Green Building Forum - Plasterboard + 1/2" EPS Tent

Plasterboard + 1/2" EPS Tent

Dominic Cooney — Tue, 08 Nov 2022 12:53:47 +0000

This is what counted as insulation to a pitched roof in 1975.
I suppose at least there was something, there was nothing in the stud walls that partitioned off the eaves storage space. Also nothing to stop the external air from the eaves blowing all the way across the room, under the floor boards, between the floor joists to the other side. Lets just say it is well ventilated!

Plasterboard + 1/2" EPS Tent

Nick Parsons — Tue, 08 Nov 2022 13:31:58 +0000

Frankly quite good for 1975. I would have expected nothing!

Yes, the floor/ceiling cooling system is alive and well and active in many many thousands of Victorian attics and modern dormer bungalows.

Maybe we should compile a list of likely 'failures' in rooms-in-the-roof. I'll start with a loft conversion done in the last few years, signed off by BC. The designer/builders seem to have paid no heed to the fact that the adjoining lofts had not been converted. Party wall finish was simply dot-and-dab plasterboard...

Even where a terrace-full of attics adjoin, people often do not seem to note that step-gables are external walls!

Plasterboard + 1/2" EPS Tent

fostertom — Tue, 08 Nov 2022 14:28:33 +0000

By 1981, in my own house where no one would stop me (Atlas) I went completely crazy with 75 glass fibre between the rafters (25 air gap above carefully vented eave to eave - not at ridge) plus plasterboard/EPS laminate like that beneath the rafters. "Why both?" they said - to kill the conduction thro uninsulated rafters. "Unh?"

Plasterboard + 1/2" EPS Tent

Dominic Cooney — Thu, 10 Nov 2022 11:17:43 +0000

The rafters here are 70mm deep x 47mm wide at approx 18" centres.

I was going to put 100mm PIR underneath & tape the joins, fixed with 25x38 battens (because I have a load spare) then plasterboard & skim.
This would be a massive improvement over what is there now.

Is there any point putting anything in between? If I have to leave a 50mm air gap below the felt then that only leaves 20mm usable space.

At some point in the future I may redo the roof, at that point I can fully fill between the rafters and fix 100mm PIR over the top to make it a warm roof (with counterbattens, breather membrane, & tile battens).

Or at this stage should I try and put say 25mm PIR inbetween (easy to get hold of), or even 30 or 40mm? (internet only order I expect) then next common size - 50mm - may be too risky. In case I don't get round to redoing the roof for 10 years or more.

Any thoughts?

Plasterboard + 1/2" EPS Tent

Dominic Cooney — Thu, 10 Nov 2022 12:10:10 +0000

which I think would give me a U-value of 0.2

Plasterboard + 1/2" EPS Tent

Dominic Cooney — Thu, 10 Nov 2022 12:24:56 +0000

whereas if I put 40mm PIR in between as well it would get me 0.16

Plasterboard + 1/2" EPS Tent

WillInAberdeen — Thu, 10 Nov 2022 14:28:48 +0000

If you have some 25mm PIR, it might be better deployed to fill the cavity directly behind the plasterboard? Helps to exclude any chance possibility of drafts/mice/fires finding their way into the cavity in future, while leaving plenty ventilation space for the tile felt to sag into. (Obvs run any wiring to lights first).

My childhood 1970s dormer bungalow had the dwarf walls of the dormer bedroom 'insulated' with some 1970s orange carpet, which someone had crawled into the triangle void to tack over the outside of the dwarf wall studs. Was as effective as you would expect. Many years later we bought an old farmhouse where the same thing had been done with fertilizer sacks.

Plasterboard + 1/2" EPS Tent

Dominic Cooney — Thu, 10 Nov 2022 17:25:52 +0000

Good idea WIll, for some reason I hadn't considered putting more PIR in the gap there.

That works out at U = 0.18

The U value of the existing roof works out at 1.936 and there is no attempt at insulation behind the dwarf walls!
There doesn't seem to be an option for carpet or Hessian Sacks in that U-value calculator, so we can't compare!

Plasterboard + 1/2" EPS Tent

Peter_in_Hungary — Thu, 10 Nov 2022 18:37:34 +0000

Dominic - whilst you don't say I presume that it is a skeiling ceiling.

In the first example you quote 12.5 plaster board, in the second you use 9 mm PB. Over here we don't use 9 mm PB much because it has a tendency to sag especially if it is not vertical. Was the change intentional or a typo? Also no one here skims PB its all tape and fill, much cheaper and quicker.

Plasterboard + 1/2" EPS Tent

Dominic Cooney — Thu, 10 Nov 2022 19:47:50 +0000

Hi Peter, it's about 900mm high at the eaves now that I have taken out the stud walls (there was eaves storage before, but cold) so that I can insulate the entire roof slope. This will make the room feel bigger, and we will probably still use the eaves for storage, but within furniture, blanket boxes etc. and now all within the insulated envelope. The room currently has a flat area of ceiling in the centre, but I am also considering opening this almost all the way up to the ridge to make it feel more spacious. (there are purlins and chords up there that would remain but then end up inside the room).

