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Insulating I-joist cathedral ceiling with Roxul

I have decided to use Roxul to insulate my cathedral ceiling constructed of I-joists on 24" centers.
The Roxul should be encapsulated on all sides.
I will be leaving a ventilation cap at the top of the I-joists, utilizing the flange as the spacer.
I was planning on using the steel stud version because it is 24.25" wide.

I first thought of using 1" XPS as the air sealing, cut to fit between the webs, caulked and taped.
I was wondering if using something like Tyvek sealed and stapled to the underside of the flange instead of the XPS?
It would be continuous, vapour permeable, and yet provide the air sealing.

Any thoughts?


Asked by Aaron Gatzke
Posted Jun 28, 2014 10:29 PM ET
Edited Jun 30, 2014 8:35 AM ET


4 Answers

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In general, energy-conscious builders usually recommend that air-barrier materials be rigid rather than flexible. Rigid materials are easier to seal at the seams; are often more durable; are usually less likely to leak at fastener penetrations; and are less subject to abuse by phenomena like wind-induced ballooning or billowing.

If you decide to use Tyvek as an air barrier, you'll need to tape the seams at the perimeter of each rectangle of Tyvek.

Answered by Martin Holladay
Posted Jun 29, 2014 5:57 AM ET


Thank you Martin. That is exactly what I needed to know.

Answered by Aaron Gatzke
Posted Jun 29, 2014 9:48 AM ET


When using flexible goods or membranes as the air barrier the seams have to be located at framing elements to have any chance of maintaining integrity over decades, but it CAN be done.

OSB/plywood or gypsum board sheet goods are the gold standard though, with sheet foam insulation coming in second. Sheet foam has long term shrinkage issues, and is not nearly as mechanically robust as wood or gypsum board, but more rugged than Tyvek or polyethylene, etc.

Answered by Dana Dorsett
Posted Jun 30, 2014 3:54 PM ET


You can indeed use a proper roof underlayment (ie not a housewrap like tyvek) that is very waterproof, like the a href="http://foursevenfive.com/product/solitex-mento-1000/">SOLITEX MENTO 1000. This material is super airtight as well, but allows outward drying at 38 perms. Giving you both an exterior airbarrier, protection against windwashing/vented air looping through your insulation as well as provides additional subroof. This extra roof works especially well when installed as indicated on the drawing - with a flying batten centered between your joists. Our ProClima sarking membrane and your suggested installation method was also used by Alex Wilson in the renovation of his house last year, see this blogpost.

Please keep in mind that an interior airbarrier does it better, which follows your mentioned intention to seal the insulation on all 6 sides, but lays the importance on your interior airbarrier. This will to keep the conditioned (and more humid) indoor air where it should stay - inside.

475 I-joist detail - venting under sheathing w ProClima SOLITEX MENTO.jpg
Answered by floris keverling buisman
Posted Jul 1, 2014 12:34 AM ET

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