The service cavity – making airtight construction easy!

In the development of Passivhaus, the focus on airtightness of the building envelope had as goals to both increase energy efficiency as well as protect the structure from condensation and all its disastrous consequences (mold, fungus, rot leading to sickness and structural failure) – while making high indoor air quality more assured, because you can’t control the quality of the air until you literally control the air.   Beyond these important benefits is one that shouldn’t be forgotten: airtightness increases comfort, as drafts will be a thing of the past. Given these pretty convincing reasons, what should our target be and how does a service cavity help us achieve it?

The challenge to reach recommended airsealing results – either the German code minimum of 1.5 Air changes per hour at 50 Pascal (in short ACH50) or to meet Passive House requirements at 0.6ACH50 (the pressurization test n50 maximum flow rate) – has been successfully achieved in the countries in the old world. There are actually competitions there, were bragging only starts at approximately 0.15ACH. In the USA we are making great progress but should use some lessons learned from the folks that have been doing it for a while.  They reduce the number of penetrations / interruptions of the airtight-layer to an absolute minimum by the extensive use of an “auf Deutsch” installationsebene (in American: a service cavity).

Wood stud wall with service cavity - foursevenfive.com

This wall inside a wall, can be a structural 2×4 wall in light-weight construction. Or you can run the a 2×2 (or 2×3 on edge) horizontally when the structure is on the exterior. This makes the electricians life pretty easy as he no longer has to drill through a large number of studs to get the outlets where they are needed. The back of this service cavity can be made airtight with:

-          an intelligent vapor retarder (INTELLO Plus or DB+)

-          interior structural sheeting (preferably plywood) with all edges taped (UNITAPE, TESCON Vana)

The service cavity can also be employed in a gut renovation of a Brownstone or other buildings with structural brick walls. The air-tightness of brick and mortar when the blowerdoor is creating a pressure difference of 50Pa is surprisingly low. By offsetting a first 2×4 at sufficient depth from the brickwork for insulation purposes, and adding airtight layer plus a service zone to the inside of this assembly. A verifiable airtight layer can be constructed quite quickly, when compared to trying to coat the brick wall with an liquid applied membrane.

Brick with INTELLO service cavity - foursevenfive.com

The larger stud walls behind the membrane, should be filled with blown-in cellulose after installation of the INTELLO Plus or any fibrous insulation can be installed before hand. The blown-in holes can be quickly and easily repaired, either on INTELLO Plus or wood sheeting with UNITAPE Patches. The building is now ready to be blowerdoor tested, which should be early enough in the construction phase to correct any mistakes and get the building as tight as a Passive House / Enerphit project.

INTELLO service cavity after cellulose blow-in (clioma.ie)

After all services are installed and before installing the drywall, optional fibrous insulation can then be installed in that cavity.

Of course you would want to make sure that during this period no new holes are made in the airtight-membrane, so instruct plumbers, electricians and other trades that might carry an utility knife to not act like Zorro. After the insulation and drywall is installed, the airtight layer behind the service cavity will be protected from nails to hang pictures, screws to hang shelves and even wires for phones, cables and internet etc. can be easily installed without compromising the air-tightness of the building for years to come.