The existing plasterboard is 9mm, I have been playing around with that U-value calculator and realised there are options for both thicknesses. Tape and fill would be preferable, I would have to space the battens to suit the size of tapered edge plasterboard exactly, which would probably mean running the battens horizontally (due to the odd spacing of the existing rafters)

Plasterboard + 1/2" EPS Tent

wookey — Thu, 10 Nov 2022 21:46:20 +0000

I'd just leave 25-30mm ventilation space - that's plenty. And lose the battens - just glue the plasterboard to the PIR. It works fine. My skeilings are done that way (100mm PUR+12mm PB)

Plasterboard + 1/2" EPS Tent

Dominic Cooney — Thu, 10 Nov 2022 22:36:30 +0000

Thanks for the comments Wookey, interestingly SWMBO suggested earlier that we just glue it on, I wasn't confident on a ceiling (even though it is sloping, not horizontal). What glue would you use? We used the pink foam adhesive in the barn covnersion - but that was on walls.

Plasterboard + 1/2" EPS Tent

WillInAberdeen — Fri, 11 Nov 2022 10:11:05 +0000

Wookey, that's interesting, did you also rely on glue to fix the PIR to the rafters, or also screw through something? Did the pb need any support while the glue dried?

Ive been gluing pb onto PIR but only for vertical wall - not tried on ceilings yet! I used instant grab polymer adhesive, got through quite a few tubes trying to run continuous beads round each board edge and criss cross across the middle.

Dominic, the 1200x900 plasterboards suit me better for working on ceilings alone. The 9mm stuff doesn't seem to be sold so much round here, maybe there are some fire or noise regs or something. The 1970s plasterboard ceiling has sagged a little round each fixing, but not really noticeable because of the swirly artex and the woodchip wallpaper..

Plasterboard + 1/2" EPS Tent

djh — Fri, 11 Nov 2022 15:46:07 +0000

FWIW, our barrel-vaulted upstairs ceiling uses 9.5 mm plasterboard, exactly because it can easily be shaped into a curve. After skimming, the joints are invisible; smooth curve all the way.

Plasterboard + 1/2" EPS Tent

Davycrocket — Thu, 01 Dec 2022 17:11:23 +0000

Could you not have boarded over the orginal PB with PIR and stuck that and the new PB on with a suitable adhesive ?

I'm thinking of doing this unless someboy has experienced a failure of such a scheme ?

Plasterboard + 1/2" EPS Tent

tony — Thu, 01 Dec 2022 17:47:04 +0000

I would advocate mechanical fixing too

Plasterboard + 1/2" EPS Tent

Jeff B — Thu, 01 Dec 2022 17:58:05 +0000

Posted By: DavycrocketCould you not have boarded over the orginal PB with PIR and stuck that and the new PB on with a suitable adhesive ?

I'm thinking of doing this unless someboy has experienced a failure of such a scheme ?

I put PIR insulation boards directly onto the existing plasterboards on my dwarf walls/skeilings. However I would not trust adhesives so I used 25 x 50mm battens to secure the first layer of 50mm PIR (screwed through) and then put 25mm boards between the battens (friction fit with Alu tape over the joins) and then screwed the new PB to the battens. Worked very well, no problems AFAIK.

Note: I am aware that purists would be horrified at the thought of all the thermal bridges caused by the screws!

Plasterboard + 1/2" EPS Tent

Davycrocket — Thu, 01 Dec 2022 20:16:43 +0000

What wouldn't you trust about the adhesive mate ?

Plasterboard + 1/2" EPS Tent

Davycrocket — Thu, 01 Dec 2022 20:19:08 +0000

Posted By: tonyI would advocate mechanical fixing too

Hi Tony did you read my reply re loaning of the cavity wall gun mate ?

Plasterboard + 1/2" EPS Tent

Jeff B — Thu, 01 Dec 2022 22:24:33 +0000

Posted By: DavycrocketWhat wouldn't you trust about the adhesive mate ?

Probably because I am a 75 years of age old codger who life's experience has shown that the chances of two items that are screwed together have a far greater chance of staying together than two that have been stuck together with adhesive!

Also it would have been quite problematic using adhesives when attaching 12mm foil backed PB onto sloping ceilings!

Plasterboard + 1/2" EPS Tent

Dominic Cooney — Fri, 02 Dec 2022 07:45:11 +0000

Posted By: DavycrocketCould you not have boarded over the orginal PB with PIR and stuck that and the new PB on with a suitable adhesive ?

