Valinor

PIR in cavity wall and avoiding air gaps

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Due to site constraints we had to do the outer skin first, and will be now adding the insulation and building the inner skin from Celcon blocks. A few questions I have which I would appreciate any advice on 😃

 

How come in some images the insulation doesn't go to the bottom of the cavity? Wouldn't that reduce the wall's insulation properties? Is there anything wrong with going all the way to the bottom of the cavity?

 

As I'll have to cut and place the insulation in the cavity before we lay each course of blocks against it, I was wondering whether it matters which side of the insulation I place tape on. As in, looking at the illustration below, from left to right, would this be ok at preventing airflow?

bricks - 25mm void - 75mm PIR - tape - Celcon blocks. 

Or should I go to the effort of trying to tape on the side with the 25mm void?

 

Any tips for making sure there is no gap between the PIR and Celcons when doing it in this order? Anything I should pay attention to when helping the brickie? I would have much rather had the Celcons up first and been able to place the PIR against them etc, but that wasn't possible. 

 

Thanks!

 

2030085730_Screenshot2021-10-10at22_08_11.jpg.b71399ec093f60641613b540db4ecba0.jpg

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The gap left at the bottom is for droppings of motar. It's a standard detail. 

When you say your going to have to cut the PIR insulation what do you mean?? 

Are you buying 8*4 sheets and cutting these up as that will be disastrous. If you intend to use PIR insulation in a cavity then you buy the cavity boards. They are 450mm high so suit wall tie spacing. Plus they are tongue and groove. Your never going to be able to cut sheets up and somehow tape the joins up with the outside skin already built. 

If the brickie  doesn't push any motar out when bedding the wall or setting down the block then the cavity boards will be tight but being realistic not every board will be. 

For this build your best bet would be getting beads blown in. 

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3 minutes ago, Declan52 said:

The gap left at the bottom is for droppings of motar. It's a standard detail. 

When you say your going to have to cut the PIR insulation what do you mean?? 

Are you buying 8*4 sheets and cutting these up as that will be disastrous. If you intend to use PIR insulation in a cavity then you buy the cavity boards. They are 450mm high so suit wall tie spacing. Plus they are tongue and groove. Your never going to be able to cut sheets up and somehow tape the joins up with the outside skin already built. 

If the brickie  doesn't push any motar out when bedding the wall or setting down the block then the cavity boards will be tight but being realistic not every board will be. 

For this build your best bet would be getting beads blown in. 

 

Thank you for taking the time to reply

 

Unfortunately we purchased all insulation before the March price rise, we've got 450mm x 1200mm though without tongue and groove as those were too expensive (we went for Recticel Eurowall - PIR Cavity Wall Insulation Board)

 

When talking about cutting I meant for all the areas around a window opening or where building control specified more brick ties (where I imagine I can insert a knife into the PIR and the wall tie will go straight through. 

 

I still want to tape the joints of the factory perfect edges etc. 

 

To prevent the brickie pushing mortar out the back - would it make sense to try and convince him to bed the mortar more to the inside, with there being less of it on the insulation side and likely to push out that way? Or would that weaken the walls too much from a structural point of view?

 

 

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Cut the boards with a hand saw.

Physically there is no way your getting your hand down a 25mm space to put tape on the joins without pushing the wet blocks out of plumb. 

They will bed the wall like that but with the best will in the world unless your paying mega bucks for him to clean each and every bed as he goes and double checks each board and lift off some blocks then some are going to have gaps.

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I’ve been put in this scenario myself-it’s an absolute #**#ache. 
As Declan said-blown in beads or full fill cavity batts are the practical solution. Otherwise,you’ll end up with an improperly inststalled,underperforming insulation system. 
If the PIR has gone up in price & is still packaged,would you not get the majority of your money back?

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11 hours ago, Declan52 said:

 

 

3 hours ago, Brickie said:

 

 

Thank you for both your replies! Yes PIR has gone up in price and that's a very good idea. Will need to mull it over as I do appreciate the insanity of trying to do it as it was originally planned!

 

 

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Valinor, my walls are going up as we speak, I'm using full fill PIR insulation, cost me a fortune. The whole thing has been a complete and utter nightmare. My first self-build, 3 years in planning, bought into the science of everything, had multiple site meetings in the months running up to the build, had everything on site ready, colour coded notes for everyone. My architect said in 50 years he's not seen anyone better prepared. Totally pointless. It just doesn't work practically with the way the trades work, and they will not adapt to you. For example if brickies are on price work, all they want to do is lay brick, and QUICK. Air gaps, mess, thermal looping, you'll get the lot, even if you try to do the insulation yourself (I've been fitting my PIR myself around them since the first row). If you have the time, sell your PIR, at a loss if necessary. Get the brickies to concentrate on laying brick straight and accurate, lay the wall ties properly, wipe mortar away on all sides. Once they're off site, pour EPS beads in from the top ensuring all gaps are filled. I've debated and been advised by our knowledgeable friends on here to take mine down and start again. Not an option for me, so I've paid fortunes for full fill PIR, now I'm going to have to pay lots more and install insulated plasterboard on the inside to reduce the cold bridge effect, and lose internal space for the pleasure. EPS beads all the way.

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Beads or batts or closed cell foam. 

 

Rigid insulation has no place in a cavity wall. Blockwork is almost impossible to get to a perfect smooth finish with no mortar droppings. 

 

 

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