Full Fill Cavity Insulation

[Mark Siddall]

There is a growing amount of interest in constructing full fill cavity walls of 100-150mm so as to prevent thermal looping (see https://aecb.net/forum/index.php?topic=745.0 for details on thermal looping.) I have a number of concerns about solution that I could do with being discussed/addressed, they are not necessarily insurmountable, but they are concerns that most people considering full fill solutions will have.

1) If water finds its way through the outer leaf and the layer of insulation gets wet what is the impact upon the U-value?
2) How is water meant to drain out of the cavity? Once concern is that is could track across the cavity insulation to the internal leaf, thus causing damp problems.
3) Related to 2, mortar snots will be more likely to form both cold bridges and weak spots for water tracking. Are there any tricks of the trade or is it simply a case of good site management?
4) Does full fill insulation increase the risk of efflorescence?
5) Does site exposure have an impact upon the success of full fill insulation?

What are peoples experiences (rather than theories) of installation and performance? Successes, failures and watchpoints would be appreciated.

Mark

[Mark Siddall]

I'm off to the AECB's “Designing genuinely energy efficient buildings that will perform” CPD Workshop next week. Hopefully I'll leave with some answers to the questions raised above.

Mark

[Mark Siddall]

News from the AECB workshop: –

1) Water will have an impact upon the U-value, but the use of full-fill cavity insulation is considered to ensure better thermal performance as thermal looping is avoided.
2) Water will drain in much the same manner as normal i.e. by gravity. All the normal considerations i.e. cavity trays apply.
3) Water tracking across the width of the cavity is considered to be unlikely as snots are likely to be discontinuous. Good workmanship will obviously help to mitigate any potential issues.
4) Risk of efflorescence does exist, thought spawling is a greater concern (see Wates House). Frost resistant brickwork is considered to offer a greater protection from spawling.
5) Full-fill insulation can be used on exposed sites. (Refer to the BRE document Thermal Insulation – avoiding risks, this documents the suitability of full fill cavity insulation to sites of varying exposure and cavity width. N.B. Weather struck joints in brickwork or tiled facades required i.e. raked joints are not acceptable and mineral fiber insulation rather than foam.) Another useful reference is BS 8104 (1992).

Mark

[Mark Siddall]

It struck me that there are two or three questions relating to full fill cavities that have not been addressed these relate to whether post filled blown insulation is more advantageous than built-in insulation.

* Will post-fill reliably fill the whole cavity?
* Compared to post-filled insulation will the horizontal coursing of the built-in insulation increase the potential for: –
i) convective bypass along horizontal joints?
ii) moisture tracking across the cavity (mortar debris)?
iii) mortar debris causing cold bridges?

To help determine which of these methods is the lesser of the two evils, has anybody seen any thermographic studies examining these two insulation installation methods?

Mark

[Mark Siddall]

With post-filled Pete's point makes a lot of sense i.e. cavity trays getting in the way etc. On Leeds Met angle, post-fill was used on Stamford Book. Which could be seen as support for this method (assuming they were not hammered in to this decision for commercial reasons by the contractors.)
I re-read the PDF on the “The End of the Cavity Wall?” thread last night….with built-in insulation one of the studies referenced notes that air gaps behind the insulation (i.e. on the warm side) of 5mm can degrade the U-value by >200%! On this basis perhaps post-fill works due to the fact that it rules out many small voids that could lead to thermal looping, even though a limited number of larger ones may exist. (At cavity trays perhaps a little built-in insulation?…..though it could be forgotten as the brickie wouldn't be in the general habit of insulating the cavity on the project.)

Mark

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