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Looking for advice on insulating tongue-and-groove timber roof deck from below

David_Gregory_CZ3_CA | Posted in Energy Efficiency and Durability on

[Apologies in advance for some redundancy with other threads here; have tried to do my homework but there are some differences in my scenario that may be important…]

There are probably thousands of ~1950’s churches in the USA with uninsulated T&G roof decks, most in places that would benefit from reducing heat gains / losses. Insulating above and re-roofing is well documented, but in some cases I’m aware of (brand new shingles, tornado / hail country, also asbestos shingles…???) that is problematic / a hard sell. I’m trying to figure out if any of the approaches to insulating below make more sense, or if above is still the best (or only) option. Or in certain zones just increase the albedo and give up on reducing winter losses (but also reduce winter gains)…

Does anyone have experience, comps, references, details, or just considered opinions, including rough cost/benefit? I have been reading threads here (Lara in PA, Michael in NH), but residential is a little different – sanctuary between-beam spans longer, access issues (need high scaffolding), aesthetics (reducing apparent glulam thickness)…

Central Oklahoma, zone 3B
Pitched Roof, east/west aspects, high cathedral ceiling
Assembly (in->out): T&G deck, felt (assumed), asphalt shingle (mid-dark color).

Intuition says it would be worthwhile to insulate, or otherwise reduce heat gains/losses. (But summer gains perhaps bigger issue than winter losses?)

‘Above’ is straightforward, lots of options, good access…but the roof was very recently replaced, so it feels like a waste to re-do the whole weather plane, besides being difficult to increment (multi-wing building). In the end, might still be best…Or, just wait until the next hailstorm, or tornado… (>.<)! Below is tricky: Could use Atlas nailable or similar and add thin T&G under (to maintain ‘look’ and acoustics); can’t just do foam panel only, because of fire ratings. Can’t nail T&G over foam of much thickness – any way to use adhesive? (seems sketchy), or is there an integrated product? Battens/furring for T&G or drywall would create thermal breaks, add weight…For aesthetics, High R/in and light would be best…vacuum panels? 😉 BUT, I wonder if the real problem is moisture / condensation…? I think we’re talking about a ‘hot roof’ here, but that term is so generic I haven’t found much yet searching with it on GBA…(just #19 Martin’s to Michael) So, risking repeats, here’s my concerns: Insulation under is the opposite of the ‘perfect (roof, wall, etc)’…structure is on the side with highest temperature swings. Adding insulation will increase the swings (expansion/contraction issues?), plus discontinuity at the roof/wall intersection. In addition, if moisture passes up through the insulation layer (i.e., if not a ‘perfect’ vapor barrier, which is hard to imagine getting), it will condense against the structural deck, absorb into the wood, or trap between the wood and the roofing (can felt+shingle allow drying to the outside? hard to imagine) – so now you’ve got insulation and structure sandwiched between two ‘impermeable’ membranes. Could you shift the dew point to ‘inside’ the wood deck? I think it’s too variable, and with the joints it’s going to move around alot. Vapor permeable insulation underneath for drying out to the inside in summer could just exacerbate the winter condensation problem, as would cycling winter heating off, while venting between a kind of ‘hung’ insulation and deck (like CrossVent, but re-arranged – furring strips etc.) would only reduce your R (unless adding foil and having only the minimum air movement could make it a tie) by speeding up convection loops, and anyway would likely lead to condensation anyway (though it might be easier to check for…) It’s enough to drive one mad. Or wish there was a software that could integrate heat and moisture flows…anyone? anyone? One option is just add elastomeric / cermaic on top of the (newish) roof, (increment problem again), but now the retrofit is maybe only ½ as beneficial (would need to do more careful balance point / heating vs. cooling ratio). All that said…with such a high space, hard to access, only periodically used (a few hours at a time, a few days a week), I wonder if any of it matters. This is where ‘comparable projects’ rather than just working out a safe detail is needed. I’m working to understand more about the HVAC setup, but if there’s no high return (or could be one with a damper you could shut down) you could just let the air stratify in summer. A high return opened in winter could recycle the trapped heat, evening out temperatures and maybe reducing in/out deltaT (and heat loss). I.e., there’s other ways to save… Sorry so lengthy. With thanks in advance for any suggestions/ideas/references. Best, David

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  1. David_Gregory_CZ3_CA | | #1

    Sorry, just to add -

    With musical equipment (piano), probably temp and humidity have to be well controlled; so it should be conditioned outside of strictly occupied times (can't let the space 'float' just because people aren't there)...making insulation more valuable? And humidity less of a problem..or more???


  2. GBA Editor
    Martin Holladay | | #2

    1. Yes, it is possible to add rigid insulation between the rafters and then a new finish ceiling. As you have guessed, however, this approach would be trickier, more expensive, and less effective than working from the top side of the roof.

    2. Sit down with the church elders and look at their annual heating and cooling bills. These bills will help you decide whether it makes sense to install a new roof (probably over nailbase or over foam plus nailbase) now, or whether to wait 15 years until the existing roof wears out.

  3. gusfhb | | #3

    While wasting a new roof is bad, I cannot imagine any alternative that is cheaper.

    My current house has the same style roof, so I have been through the same thought process for various reasons, but luckily[?] my roof was dead when we bought it. It was very high tech tho, whole inch of foam above. Oil bill would make you faint.

    I think once you do the math on materials labor etc without even knowing how you would pull off the final finish, you will see that Martin is correct

  4. user-723121 | | #4

    I just did a ceiling like this and insulated from the inside with R-37.5 extruded. I strapped the ceiling with 2x4 and attached drywall for the finish. This particular home has a rubber membrane roof and would cost 20k plus to replace, insulating from the inside cost a fraction of that.

    If the roof needs replacing, insulate from the outside, otherwise I would consider both methods by doing a cost assessment.

  5. David_Gregory_CZ3_CA | | #5

    Thanks for the feedback. Given access and aesthetics, even before moisture issues, everything seems to point toward waiting until the time is right - either the roof dies of natural causes, is killed by natural disasters, or the savings can justify wasting the 'sunk cost' - and doing it from above...while in the meantime looking for lower-hanging fruit. Which I am quite sure there is some...

    And in the mean time, doing our best to make sure that no new roof gets put on any other structure like this without adding insulation!

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