Green Building Forum - Why can't blob and dob work?

Why can't blob and dob work?

Mike George — Tue, 08 Jan 2008 23:44:01 +0000

Nick's fundamental topic gave me this idea, not sure if it belongs in that thread?

Anyway, there is a lot of valid criticism of dot and dab [or blob and dob as its commonly referred to here] However, I think there is a place for such a method in the renovation of buildings and I don't see why it cannot be done without compromising air tightness.

For example, I am about to do some work converting the space above an integral garage. The walls are cavity construction, with a token bubblewrap insulation within the cavity. I propose to do the following to the gable wall. Note that Part L1b now requires the external walls to be insulated.

1. Ensure the wall is free of cracks and missing beds/perps
2. Parge coat of 6:1:1 sand:Cement:Lime. [Nice and flat]
3. Dot and dab PUR insulation backed platerboards, taking extreme care that the perimeter is covered with a consistent, uninterupted band of adhesive. Bands of adhesive also at 400mm ctrs
4. Ensure a gap between floor and boards of say 30mm
5.When adhesive is dry, fill the gap between the floor and the plasterboards with expanding foam.
6. Fix skirting, then plaster walls.
7. Allow timber to acclimatise then seal the junction of floor and skirting with a mastic.

Not very environmentally friendly materials I'll grant you, but the only method I can come up with which I believe will work, while satisfying Building Regulations and the Client's spatial and budgetary requirements.

I think this will work fine, anyone have an opinion or suggest an alternative? Filling the cavity is not an option, neither is external insulation

Why can't blob and dob work?

tony — Wed, 09 Jan 2008 08:55:04 +0000

If you do all that you will be fine, still today on the big sites they are still not parging and almost never using ribbons of adhesive.

Why can't blob and dob work?

Peter A — Wed, 09 Jan 2008 16:31:11 +0000

Mike, you will have no problem with that spec.
Tony, with the air leakage testing just beginning to kick in on sites, some might begin to think about parging and better quality control (you'll notice that I said "begin to think"!)

Why can't blob and dob work?

Mike George — Wed, 09 Jan 2008 19:30:32 +0000

So the problems associated with dot and dab are not with the method but with the quality of specification and/or workmanship?

Why can't blob and dob work?

Peter A — Wed, 09 Jan 2008 20:02:06 +0000

Mike, I would say it's all down to workmanship, that and the fact that it's quicker not to put all the dabs on, so trades can earn more in a day, if the site supervision wants it done quickly they don't look too closely so trades know they can get away with it!

Why can't blob and dob work?

Mike George — Wed, 09 Jan 2008 20:08:15 +0000

Also this maybe? [from the fundamentals thread]

I have come across the howling wind behind scenario but the problem was not due to unjointed blockwork, but to air infiltration from between the block wall and the sub floor void. This I think is a common failing on many new sites which are built on sloping ground. Because of the gradient, sub ventilated beam and block flooring seems to be a common method, and as the screed is normally laid last, the perimeter behind the dot and dab walls is not sealed by the screed resulting in air circulation behind the plasterboard.

Why can't blob and dob work?

tony — Wed, 09 Jan 2008 20:18:17 +0000

and people living in plasterboard tents instead of well insulated houses

Why can't blob and dob work?

tony — Wed, 09 Jan 2008 21:00:46 +0000

I have never seen blob and dob without draughts behind it. Taken plenty off too. Never seen damp or mould there either but then it it too well ventilated

Why can't blob and dob work?

howdytom — Sat, 12 Jan 2008 19:42:26 +0000

Tony,
We hauve to leave all our sockets with plugs in in the kitchen otherwise we get draughts coming out of them !!!.
tom.. in the wind north

Why can't blob and dob work?

Chris Wardle — Sat, 12 Jan 2008 20:17:18 +0000

Regardless of whether you can stop the drafts or not, what is the point of building a new house out of blocks and then covering them with plasterboards? You get no more thermal mass than a timber frame house, so why not just build a timber frame one which I would think would be cheaper, quicker and have a lower embodied energy? Shouldn't the building regs be changed to require all block built new builds to be wet plastered? While we're at it, lets stop the practice of giving timber frames a brick or stone veneer - a wasteful practice and dishonest in my opinion.

Why can't blob and dob work?

chuckey — Sat, 12 Jan 2008 21:27:02 +0000

Chris, if your blocks are inboard of the insulation, can you explain to me exactly how they are going to add to comfort compared with a timber framed building.
Frank

Why can't blob and dob work?

Chris Wardle — Sun, 13 Jan 2008 13:33:18 +0000

Are you referring to the lightweight verses heavyweight argument Frank? If so, I don't really think I'm qualified to make an argument either way. I've got my own ideas based on personal experience as we all have I'm sure.

I was just making the point that, on new builds, I can't see any good reason to build a heavy weight structure (which involves costs in terms of time on site and embodied energy compared to a lightweight structure) and then effectively turn it into a lightweight structure from an internal comfort point of view by dry-lining it with plasterboard. If I've overlooked something then someone please enlighten me! (I think I must have because there tens of thousands of house being built this way every year...)

