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Case study – making the underside of truss roof in a SIP house airtight with the Pro Clima system

February 29, 2012

3d model showing South side of the house with extension leading to garage

In October we were contacted by Bob in Pennsylvania, who was building a Passive House.  Bob wanted to show that a Passive House can look like a conventional American home – but of course also much more comfortable and energy efficient. The house is built with SIP-walls,  and basement with superior walls.However, it deviates at certain points from PH “guidelines”- it has a substantial amount of windows on the North side, a small wing connection to the garage and double high entrance and living – i.e. not very compact.  To offset these inefficient elements, the house uses HRVs from Paul/Zehnder that are >90% efficient according to their PHI certification, which is further enhanced by an earthtube.

Double height north facing living room with service cavity and roof trusses before INTELLO installl

However, this is not why we were called – the issue at hand was the truss ceiling. The intention was to insulate it with cellulose. However, an airtight layer hadn’t been clearly defined. Since there were a number of recessed can lights and other penetrations through the ceiling, and the fact that Bob really wanted to get an airtightness far lower than 0.6ACH50 – airtight drywall (ADA) certainly wasn’t going to get him there. First of all, drywall tends to crack in corners over time – compromising it’s airtightness – it also cannot achieve 100% tightness to light fixtures etc.  Actually airtight recessed cans aren’t airtight enough – so the number of ‘air-tight” cans was reduced. For good measure, of those that were left, Bob taped all their seams with TESCON VANA to make sure they were tight enough (see comment 7 on GBA, that airtight cans do indeed leak).

Furthermore there was a service cavity inside of the SIP, which is of course a good idea,

Pipes through top plate connected airtight with pieces of TESCON No.1

making it easy to run wires and plumbing. But there was no service cavity planned in the ceiling, making the wall cavity into a cellulose filled void that would become an air conduit to the vented attic – not a good idea.  To make the 1.5″ gap between the top of the SIP and the top-plate of the service wall airtight, a piece of CONTEGA FC was taped to each side. Further more all the wiring, pipes and ducts that were already installed were connected to this top plate with TESCON VANA.

Cove ceiling made airtight with INTELLO and TESCON, UNITAPE

These top plates – since all interior walls were build already – became part of the airtight layer. Of course, butt-joints and junctions had to be taped on the underside of these plates to create an continuous seal to the INTELLO Plus membrane, which was installed on the underside of all the trusses: in each one of the 3 bedrooms, 6 closets, hallways and 2 bathrooms.   The most complicated parts were the curved stairway-wall that had a large number of studs to be sealed and the re-entrant corners above the slanted ceiling of the double height living-room (see below).

Curved stairway wall airtight connection at sealing – note number of studs

When the ceiling/sealing was completed, the house was tested with a blowerdoor and reached 0.9ACH50.  It was discovered that the central vacuum connection to garage and some other holes were the main culprits.  Small leaks were found at some open ends of SIP walls (all OSB joints had been taped) and that the door to the garage, after the large holes were fixed the house came easily under 0.6ACH50.  Bob and his wife continued the to seal the house as his goal is to be around 0.2ACH50. The sheetrocking straight on the INTELLO caused some punctures as some screws missed 2×4’s, but as the attic wasn’t insulated yet, those leaks could be taken care off from above. It shows that the Pro Clima system can cope with recessed cans (gasket-ed with ROFLEX/TESCON No.1) and interior walls to make an American building as tight as a Passivhaus.

bathroom – recommendation to build hutch after membrane install

Watch on Youtube

INTELLO membrane after install during blowerdoor

Re-entrant corner sealed with INTELLO to ceiling and around studs at connection of double height living to bedroom

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One Response to Case study – making the underside of truss roof in a SIP house airtight with the Pro Clima system

  1. zeropassiv March 1, 2012 at 4:24 pm #

    Awesome detail! Post the final number when you have it.

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