Airtightness in 1950s refurb

[Robert Rickey]

I am now drawing up our house in anticipation of starting detailed design using the PHPP. I have an understanding of the issues regarding insulation and thermal bridging and how to assess them. Airtightness, however, is terra incognita for me in a refurb. At the moment I am supposing external wall insulation, enhanced loft insulation (clever detail needed at junction with wall insulation) and replacement of the suspended timber floors with EPS and concrete. Two main areas are of concern are the first floor to wall construction (built-in joists in cavity wall construction) and the ceiling. Am I looking at tearing out the plasterboard ceilings to install an air barrier, or just a thorough-going sealant job?

Any examples of tested refurbs? Infiltration rate will obviously affect decision-making about HRV vs MEV and so on.

[Nick Grant]

Hi Rob

With external insulation retrofit, it is usual to do the airtightness layer externally before putting on the insulation. This sorts the floor joists, stairs etc etc.

Need to join this to roof/ceiling airtighness layer at some point.

Alan Clarke and Andy Simmonds have both achieved very respectable airtighness this way.

[Nick Grant]

Whoops I missed the cavity wall bit but Alan has covered it.

[Anonymous]

Alan ,was the airtightness barrier with the parge/render applied before the EWI boards or is it just the EWI finished top coat that does the job ?

cheer Jim

[Nick Grant]

I'm not Alan but is the parge/render under the EWI.

[Anonymous]

Slighlty confused , did you mean ” it's the parge/render under the EWI ” ?

cheers Jim

[Robert Rickey]

I saw Andy Simmonds' slide show on his house, and I appreciate the value of building the roof insulation up from the rafters to avoid a thermal bridge at the wall junction. I am wondering if there is a workable way to leave the roofing in place and insulate inwards. The wall junction becomes difficult, and tying in the air barrier likewise. Or should I just forget it and go for the larsen trusses on top?

[Nick Grant]

Yes, the render under the EWI forms the airtight layer. Windows and roof membrane are taped to this before adding the insulation on top.

Rob, as you say, if you insulate at ceiling level then there is a thermal bridge at the ceiling to wall insulation join and an issue with air-barrier continuity. I would not rule out a clever bodge but if your roof blows off, see it as a blessing in disguise!

🙂

[Author]

Posts