Foam-free attic insulation?
Hi, I understand spray foam (or possibly rigid foam) is strictly recommended when insulating between the rafters of the attic Both prevent wintertime condensation at the sheathing and eventually rot and roof failure. However, I am failing to see what would be wrong with the following foam free approach to insulating my attic in climate one 4A . Leave 2 inch ventilation gap between underside of the rafter and then install water resistant barrier (but highly vapor open) such as Solitex Mento 1000. Then rockwool insulation batts, followed by a smart vapor barrier such as Intello plus, followed by drywall. I would think the risk of moisture condensation against the underside of the roof is tiny, possibly zero. The vapor barrier should prevent vapor traveling from the interior against the sheathing and if any small amount of vapor does make its way through, there is the vent channel to transport it away. Are there any other problems? Maybe this approach is okay? Your views are much appreciated, thanks in advance. – Matthias.
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Matthias. Do you have ventilated or unventilated attic? Do you have HVAC ducts or equipment in the attic? Your responses will help us to provide useful feedback.
Matthias,
The insulation method you describe -- creating a vent channel under the roof sheathing, and filling the rafter bays with fluffy insulation -- is a time-tested and code-approved method of insulating a sloped roof assembly. It's been the standard way to proceed for 40 or 50 years.
Of course, this method assumes that you have soffit vents and a ridge vent. The main caveat is that the method doesn't work on roofs with valleys, hips, dormers, or skylights. You need a simple gable roof or shed roof for the method to work.
For more information on this approach, see these two articles:
How to Build an Insulated Cathedral Ceiling
Site-Built Ventilation Baffles for Roofs
Steve, I plan on creating a vent channel under the roof. There are no HVAC ducts in the attic anymore, I replaced the central AC with mini-splits and decided not to put any heads in the attic, I might put a window AC in the attic though.
Martin, thanks. I was not sure whether rigid foam was required immediately beneath the vent channel. I interpreted what I read in GBA on this issue as: If you do not have foam, any moisture in wintertime might create mold at the first condensing surface, which might be my WRB that forms the vent channel . But maybe since it is vapor open, it is not actually a condensing surface. In any case, thanks for clarifying things.
The other caveat is that the depth of the rafters may prevent fitting enough insulation, so you might need to add depth below the rafters.
Matthias. So you have soffit vents and ridge vents (or can install them to facilitate ventilation)?
If you are considering installing a window air conditioner, does that mean you want to use the attic as living space?
To meet code, you need r-49. With rockwool and a ventilation space, your rafters would have to be something like 13 1/4 inches deep (assuming r-4 per inch and a minimum 1 inch gap). You could make the rafters deeper, but it might be easier to air seal and add insulation to the attic floor. That's assuming you don't want to use the attic as living space.
PS. Martin and Charlie certainly know more about best practices. They will give you the best advice on how to proceed.
Charlie and Matthias,
One of the articles I linked to, How to Build an Insulated Cathedral Ceiling, discusses several possible ways to address the problem of "my rafters aren't deep enough."
Thanks ! Yes, I want to keep the attic conditioned living space, hence floor insulation is not an option. I was thinking of adding 2x6s perpendicular to the rafters to create 5.5 inches extra space for insulation. I would need some kind of metal corner angle for them to hold on to the rafter, I am sure Home Depot sells something suitable. That gives me effectively R-38 ( R-15 Roxul of 3.5 inches in the original Rafter Bay with the vent channel and R-23 Roxul of 5.5 inches in the new perpendicular bay). That is less than code, but much better than what I currently have. Do I need to worry about my rafters being able to sustain the extra weight of all this Roxul? Martin, thanks for the link
Matthias. You should read "How to Build an Insulated Cathedral Ceiling," which Martin calls out in Post 6. I suspect your best option will be to used closed cell insulation in combination with air permeable insulation to create an unvented roof assembly. Your original idea might work if the roof is simple, if the ventilation channels is installed and detailed correctly, and if you create extra depth for insulation.
I will be interested to read Dana's response (if he is available to review this thread).
Using stamped sheet metal framing ties rather than nails to hang the perpendicular 2x6s increases the thermal bridging, but the approach can work. If nailed the performance will be quite a bit better than R38 batts between joists/rafters due to the enhanced thermal break, but that is undercut if you use framing ties.
You won't be quite to IRC 2015 U0.026 code max for the assembly (which it would, if it was R38 continuous insulation, or even R36 after adding in air films and ceiling gypsum, etc), but it wouldn't be terrible. It would still be under U0.030 (R33 whole-assembly), which was code max for zone 4 under IRC 2009.
With a 2" vent gap and soffit-to-ridge ventilation in every rafter bay there is no need for an interior side vapor retarder- standard latex ceiling paint is more than adequate, especially for climates as temperate as zone 4A. If it were zone 7 or 8 the Intello Plus might have some rationale even in a vented cathedral ceiling, but not zone 4.
If you can't achieve soffit to ridge venting in every rafter bay, it may be advisable to do something else, at least in those blocked bays that can't be properly vented.
Dana, thanks so much. So would you suggest I just drive long nails (about 10 inches?) though the 2 by 6 to attach them to the rafters? Will that be durable? At a length of 10 inches they would go 4.5 inches deep into the rafters. I would not expect that the small amount of metal framing ties makes a big difference for thermal, but I am not an expert. I was thinking of adding the smart vapor barrier interior side, because I do not have ridge vents, but a highly vapor open slate roof assembly, But strictly speaking it cannot vent as well as proper ridge vented roofs. Thanks again !
Mathias,
Another way of increasing the depth of the existing roof structure is to use 2"x4"s fastened with plywood gaskets. This provides a thermal break and uses lighter framing lumber.
You can see this illustrated in a house built by Dan Kolbert, a frequent contributor to this site:
http://www.kaplanthompson.com/_images/publications/09.06-jlc.pdf
Matthias: Not ten inch nails, just toenail the 2x6 to the rafter edges.
Carbon steel is about 350x - 450x more thermally conductive than wood, so any steel bridging the exterior of the wood from the 2x6s to the existing rafters moves dramatically more heat than would be going through the wood. A couple of toenailed nails have far less surface area in contact with the wood, and far less cross section to the steel than a framing tie.
A slate roof on skip-sheathing usually provides better ventilation than soffit to ridge venting, and it should be fine to leave just barest 1/4" of gap between the Solitex and the bottom of the skip sheathing as a capillary break and drainage path for liquid water that get's blown past the slate. Code officials unfamiliar with slate might still insist on the 1" code minimum necessary for plywood sheathed roofs. See:
https://www.greenbuildingadvisor.com/community/forum/energy-efficiency-and-durability/21399/proper-way-insulate-unvented-slate-roof
If you take Dan Kolbert's add-on-truss chord approach, use 2x2s rather than 2x3s for a better thermal break. That works great with blown/sprayed insulation, but with batts it ends up being an endless amount of detail sculpting with a batt knife to get a good fit.
[edited to add]
With 2x3 truss chords you would have 3" between the rafter & chord, and it may be possible to cleanly compress & stuff and cleanly trim R15s flush with the rafter & truss edge in that space for a good fit. I'm not 100% sure how well that would really work in practice.