Comments on: Building Science Term of the Week: Ratio Rule https://www.energyvanguard.com/blog/building-science-term-of-the-week-ratio-rule/ Building science knowledge, HVAC design, & fun Thu, 04 Nov 2021 17:19:21 +0000 hourly 1 https://wordpress.org/?v=6.6.2 By: Jonathon Neville https://www.energyvanguard.com/blog/building-science-term-of-the-week-ratio-rule/#comment-17550 Thu, 04 Nov 2021 17:19:21 +0000 https://www.energyvanguard.com/?p=6228#comment-17550 1. Why does this apply only if insulating above-code?
2. If the insulation inside the cavity is vapor-impermeable (closed cell spray foam), do these or other ratio rules apply?
3. Mold requires warmth to grow. Might sandwiching the roof deck between insulation mean it is warm enough for to grow mold in winter?
4. My zone 5b vaulted ceiling should have 41% of the insulation exterior, according to this table, but will have 62% exterior. (R10-R15 spray foam is against the underside of the roof deck, and the plan is to add R-20 on top of the roof deck, then a ventilation channel, …) (R31 is Code for cathedral ceilings in Ontario.)

]]> By: Allison Bailes https://www.energyvanguard.com/blog/building-science-term-of-the-week-ratio-rule/#comment-16137 Sat, 31 Jul 2021 15:39:20 +0000 https://www.energyvanguard.com/?p=6228#comment-16137 In reply to Jon R.

Jon R, yes, that’s a good point. It’s stated in a footnote to Table R806.5. Here’s the full text:

Alternatively, sufficient continuous insulation shall be installed directly above the structural roof sheathing to maintain the monthly average temperature of the underside of the structural roof sheathing above 45°F (7°C). For calculation purposes, an interior air temperature of 68°F (20°C) is assumed and the exterior air temperature is assumed to be the monthly average outside air temperature of the three coldest months.

That’s where the science for the ratio rule comes in, as explained by Joe Lstiburek in the paper I mentioned above and first (I believe) given in a paper by Canadian building scientist Gus Handegord. Joe gives the reference to Handegord’s paper in a footnote in his article.

]]> By: Jon R https://www.energyvanguard.com/blog/building-science-term-of-the-week-ratio-rule/#comment-16136 Sat, 31 Jul 2021 15:22:16 +0000 https://www.energyvanguard.com/?p=6228#comment-16136 It’s worth noting that code may allow no foam with “Alternatively… maintain the monthly average temperature of the underside of the structural roof sheathing above 45°F”.

]]> By: Allison Bailes https://www.energyvanguard.com/blog/building-science-term-of-the-week-ratio-rule/#comment-16069 Wed, 28 Jul 2021 14:58:59 +0000 https://www.energyvanguard.com/?p=6228#comment-16069 In reply to Stephen.

Stephen, this section of the IECC isn’t relevant to the ratio rule I discussed in the article. This section is talking about insulation on the floor of an attic, not at the roofline.

]]> By: Stephen https://www.energyvanguard.com/blog/building-science-term-of-the-week-ratio-rule/#comment-16068 Wed, 28 Jul 2021 14:24:39 +0000 https://www.energyvanguard.com/?p=6228#comment-16068 In reply to Allison Bailes.

Thanks
But for reference the R38 is in the code.

R402.2.1 Ceilings with attic spaces. Where Section
R402.1.2 requires R-38 insulation in the ceiling, installing
R-30 over 100 percent of the ceiling area requiring insulation
shall satisfy the requirement for R-38 wherever the
full height of uncompressed R-30 insulation extends over
the wall top plate at the eaves. Where Section R402.1.2
requires R-49 insulation in the ceiling, installing R-38
over 100 percent of the ceiling area requiring insulation
shall satisfy the requirement for R-49 insulation wherever
the full height of uncompressed R-38 insulation extends
over the wall top plate at the eaves.

]]> By: Allison Bailes https://www.energyvanguard.com/blog/building-science-term-of-the-week-ratio-rule/#comment-16066 Wed, 28 Jul 2021 11:34:37 +0000 https://www.energyvanguard.com/?p=6228#comment-16066 In reply to David Butler.

David, thanks for adding that. I should have mentioned that in the article. And thanks for the correction on the table number. I’ve corrected the typo.

]]> By: David Butler https://www.energyvanguard.com/blog/building-science-term-of-the-week-ratio-rule/#comment-16046 Tue, 27 Jul 2021 20:19:19 +0000 https://www.energyvanguard.com/?p=6228#comment-16046 It’s worth pointing out that a ‘hybrid roof assembly’ in the context of the IRC also (and importantly) covers the configuration where both air impermeable (i.e., foam) and air permeable insulation are installed in combination on the underside of the roof deck. In that case, the IRC table you refer to specifies minimum R-values of air impermeable insulation that must be in contact with underside of roof deck.

BTW, that table is actually R806.5 in the 2018 IRC, not R806.4. It changed in 2012.

]]> By: Allison Bailes https://www.energyvanguard.com/blog/building-science-term-of-the-week-ratio-rule/#comment-16034 Tue, 27 Jul 2021 15:23:08 +0000 https://www.energyvanguard.com/?p=6228#comment-16034 In reply to Stepehn.

Stephen, if you’re in CZ 5 but your local code requires only R-38, your code is different from the model code. Of course, you should check with the building department and see what they require if you’re doing a hybrid assembly like this. If they let you do whatever you want, I’d use the CZ 5 percentage (41%) to determine how much needs to go on top of the roof deck. That would be R-15.6. Or you could do some modeling. Find the average temperature for the three coldest months in your area, and determine what R-value above the roof deck will keep the sheathing at 45° F or higher with a total R-value of 38.

]]> By: Stepehn https://www.energyvanguard.com/blog/building-science-term-of-the-week-ratio-rule/#comment-16032 Tue, 27 Jul 2021 14:43:21 +0000 https://www.energyvanguard.com/?p=6228#comment-16032 How do the ratios work if the house specs meet R402.2.1 (IECC 2018)?
In climate zone 5 the roof would only need R38 to meet code.

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