Draft behind plasterboard in new extension

Splamone · 8 Dec 2018

Hi,

I am having an single storey extension built out the back of my house and it’s almost finished. However, the cold weather and cold wind has highlighted a potential issue.

The walls are dot and dab on insulated cavity walls. The roof it pitched at each side to meet in the middle and has 100mm of celitex. With the cold wind blowing, the walls feel quite cold, colder than my original walls that are not boarded, and ther is a draft coming from any hole in the wall. Sockets, holes for cables, gaps under the plasterboard where skirting is yet to be fitted.

My question is, has my builder screwed up here and is this a mahor problem that needs to be sorted before decorating? Every wall is already plastered and my kitchen is in already. The wall cupboards are very cold inside which I wasn’t expecting and I can feel drafts behind the cupboards where the wires come out for the lights etc.

I may be able to fill the holes with expanding foam or bits of insulation fluff and foam around the skirting gaps. Would this be ok or am I potentially going to have problems with the cold air behind the walls?

Is there anything I can do at this stage to fix it or am I being too picky about stopping the cold air?

Thanks in advance!

Yeahaa · 9 Dec 2018

https://www.designingbuildings.co.uk/wiki/Defects_in_dot_and_dab

Id personally suspect the eaves detail to be a source of drafts , obviously is getting in from somewhere

noseall · 9 Dec 2018

A lot of it is down to misunderstanding how one should go about fixing the boards and preventing air movement. Here is how we do ours, solid bead around the perimeter plus solid dabs around switches etc......

Untitled

noseall
15 Aug 2014
3

Dabber2

noseall
9 Dec 2018

Dabber1

noseall
9 Dec 2018

Splamone · 9 Dec 2018

Hi, thanks for the pics. I agree that this method is much better than the way mine has been done! Any idea how I can fix mine or if I even need to? I could seal around the holes and live with the switches being a bit cooler as not much air gets through them. However, could this potentially cause inefficiency in keeping the room warm in winter if the walls are cold? I guess there is also the potential issue of damp but the rooms have trickle vents and VELUX Windows that open the vents a few times a day so should be pretty good for ventilation.

If it is going to be a problem, what are my options? Do I need to cut a channel across the top of all the plaster boards on the wall and fill them in? I assume the air is coming from above as there are no holes in the wall and the floor is sealed up to the walls. Maybe I could drill some holes, fill with some foam and then patch them up? It would be a lot of work though as it’s a fairly big room.

Thanks for your help

ivixor · 9 Dec 2018

For what it's worth, my rear extension has been done with standard dot and dab, and there were no drafts after or before the plastering was done.

It seems that as has been mentioned, with yours there is a problem with the detailing where the walls meet the ceiling.

Splamone · 9 Dec 2018

ivixor said:
For what it's worth, my rear extension has been done with standard dot and dab, and there were no drafts after or before the plastering was done.

It seems that as has been mentioned, with yours there is a problem with the detailing where the walls meet the ceiling.

Hi, do you know if the dot and dab had a seal along the top (at least) to prevent any air coming down the walls?

Do I need to get the builder to fix this some how before he finishes completely? I have a feeling that won’t go down well if it’s a tough job to repair!

Thanks

noseall · 9 Dec 2018

Also note how full our block perp joints are. A bit anal I know but I always like to see full joints.

ivixor · 9 Dec 2018

Nope, no need. There wasn't any draught before. Though I did go round and stuff rockwool in some poorly insulated bits in the ceiling, for example at the edge of the flat roof where there was only fascia between the inside and outside.

garyo · 9 Dec 2018

You could try drilling a 10mm hole every 100mm around the top of the walls and inject expanding foam.

John D v2.0 · 10 Dec 2018

The air barrier should be more or less continuous and there should be minimal air movement between it and the insulation layer to prevent thermal bypass.
You can block the holes in the plasterboard but the plasterboard isn't the air barrier, as the insulation is no use floating in the wind. Google plasterboard tent.

Munzy · 11 Dec 2018

noseall said:
A lot of it is down to misunderstanding how one should go about fixing the boards and preventing air movement. Here is how we do ours, solid bead around the perimeter plus solid dabs around switches etc......

Untitled

noseall

15 Aug 2014

3

Dabber2

noseall

9 Dec 2018

Dabber1

noseall

9 Dec 2018

Sorry to hijack slightly but what adhesive do you use for dot and dab?

noseall · 11 Dec 2018

Munzy said:
Sorry to hijack slightly but what adhesive do you use for dot and dab?

Gyproc/Knauf/Lafarge bonding compound (board adhesive).

Munzy · 11 Dec 2018

noseall said:
Gyproc/Knauf/Lafarge bonding compound (board adhesive).

Knauf states 4.6m2 coverage per 25kg bag... as someone who has never fixed with the dot and dab method, how many boards would a bag fix if applying as you have above?

noseall · 11 Dec 2018

Munzy said:
Knauf states 4.6m2 coverage per 25kg bag... as someone who has never fixed with the dot and dab method, how many boards would a bag fix if applying as you have above?

About 3x 1200mm x 2400mm boards. Cuts and thickening brings the numbers down though.

Splamone · 11 Dec 2018

John D v2.0 said:
The air barrier should be more or less continuous and there should be minimal air movement between it and the insulation layer to prevent thermal bypass.
You can block the holes in the plasterboard but the plasterboard isn't the air barrier, as the insulation is no use floating in the wind. Google plasterboard tent.

Thanks for the tip. Useful information and brings up a lot more on the issue. I think I will try to seal the holes behind the plasterboard that I know about (waste pipe through wall, cables coming through insulation at edge of wall/sealing) and see how that goes. If it’s still petty drafts in the wind then I may try the expanding foam through holes near the top of the walls as I know that the ceiling insulation doesn’t go right up to the wall and is probably an issue. If the draft is reduced to almost nothing then I guess that would be acceptable and I could then seal up the holes in the plasterboard. I assume that it’s unrealistic to expect a dot and dab wall to be 100% draft free unless it’s properly sealed at all sides. That’s not really possible now. Wish I knew about this before they boarded so I could spot it!

Thanks for the advice.

Added a couple of images to show the boarding on the ceiling before the walls were done. Could have foamed up the edges if I had known about the draft issues

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Draft behind plasterboard in new extension

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