Why can't blob and dob work?

tony — Sun, 13 Jan 2008 19:58:54 +0000

As howdytom says,"We have to leave all our sockets with plugs in in the kitchen otherwise we get draughts coming out of them !!!.
tom.. in the wind north

This is very common and all these homes will have to be wet plastered to stop massive heat losses.

Blob and bod should be outlawed for all except Mike George who does it properly.

Why can't blob and dob work?

Mike George — Sun, 13 Jan 2008 20:23:57 +0000

Ah, but what if we could devise a blob and dob system which was airtight?

Why can't blob and dob work?

tony — Sun, 13 Jan 2008 20:40:25 +0000

And cowboy proof ?? unlikely

Why can't blob and dob work?

Mike George — Sun, 13 Jan 2008 20:58:13 +0000

I don't see why not. Anyway, cowboy proof is something we need across the board. There needs to be more on site education and supervision

Why can't blob and dob work?

tony — Sun, 13 Jan 2008 21:31:36 +0000

It would be nice to think so but I cant see it.

Why can't blob and dob work?

Chris Wardle — Mon, 14 Jan 2008 12:18:22 +0000

Even if you could get it airtight, Mike, why would you want to dryline a block wall in a new build? It could be the best option on a renovation job where external insulation not possible, if it is done as you describe, but otherwise I can't see any circumstances where it would the best option.

Why can't blob and dob work?

Mark Siddall — Mon, 14 Jan 2008 13:05:42 +0000

I've been following this thread for a while now. Mike, blob and dob as you call it can in threory be made to work but the skills issue is a major hurdle. The parging approach was used at Stamford Brook and was capable of achieving 2 m3/m2@50pa though the average was nearer 5 m3/m2@50pa. The reason that parge rather than full plaster was spec'd was to reduced drying times and therefore shorten the construction programme and minimise prelims costs. They found that one of the biggest challenges in maintaining airtightness was ensuring correct sequencing and avoiding out of sequence working that can prevent getting into all the nooks and crannies (behind the bath, stairs, first floor ceiling voids etc.)

If using masonry I'd certainly use a wet finish rather than standard blob and dob. On a project that we worked on we achieved ~3.6 m3/m2@50pa using a parging technique. With a little more pre-planning on the next job I reckon that we could get to sub 2 m3/m2@50pa if we want to.

Mark

Why can't blob and dob work?

tbhulse — Fri, 20 Jun 2008 12:34:37 +0100

Mike,

I have a slightly different scenario to your cavity wall situation and would appreciate your thoughts on modifying your method.

I'm re-doing my kitchen and want to upgrade the insulation (ie. put some in!) at the same time. I have solid brick walls about 400mm thick. Only two walls need doing as the gables abut other rooms. It's currently plasterboarded (10mm board with 10mm polystyrene on the back), with dot and dab on one side as far as I can make out but appears to be a 50mm gap on the other so may be some wood in there. I'm suffering from Howdy Tom's problem of huge draughts through the plug sockets as well as under the pasterboard where it hasn't been sealed to the floor (under kitchen units). (Same problem in rest of house but that's a problem for another day). The ventilated eaves under exposed rafter ends are clearly venting straight into the walls (though the original airbricks don't help on the rest of the building either). When there's no wind everywhere is nice and warm, when there is a wind, it's draughty and cold. Not sure what's in the ceiling, probably not much, but planning to do that at the same time.

So, I plan to rip everything out and start again - the new kitchen will be in for a long time so it will be a long time before I get the chance again (current one being recycled to two happy recipients before anyone says anything). The barn is in a conservation area so can't add insulation externally (Tony).

Question is, is the parging required to ensure that the breeze blocks in your job are airtight? Is it necessary on the bricks if the're sound (they currently appear to be painted or limewashed)?

Am planning Hemcrete on another project but can't afford to lose a foot internally in this case (Chris).

Once I've got the insulation and air tightness sorted out, I'm anticipating another problem - I've got an Aga (came with the wife and if it goes so does she!) and the place is going to be too hot. I was thinking of putting in some ductwork with a fan to pull the heat through to the rest of the house and would appreciate your comments. As the rest of the house is not draughtproof yet I figured there's no point in thinking about an HRVC???

Thanks,

Tim

Why can't blob and dob work?

tony — Fri, 20 Jun 2008 14:31:17 +0100

Good idea to rip it all off and start again. The basics are to stop outdoor air (wind) getting inside your new kitchen. So seal all gaps and cracks to the outside. May be put up ceiling first - seal it to the walls then if you must thermally line go ahead. But seal all holes, gaps and cracks first.

Re fan -- beware as a fan is likely not allowed in the same room as the Aga as it could suck fumes back down the flue! Any change in pressure in the room could effect the combustion/efficiency of the Aga.

Why can't blob and dob work?

tbhulse — Fri, 20 Jun 2008 18:12:38 +0100

Thanks Tony. The Aga is gas with a balanced flue so presumably that shouldn't be a problem.

Why can't blob and dob work?

Mike George — Fri, 20 Jun 2008 21:03:49 +0100

Hi Tim, thats a tricky one. I have a feeling your aga will make the temperatures in the room very hot, as insulating internally makes for a very quick response in air temperature. If it were me, I would be going for a heat source with much greater control - something you can turn off quickly when the room starts to get too warm.

