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Community and Q&A

Moving building envelope in Century home attic

bdharv | Posted in General Questions on

I’ve recently purchased a 100 (ish) year old home in Ontario, Canada (on Lake Ontario directly north of Rochester, NY – climate zone 5A or possibly 6A). The attic currently has roughly R20 of old blown in insulation and I am looking to improve the insulation up there. 

To complicate the matter, the home has no ducting for air conditioning and is heated using hot water radiators. I believe the best route to provide air conditioning to the second floor bedrooms is some sort of ducted system in the attic (I am not interested in having an individual mini-split head in each room). Which has lead me to conclude that my best option is to move the building envelope to the roof deck. 

I am am having a hard time however, determining what the best method of executing this is. The attic currently does not have soffits and is served by a single roof vent. I had thought that I would get spray foam applied to the underside of the roof deck, but I’m unclear if I need venting added in each cavity or if I would just close the current vent off and have an unvented, conditioned attic space. 

One last question is the roof is fairly old and I’m not certain of the condition of the roof decking underneath. Should some of it need replacing when the roof is redone, would that damage/affect the insulation underneath? How is this best mitigated?

Thank you for any assistance your able to provide.


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  1. Expert Member
    Dana Dorsett | | #1

    At the moment Cobourg, Trenton, and Belleville Ontario are just barely in zone 6, but won't be by mid century:

    To hit R50 with closed cell foam alone would take 8" of climate-damaging HFC foam, or 7" of the more benign HFO blown foam. Neither would be considered a "green" solution, due to the high environmental impact of that much polymer. It's quite expensive too- CDN$11-12 per square foot.

    Per the IRC's prescriptives for zone 6 it would take at least 50% of the total R to be impermeable insulation fully in contact with the underside of the roof deck to do a combination of foam/fiber. That too is expensive.

    If re-roofing you COULD go ahead and install rigid foam above the roof deck held in place with a 5/8" plywood nailer deck through-screwed to the structural roof deck, and up to an equal amount (R-value-wise) of fiber insulation snugged up to underside of the roof deck. To hit ~R50 would take 5" of roofing polyiso up top with R25 fiber insulation between the rafters.

    It's often possible to build out a service chase box for running ducts and housing a mini-duct cassette above the existing attic floor joists for less money than converting it to a conditioned attic, since it can be insulated without using expensive foam. It's basically building an air tight platform tall enough to fit the mechanicals inside, and insulating over the sides and top. That keeps it all inside the thermal and pressure boundary of the house where it can work more efficiently. Every floor joist bay would need an air dam where it crosses under the platform walls, and the interior walls & ceiling of the service chase would have to be air tight (gyprock would be fine.) Access could be from an insulated weather stripped hatch up top, or a door in the platform walls if it's tall enough.

    In new construction this sort of approach can be done with a "Plenum Truss":

    Since the top of the platform isn't structural (it's only holding up insulation), it can be done with 2x4 framing lumber for pretty cheap. It's time consuming but for a smaller house and simple duct layout it's not bad.

    A soffit-to-ridge venting scheme requires that every bay be vented at both the top and bottom, which makes valleys and hips pretty much impossible to vent properly (without doing a lot of rework.)

  2. Expert Member
    BILL WICHERS | | #2

    I second Dana's suggestion of building a chase. You can hide the vertical ductwork in the back of a closet, or the corner of a room depending on your floor plan. It's often possible to run horizontal runs in joist cavities in the floor, but you will have to open some walls/ceilings to do this. Overall, you'll probably spend less money sneaking the ductwork in this way, but likely more time compared to spray foaming an attic (which usually takes a day or less).


  3. MattJF | | #3

    I think these insulated mechanical chases make a lot of sense and they seem under utilized. It is a very cost effective solution.

    While pretty straight forward, it seems like a good topic for an article here showing design examples for the insulting structure, duct layout, and cost comparisons.

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