Will my insulation still work if it is not hard against the blockwork?

[AliG]

Scared to start this.

 

The builders started to put the Celotex on the outside of the inner leaf last week.

 

There are plastic ties which should hold it close to the Porotherm blockwork, But I noticed a couple of pieces were maybe 2cm away from the blocks.

 

I have asked the builder to tape all the joints and the cavity is not ventilated so it should be pretty hard for air to blow behing the insulation.

 

In this scenario do you think the gap materially affects the U-value or as long as the air cannot move does it not make a difference?

 

There will also be insulated plasterboard on the inside walls which means there will be some insulation everywhere no matter what.

 

FYI Wall build up is -

 

20mm render, 100mm porotherm, 50mm cavity, 100mm Celotex CW4000, 100mm porotherm, parge coat, dot and dab 37.5mm insulated plasterboard, plaster skim

 

Thanks

Edited February 6, 2017 by AliG

[TerryE]

The main risk with poorly fitting insulation is that there can be a pretty high temperature delta front-to-back and if there are gaps then air convection will start up in the cavity with the air (in your case) rising up between the Celotex and the inner porotherm leaf crossing over the Celotex in the cracks and dropping down the colder gap between the Celotex and the outer porotherm leaf.  This circulating air then effectively acts as a heat pump pumping heat around the main insulation layer and reducing its effectiveness. 

 

The main way to prevent this type of internal convective circulation is to fix the insulation tight to one surface and to fill any gaps with foam. If the outer leaf hasn't gone up yet, have you though about using something like sticky foam to anchor it back.  Might be worth a try.

[AliG]

Thanks Terry.

 

They have literally just started putting it on. Porotherm lets you build one leaf at a time so the entire ground floor inner leaf is built but they are only just starting the outer leaf.

 

I had read about the issues with the convection currents behind the insulation. But if it is only some pieces, I think it is probably either where the Porotherm blocks aren't flat or the ties are getting in the way, then the next piece of insulation being hard against the wall would presumably stop the convection current? Also if the gaps are taped the air would have to rise up the whole height of the building without hitting anything which seems unlkely? Or am I missing something.

 

I have impressed numerous time that I want it as tight as possible. I did wonder if you could infect stick all the outer insulation on but I haven't heard of that being done and decided that taping the gaps to prevent air being able to easily move around was the most cost effective solution.

[TerryE]

If you are on the case and working the issue with the builder then that's the main thing IMO. 

 

I would have though foam filling would be a safer bet than taping, but if you do tape then you'd need to use a decent tape with a 20 year guaranty or whatever.  @JSHarris was the trained chemist, so he's probably got a more informed view here.

[readiescards]

Mistake I made was taking my eye of the insulation installation as the build went on.  Initially the insulation on my build was poorly fitted and I talked it through with the main contractor and the salesman from the PIR foam supplier (Xratherm in my case) and things improved and I relaxed but the builder took on a new brickie who was slap dash and it was too late when I noticed the huge gaps.

 

So my tip to you is keep checking it continuously.

[Jeremy Harris]

Terry's right, foaming would be far better than trying to tape it, as I doubt that ordinary foil insulation tape (it that's what you're thinking of) will stay stuck for long inside a cavity.  Low expansion foam, applied with a proper gun, will be a permanent solution for sealing the gaps.  The only sort of tape that might stay on long term is the very expensive airtightness tape, like Siga, but that would cost more than a few cans of foam and a decent gun and not do as good a job.  It's easy enough to just go around with low expansion foam and do the sealing, as the low expansion stuff both adheres very well and, more importantly, won't carry on expanding and risk moving the insulation.

[AliG]

Thanks Jeremy. I will speak to the builder about this. I did think about it initially but my feeling was in the pieces that fit tight against each other it would be difficult to get foam between the insulation boards.

[Jeremy Harris]

If you can persuade the builder to foam the edges as they are installed than the problem of trying to get the gun in afterwards goes away.  FWIW, I found that the Soudal low expansion foam was about the best balance in terms of price and performance, and it's usually available pretty widely.  The guns come with conical nozzles with a tip around 2mm wide that will usually work fine with board gaps.  The plastic conical nozzles that come with the guns are a push fit and I found that they were easy to dislodge, so tape around the outside before use to help keep it in place. 

Edited February 7, 2017 by JSHarris

[AliG]

That was exactly my thought on the way into work. If they put the boards on in rows then put a foam bead at the edge where they meet the wall then effectively they are gluing them together

[Nickfromwales]

5 minutes ago, AliG said:

That was exactly my thought on the way into work. If they put the boards on in rows then put a foam bead at the edge where they meet the wall then effectively they are gluing them together

And if it's a continuous bead, then you almost certainly will stop convection airflow behind the insulation. That's what I'm doing to the insulated plasterboards I'm currently fitting on a refurbishment, continuous horizontal beads plus full beads at the sides, and head. Saves going mad at the foot where I can't see properly ( and want left clear to stave off damp bridging ( it's a basement )). 

 

Id stop the builder now and sit him down and directly address this with him letter by letter. I always find its best to offer an extra day's rate / whatever for any downtime this causes as if you add to the remit then they're entitled to be compensated accordingly imo.

 

[Declan52]

I would be asking why are the blocks not plumb and why are the ties stopping the insulation???

Are the blocks poorly made and need returned or are they from different moulds with one slightly wider??

Why are the ties in the way, are the beds to tight so the ties are getting bent to get the boards in???

[AliG]

I couldn't get close to see exactly. It was the weekend and I was looking through the fence. Most pieces looked fine, but not all and one piece in particular was far off the face at the corner. It may be that it just didn't have the clip on it yet.

 

I do know previously that although Porotherm blocks are ground to be flat at the top and bottom some of the blocks were not flat on the side faces so where they bulge out it may stop the insulation being flush.

 

Porotherm only has 1mm of mortar between the blocks. This does mean that the blocks seem to fit very tightly against each other it is more whether they are hard against the wall I am worried about.

[AliG]

I spoke to the builder and he believes that the pieces that appeared proud of the blockwork don't have the clips in place yet. Labourers put on some of the insulation, but they aren't allowed to mess with the bricklayers ties.

 

We agreed on using foam at the corners, around the steel beams and anywhere where the insulation is not hard up against the blockwork.

 

The bricklayers will be off site for the next week while the first floor slab goes on. So I will be able to more closely inspect the insulation when I am off the week after next.

 

He seems to understand the importance of making sure that the air behind the insulation cannot move around.

[AliG]

I went up today and had a good look around at the insulation.

 

The vast majority is tight up against the wall and looks pretty neat. The main issue seems to be on outside corners and at the ends of runs and I have asked them to seal these off.

 

I think this looks tight enough?