I'm filling the voids under suspended floors with EPS beads and need to seal the vents, which appear to be galvanized cast iron (they attract magnets). The vents date from the late 1960s.

As you can see access is limited (just under 30mm) but I have 340mm long nozzles (for silicone etc cartridges) which could apply sealant to the vents.

Which sealant might last longest? (Mastic, Tec 7 etc.)

Any other ideas welcome.

<blockquote><cite>Posted By: GreenPaddy</cite>Is it possible to access them from the outside? Looks like daylight peeking through?

If they are grilles, remove them, insulate with rigid board and foam it in place. Then refit the grille and repoint. If clay "solid" vents, still fill the holes from the outside. Only need enough to stop the beads coming out, and air washing in.</blockquote>
Yes there is very good access from outside.

How do you remove the grill?

Posted By: tonyHit it with a club hammer !

Posted By: Peter_in_HungaryI would be inclined to leave it as it is and fill the gaps with mortar using a pointing trowel or similar pushing the mortar up to fill the vents.

Or just render over the whole thing - the quicker option

There might be an issue with mortar/render adhering to the metal. I would remove it entirely., insulate what you can behind and brick it up. EWI is definitely the best way IMHO

Hi PiH,

maybe you are correct only my experience is that mortar often struggles to remain stuck to metal during the winter when the metal gets colder than the surrounding wall. Though even if there is no problem for the small amount of effort, time and money it seems sensible to replace the vent with a brick..