In this particular case I removed all the old plasterboard because I have also removed the dwarf wall on each side of the room, as well as the flat ceiling in the middle. So the original skeiling part was less than one third of the plasterboarded area of this room.
This has gained us 800mm each side of the room, so 1600mm in total, less 100 each side when I put the PIR on the block walls plus the plasterboard so more like 1.37 metres total extra on this room dimension. It will look and feel bigger and more spacious, it will bring the previously cold ventilated-to-outdoors eaves storage inside the warm envelope. The whole job will prevnt cold outdoor air from blowing all across the room underneath the floorbards (Kitchen is below).

We will likely still use the eaves area for storage but in furniture (blanket boxes etc.) and also form a rearranged airing cupboard that will now be properly insulated!

Plasterboard + 1/2" EPS Tent

Dominic Cooney — Fri, 02 Dec 2022 07:46:33 +0000

This is the 40mm layer going in between the rafters

Plasterboard + 1/2" EPS Tent

Dominic Cooney — Fri, 02 Dec 2022 07:54:35 +0000

Helpfully the 1200 x 2400 boards I cut lengthwise to 400 wide and they fit between with a very small gap, which was then foamed in (held in place temporarily by nails). In one place I had to cut 390mm / 410mm but I'm glad I noticed it before I just cut them all to 400!
I cut both sides of the board with a sharp knife and snapped the board, so almost no dust created.

I still have the top bit to do above the chords & purlins, but helpfully this is approx 800mm long so my pieces cut 400 x 2400 will then get cut into 3 lengths of 800 x 400

The 100mm layer will go over these held in place with 25mm battens.

The radiator under the window will probably go somewhere else, as it will need moving inwards anyway to make room for 100mm PIR on the end wall.

Plasterboard + 1/2" EPS Tent

Dominic Cooney — Fri, 02 Dec 2022 08:00:05 +0000

At least I know when it was built.

My parents would have been celebrating the birth of their new baby boy a few days earlier. (although disappointed that I wasn't a girl, after my 3 older brothers!)

Plasterboard + 1/2" EPS Tent

Dominic Cooney — Fri, 02 Dec 2022 08:15:54 +0000

In my recent exerience with the pink foam adhesive for PIR and Plasterboard, it sticks very very well.
It is almost impossible to remove without wrecking either the PIR or the P/B or both. I did manage to carefully peel a 25mm sheet of misplaced PIR off of a very dusty stone wall so that I could re-do it (it was sticking out proud of the one next to it - I was just using it as gap filler in this location on an internal wall).
We used it on the walls of the barn conversion we did recently, but we also fixed the PIR mechanically with plastic insulation fixings, then pink foam glued the P/B to the surface.
I can see the appeal in using it for skeilings, but it would need to be held in place while it goes off, for which screws are an easy solution anyway. You could use just a few screws and take them out again after, but this would seem a bit pointless.
My concern with adhesive-only on existing skeilings or ceilings is that any paint layer could act as a release agent?
Also longevity, although it stuck very very well when we used it last year, would this still be the case in 10, 20 or even 47 years time?

Plasterboard + 1/2" EPS Tent

Davycrocket — Fri, 02 Dec 2022 10:27:04 +0000

Posted By: Jeff B
Posted By: DavycrocketWhat wouldn't you trust about the adhesive mate ?

Probably because I am a 75 years of age old codger who life's experience has shown that the chances of two items that are screwed together have a far greater chance of staying together than two that have been stuck together with adhesive!" title="">

Also it would have been quite problematic using adhesives when attaching 12mm foil backed PB onto sloping ceilings!

Fair do's Jeff

Plasterboard + 1/2" EPS Tent

Davycrocket — Fri, 02 Dec 2022 10:35:02 +0000

Thanks for the replies Dominic Tom and Jeff, very informative.

Plasterboard + 1/2" EPS Tent

Dominic Cooney — Mon, 12 Dec 2022 09:41:19 +0000

Who actually thought this was a good idea?
I have managed to push it away from the roof felt a bit and squeeze 25mm of PIR behind it.

Plasterboard + 1/2" EPS Tent

Dominic Cooney — Mon, 12 Dec 2022 09:49:09 +0000

These ones are even worse. The one at the top was pressed hard against the roofing felt.
I have managed to pull it away a small amount and squeeze 12mm of Aerogel board behind it.
The location of these pipes are the reason that during the "Beast from the East" in 2018 we had no hot water.
(this part of the roof is on the east side)

Plasterboard + 1/2" EPS Tent

Davycrocket — Tue, 10 Jan 2023 15:08:12 +0000

We had frozen pipes over xmas, thankfully they didnt burst.

Whilst trying to find the culprit I discovered that the fibreglass was wet presumably caused by warm air escaping from below ?

The insulation is touching the felt in the eaves.

If I pulled the said insulation out could it be replaced with eps beads between the joists ?

I think ive stopped the air escape from below by changing the downlights to fire rated alternatives.