4" Insulation Board in a 6" Cavity

PC00123424

Hello All,

In our building project we have ended up with 4" cavity board in the 6" cavity. I was under the impression the cavity would be completely filled but, here we are. Not going to go into detail as to how it ended up like this, not enough time to monitor the site from overseas basically and the walls and roof are not getting ripped down now.

So on to the question, what are opinions on attempting to fill the 2" gap with spray foam working from the inside? Outside is rendered so would prefer to not disturb that. There's nothing on the blocks inside yet. Roof will be spray foam insulated and insulated plasterboard will go on all external walls. Heating will be air to water UFH and MVHR will be fitted.

What do we think, is there any value in trying to fill that 2" gap in the cavity?

Are there technical issues which would make attempting such a thing dangerous?

Thanks!

sydthebeat

What exact product was installed in the cavity?
And what u value have you to meet?

Water John

You could inject it now with bead, on the inside before plastering. Older houses would do that to upgrade the insulation. Many houses have 150mm cavity and only 100mm insulation sheet.

BryanF

Water John wrote: »

You could inject it now with bead, on the inside before plastering. Older houses would do that to upgrade the insulation. Many houses have 150mm cavity and only 100mm insulation sheet.

If you inject from the inside you upset the insulation in the cavity. Should be pumped from outside. Sounds like builder took shortcut. Pumping cavity now may not give the necessary u-value. Internal insulation may be necessary, but air-tightness must be addressed before proceeding.

Calahonda52

which face of the cavity is the 4" attached?

PC00123424

Thanks for your replies. Yes it's tied to the inside leaf so, I guess that would be a problem if working from the inside. So would pumping liquid foam from outside be an option now?

I don't know what U value has to be met, I've asked the engineer, also finding out exactly what product is in the cavity now.

BryanF: can you elaborate when you say air tightness must be addressed now?

Calahonda52

PC00123424 wrote: »

Thanks for your replies. Yes it's tied to the inside leaf so, I guess that would be a problem if working from the inside. So would pumping liquid foam from outside be an option now?

I don't know what U value has to be met, I've asked the engineer, also finding out exactly what product is in the cavity now.

BryanF: can you elaborate when you say air tightness must be addressed now?

What I expect he means is that you need decide where the A/T layer is going to be?
With the walls unplastered internally they are porous so if they get a coat of sand cement, that will be your A/T layer, which needs to be continuous.

PC00123424

The product in the cavity now is 100mm Kingspan Kooltherm Phenolic.

Ok. So sand cement continuous inside on all the external walls, insulated plasterboard then plastering, and do nothing else? Does this sound like the best course of action now? Or in addition to this, also try to get something in the 2" gap in the cavity?

Biker1

PC00123424 wrote: »

The product in the cavity now is 100mm Kingspan Kooltherm Phenolic.

Ok. So sand cement continuous inside on all the external walls, insulated plasterboard then plastering, and do nothing else? Does this sound like the best course of action now? Or in addition to this, also try to get something in the 2" gap in the cavity?

Surely someone has done a provisional DEAP Part L report and if so they are best positioned to advise.

sydthebeat

PC00123424 wrote: »

The product in the cavity now is 100mm Kingspan Kooltherm Phenolic.

Ok. So sand cement continuous inside on all the external walls, insulated plasterboard then plastering, and do nothing else? Does this sound like the best course of action now? Or in addition to this, also try to get something in the 2" gap in the cavity?

Why do you think the 100mm board is not good enough?
Do you have any professional advising / certifying

PC00123424

I have little knowledge in this are so I don't have a strong opinion on whether the 100ml cavity insulation is bad or wrong, I'm just asking for advice on whether the 2" gap should be filled or not? Other people have questioned why the gap was not filled fully with 6", as it was in their houses that were recently built.

We are just trying to make decisions to achieve as good as possible air tightness (working with the MVHR) and insulation as possible, with what we have so far.

We have been asking the engineer repeatedly about this but at the moment we are not getting responses.