractor fans in the bathroom, kitchen or utility.
It is also useful whatever detail you use to ensure low level heat is maintained even in bedrooms during the day.
This will help avoid condensation on walls but also on the cold [outer] face of the insulation if internal insulation is used.
This hidden condensation on the inner face of outer walls behind the internal insulation may dissipate but heat helps this process.
So <takes deep breath> you can see that insulation, ventilation and protection of occupants from fire are all interlinked, even in relatively simple buildings like traditional houses.
Its why I keep telling people to talk to professionals.
ONQ.
[broken link removed]