Re the parging. If the wall is very flat, and has no cracking, then probably unnecessary. I would go with what Tony says above.

An extra advantage of parging is getting the flat surface, meaning that the drywall adhesive can be used thinlly and evenly, reducing the volume of voids behind. On a very flat wall, I use the adhesive as wet as possible, almost like tiling but on a bigger scale. Plenty of mechanical fixings needed as well though.

Why can't blob and dob work?

tbhulse — Fri, 20 Jun 2008 22:33:26 +0100

Thanks for the input Mike. Will have to think about the heat situation some more - little chance of the aga going! The kitchen is quite long - 26 feet - with a pitched ceiling so a fair volume. Thought if I could get the air warm air out from the apex of the gables I can at least get some free heating in the rest of the house!

Why can't blob and dob work?

arthur — Wed, 17 Sep 2008 18:42:14 +0100

Posted By: Mike George

1. Ensure the wall is free of cracks and missing beds/perps
2. Parge coat of 6:1:1 sand:Cement:Lime. [Nice and flat]
3. Dot and dab PUR insulation backed platerboards, taking extreme care that the perimeter is covered with a consistent, uninterupted band of adhesive. Bands of adhesive also at 400mm ctrs
4. Ensure a gap between floor and boards of say 30mm
5.When adhesive is dry, fill the gap between the floor and the plasterboards with expanding foam.
6. Fix skirting, then plaster walls.
7. Allow timber to acclimatise then seal the junction of floor and skirting with a mastic.

Mike, if you have a parge coat on the external side of the insulation plus further plaster on the inside, why does it matter about insuring a continous, uninterrupted band of adhesive with the dot and dab?

Why can't blob and dob work?

Mike George — Wed, 17 Sep 2008 19:03:47 +0100

Hi Arthur,

Because doing so minimises air circulation which can cause convective heat loss. Still air is a good insulator, moving air is not. I try and get the parge coat as flat as possible - less drywall adhesive and also easier to fix the boards.

Why can't blob and dob work?

arthur — Wed, 24 Sep 2008 18:33:43 +0100

Posted By: Mike GeorgeHi Arthur,

Because doing so minimises air circulation which can cause convective heat loss. Still air is a good insulator, moving air is not. I try and get the parge coat as flat as possible - less drywall adhesive and also easier to fix the boards.

Mike, if that's the case, then is the effectiveness of loft insulation seriously inhibited by having ventilated lofts? My loft has bare slates above the insulation so plenty of air moving over it.

There's another (rather agressive) thread which may be about just this question but I was scared to get involved

Why can't blob and dob work?

tony — Wed, 24 Sep 2008 19:24:49 +0100

Me too! The danger with loft insulation is that as warmth and warm air escape and breathe up out through the insulation there comes a point where condensation can and does occur ( the dew point ). When this happens usually in or on the uppermost layers of the insulation, and in winter, it needs to ventilated away which is why there is all this fuss about lofts needing ventilation. Unchecked this could lead to mould forming, fungus growing, smells etc.

Keep the loft ventilated.

Why can't blob and dob work?

Mike George — Wed, 24 Sep 2008 19:38:34 +0100

Hi Arthur, that's the million dollar question. I have seen research which suggests that air circulation does indeed have an affect - I'll try and remember where it is. My feeling is that it depends really on the situation. An example I saw recently was in a converted loft.

The floor joists had been upgraded by means of adding extra timbers on top of them.
The spaces between them were insulated, but not completely, with fibrous insulation
A new floor was fixed above this.
Now here's the problem,

There was also continuous low level ventilation [at the eaves] The presence of a void like this was like having a mini wind tunnel from the front of the house to the back.
The visible signs of this below was a nice black line on the ceiling/wall junction caused by condensation.
Quite how much heat was being wicked away I don't know. But filling the voids at either end of the floor certainly made a big improvement to the rooms.

I don't know how much heat is lost in a more conventional loft in this way. But I think it is more than the steady state u-value would predict. I will try and find the report I mentioned. The other method is via Dynamic Simulation software which would take a fair amount of work. Its why most people tend to add as much as they can I believe. If you have 250 -300mm I think you will be fine. Even less is still good as it is the initial thickness which saves the most energy. Doubling the thickness does not double the savings - there is a law of diminishing returns.

I apologise for my aggressive post in the other thread. I try not to respond to provocation normally.

An important issue for me is this: The trouble with u-values is that as they get lower the margin for error in calculation error becomes more critical. When regulations were first introduced. requirements for walls were around 0.6W/m2K. This compared to an uninsulated wall of say 1.0W/m2K or even 2.0W/m2K. This was achieved with quite a minimal thickness of insulation.

It didn't really matter if the calculation was out by say 0.05W/m2K. Now though, when we are suggesting changes from say 0.18W/m2K to 0.13W/m2K an error of that magnitude becomes more important.

Why can't blob and dob work?

Mike George — Wed, 24 Sep 2008 19:42:49 +0100

Like tony says - keep it ventilated.