Adding insulation to stud wall

fiona111

Hi
We have timber stud walls covered by plasterboard separating our upstairs bedroom from the loft. At the moment there is a layer of plastic sheeting, then 6" of glasswool between the batts (which is basically the depth of the batts. We would like to upgrade the insulation and were thinking of attaching polyurethane (or similar) foam sheets to the batts on the loft side. Would there be any condensation issues? Should we have a vapour barrier between the glasswool and the foam sheets? Any other problems that we could have?

So to clarify we are looking at: Plasterboard - plastic sheeting - timber batts with glasswool infill - polyurethane foam sheets.

Thanks

Calahonda52

lots to help you here
https://www.nsai.ie/S-R-54-2014-Code-of-Practice.aspx
yes u will have condensation issues as the PU will most likely have an alu foil surface which is no breathable
Any vapour barrier should be on the warm side

Is the glass fib=re covered on the inside with anything?
is the attic space draughty?
Is it in good nick will full fill and no gaps, sagging, etc?
If in good nick, then consider a sheets of a wood fibre insulation.
This will do three things.
increase the insulation value over all
is breathable but wind tight
will improve the U value of the glass fibre by keeping the wind from blowing through it.
Think wooly jumper in the wind outside with out a windbreaker jacket

fiona111

Thanks Calahonda52
To answer your questions:

Is the glass fib=re covered on the inside with anything?
On one wall the glassfibre is not covered. On the other we covered it in with OSB boards so we could hang stuff off hooks in the loft.

is the attic space draughty?
It is certainly cool - the floor of the attic space has 12" of glass fibre but we were careful to leave a bit of a gap at the exterior walls - I think you are meant to do that for ventilation. On a windy night if you open the door into the attic you can hear and feel air moving.

Is it in good nick will full fill and no gaps, sagging, etc?
Yes - it was put in about 15 years ago and looks ok.

Calahonda52

okay, in addition make sure attic door is airtight, insulation on its own won't help, u you need controlled air movement as well

I would take off the osb and put on the fibre board and mark where the uprights are and then put strips of OSB back for hooks

fiona111

Thanks - that's great. Do you know how much per m2 it is for wood fibre insulation? I can't find prices on the net anywhere. Also, I see wood fibre board and wood wool board being talked about - do you know what the difference is?

Calahonda52

I have attached a picture of one attic I did, in this case the boards were laid vertically as there were already horizontal runners in place to keep in the glass fibre which was well proud of the uprights.
These sheets were tongued and proved for the vertical joints.

Re pricing, is a fn of thickness so call the guys in Athboy that I sent you a PM.
There are others that do it also.

hexosan

Anyone know suppliers of hemp or similar thermally dense insulation other than the guys in athboy.

fiona111

Thanks Calahonda52 - very useful photo. A question on moisture - in your first reply on putting polyurethane-type foam boards on the glassfibre you said:
"yes u will have condensation issues as the PU will most likely have an alu foil surface which is no breathable. Any vapour barrier should be on the warm side"

- if there is plastic sheeting between the plasterboard and the glassfibre then how would moisture get in? Surely the hot, moist air would be stopped by the plastic before it gets to the glass and PU?
Thanks

Calahonda52

The moisture will get through, nail holes, tears, sockets, switches, overlaps, etc as well as just the vapour pressure differential due to warm moist air versus cold drier air.

Best practice suggests that yes, try keep it out, but accept that some will get through so when it does, let it go all the way and don't block it again with Alu faced insulation.

The issue is the temperature drop across the wall from maybe 18 Degrees surface temp on the PB to maybe 10 in the attic
http://www.dpcalc.org
does the dew point cals but its generally accepted that once you get to 12.6 degrees C the water vapour condense out somewhere in the wall structure.
This could be in the glass fibre which will then get wet and eventually sag..

I know it seems like a long shot but thats the theory

fiona111

Thanks Calahonda52 - that makes sense. :-)