Help! Building a log home in southeastern British Columbia, Canada
If anyone out there can help us – we would be very grateful. We are building a 2400 Sq Ft home in SE,BC Canada. 40×60 Cross Gable Roof. We are in Zone 6 – would like to insulate the roof to R52 or higher. Have decided to proceed as follows. Over Roof Purlins – 2×6 T&G Decking – followed by a Vapour Barrier (MemBrane) – followed by 16″ I-Joist Truss Rafter 24″ Centres – To be insulated with Closed Cell Urethane Foam (1-2″ only as a Vapour Barrier) – followed by offset Roxul Batts – leaving a 2 ” air space (cold roof installation) just below the 5/8″ T&G Plywood Sheathing to be Taped – Followed by Grace Ice and Water Shield HT along valleys, rakes, eaves and penetrations – followed by Grace Triflex/Titanium UDL 50 – Topped off with a Standing Seal Seam Green Metal Roof!!! Is this overkill – good or am I missing something? I’m acting as a General Contractor and have researched and read dozens of articles of the internet and in roof books trying to this the best and most economical manner possible. I would like to make this ceiling as airtight as possible but concerned with moisture/insulation?
Please advise. Thank you – Leslie and Mark Kozlowski, Delta, British Columbia.
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Replies
Leslie and Mark,
I doubt if this is the least expensive way to go. The inclusion of a layer of spray foam almost guarantees that this will not be the least expensive approach.
You might want to come up with a system that uses cellulose rather than Roxul mineral wool if you are interested in a low-cost solution.
A few other comments:
1. You forgot to include an air barrier between the top of your mineral wool and the ventilation cavity. As described, there will be some degradation in thermal performance due to wind-washing.
2. I-joists are quite thin, so this type of framing requires extra-wide insulation batts. Make sure that Roxul makes batts that are wide enough for I-joists before you settle on this approach.
3. You can't vent a cross-gable roof (see image below), because the valleys interfere with soffit-to-ridge venting. You will need to come up with an unvented approach rather than a vented approach.
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Hi Martin:
Thank you for your prompt reply! I am panicking right now - It never crossed my mind that I can't soffit vent a cross gable roof!!! So now my Final Question is: Given that this is a Log Home in Zone 6 in South Eastern BC, Canada - What would be your professional recommendation for insulating this type of roof to maximise energy efficiency, minimise and form of condensation given the fact that we will go with a standing seam metal roof - The house is 40x60 and because of the long spans we will have to use I-Joists? Do we still need a vapour barrier above the T& G Decking. We will not be installing any Pot Lights in the ceiling - we will run outlets from the Log Ridge Pole 18" in Diameter and use floor and wall lighting to minimise e any ceiling penetrations. The only penetrations we will have will be the plumbing vents and the stainless steel chimney vents from the main and second floor fireplaces. We are environmentally conscious - but it is my understanding that with an unvented roof we will to insulate the joist with closed-cell polyurethane to prevent air leakage and moisture problems. Thank you for taking time out of your busy day just before Christmas to answer these questions.
Leslie and Mark Kozlowski, Delta, BC Canada
Log homes are notoriously difficult (impossible?) to air seal adequately, due to the seasonal dimensional changes of the wood with humidity & temperature. The roof may be the least of your thermal-efficiency problems!
How committed are you to the log home concept?
Leslie and Mark,
Q. "What would be your professional recommendation for insulating this type of roof?"
A. If you want the insulation to follow the roof slope, and if you are limited (as you are) to an unvented roof assembly, then the type of roof you will be building is called an unvented insulated roof assembly.
Here is a link to an article that describes all of the different ways that you can insulate this type of roof. (Keep in mind that you should limit yourself to unvented assemblies): How to Build an Insulated Cathedral Ceiling.
You may also want to read this article: How to Install Rigid Foam On Top of Roof Sheathing.
Leslie and Mark,
I'm guessing that you may be too far along in your project to reconsider your log walls, but Dana Dorsett is 100% right. Log walls are almost impossible to air-seal. This creates a big problem for green builders interested in energy efficiency.
The only satisfying solution is to create a "house within a house." The log walls end up as siding. (Very, very expensive siding.) Inside the log walls, you build stud walls. The stud walls include insulation, and allow the installation of an air barrier.
The "house within a house" approach can work, but there are still a lot of tricky details -- especially at intersections with joists and beams. Good luck.