u/Risinglight0123

Hi all. I am trying to wrap my mind around the latest thinking around insulating unvented short-slope cathedral ceilings in Cape Cods that open up to a small vented top attic above, and would really appreciate any guidance folks can provide!

I have read a lot of articles and presentations and posts and am still not quite sure I fully understand what is considered reasonably safe and why, especially when it comes to cellulose vs fiberglass and whether the insulation should be up against the roof deck or if there should be baffles. (I know there has been a fair amount of research in the last 10-15 years or so on this from places like BSC and Byggmeister, although I haven’t seen any updates since 2023ish and some of the findings seem confusing to me as a layperson.) Assuming a retrofit situation where rigid exterior foam and spray foam are off the table (which I understand are by far the safest and most code-compliant options), what are the least risky ways to handle sloped ceilings where the rafter bays open up to a top attic in region 4A?

— Is dense-pack cellulose in these slopes now considered reasonably safe as long as they open up to a top attic up above, based on recent research (such as New England’s Favorite Roof Retrofit: Moisture Data from Three MA Case Studies)? What are the most important factors to minimize risks in this situation? It’s important to make sure the cellulose is packed very densely, right? Does the ceiling drywall provide enough of an air barrier to keep moisture out (assuming no recessed lighting or other penetrations), or are there other steps that should be taken to protect against air/moisture traveling into those slopes in a retrofit situation? Does it matter if the top attic only has gable vents?

— What is the building science behind baffles/air gaps and whether insulation should touch the roof deck or not (in a situation without soffit vents from below, but where it’s an old house with a wood plank roof deck that probably has some outdoor air leaking in through cracks and gaps, and then the baffles go up into the vented top attic)? Does this differ for cellulose vs fiberglass? I would have thought that baffles would help or at least not hurt (except decreasing the R-value a little), but that doesn’t seem to align with the importance of making sure cellulose is really densely packed without any airflow. Or is it okay for the cellulose to be densely packed against baffles rather than the sheathing itself? Does it matter what the baffles are made out of?

— The modeling study “Moisture-Safe Unvented Wood Roof Systems” seems to suggest that faced fiberglass batts would be significantly safer than cellulose in a regular unvented rafter bay in zone 4 (table 3 on page 11)– does that sound right? But then is table 4 on that page saying that it is too risky because if air flows through the insulation there will be too much time with condensation potential at the roof deck? If so, is that still true in the Cape Cod situation where there is an attic above that the moisture can rise up into? How about if there are baffles? Is fiberglass generally considered safer or less safe than cellulose in these situations?

— Vapor diffusion ports are allowed in un-vented roofs as an alternative to foam in zones 1-3, but they were tested by BSC in zone 5 in MA and failed, correct? Have they ever been tested in zone 4A? Here near DC it feels like our climate (winter monthly average temps around 40) is closer to, say, Charlotte, than MA. I saw that ages ago Joe Lstiburek said he thought they would probably work in zone 4 but needed more testing– were they ever tested or was it assumed that the zone 5 testing went poorly enough that they should be avoided in zone 4 as well? Also, is the top attic of a Cape Cod considered to be kind of like a giant vapor diffusion port, and if so, does it matter if the attic only has gable vents and no ridge vents?

Thanks so much in advance for any insight you can share!

We have a 1951 Cape Cod in the DC area (climate zone 4A)-- the north side-attic has no formal venting (but is likely pretty leaky) while the south side appears to have soffit vents but along only about 1/3 of that side thanks to a one-room addition. The top attic has gable vents, not 100% sure if the soffit vents from the south side connect up through the sloped upstairs ceiling to the top attic or not. No obvious signs of moisture/mold in any of the attics thus far, but we're not 100% sure what to look for (and there have been some changes in recent years.)

I understand that to do things "right," we ought to use either exterior rigid foam on top of the roof deck or spray foam under the roof deck to make all 3 of these spaces into unvented conditioned attics. Assuming those options are off the table, I am trying to figure out how risky the situation is and what is likely to make the situation worse rather than better. A few questions:

1. Should we go ahead and improve insulation and air sealing/air barriers on the side-attic floor and kneewalls and the top-attic floor (we can get rebates to make this fairly affordable)? Is there any chance this will make things worse moisture-wise? Presumably there will be a benefit in that less of the moisture from the indoor air will get into the attics (although I'm sure some still will), but on the flip side, those spaces will likely now be colder (currently they tend to run about 10-15 degrees different from the outdoor air)-- might that increase the chances of dewpoint issues with the sheathing? And is there a chance that the air leaks through the ceiling are actually contributing to ventilation and blocking them would cause problems?

2. If need be, I think adding additional vents is probably a better solution for us than using spray foam or exterior rigid foam to go unvented, but it will be fairly complicated and expensive itself (we have a metal roof.) Would it be reasonable to make the air-sealing and insulation changes first and then monitor the spaces and wait to add better venting in future years if it seems to be needed? If so, what should we be looking out for?

3. Anything else to be aware of that would help or hurt? Does having batts on the side-attic roof slopes (with baffles or another form of gap) make it more likely for the sheathing to get moist or harder for it to get dry, or does it not make much difference? Is using blown-in loose-fill cellulose on the floors more likely to be helpful or harmful to the humidity/moisture/mold situation in the attics compared to using blown-in fiberglass given that it absorbs moisture, or does it not matter? Would a smart vapor barrier make a difference, and if so where should it go? How about using a dehumidifier? Anything else you'd suggest?

Thanks for any advice you can give! Let me know if you have any questions or want pictures of anything. More details about the house below-- but feel free to skip.

(Details about the house for the curious: Roof deck is wood planks. Roof is metal-- dark grey metal shingles on the north side and most of the south side (with I believe typical black felt underlayment, put on in the early 90s), except for white standing-seam over the addition over ice + water shield with solar panels above it-- the slope of the addition roof is also on the low side at 3:12. Blown in insulation (I think fiberglass) on the top attic and side attic floors, poorly fitting fiberglass batts in the kneewalls and cathedral ceiling between side and top attics, ancient mineral wool batts in the side attic slopes that is probably original to 1951. The house is quite leaky (blower door test around 4200 CFM50, and I think the ACH50 is about 16?) The side attics don't get that hot (generally in the 65-85 range during summer)-- I've had a gauge checking RH in the north side-attic for about 6 months and it's generally around 45-55%; just moved it to the south side-attic this week and it's been around 55-60% but with some short spikes around 70%. Humidity inside the house is generally around 30% in winter and 60% in summer. Average monthly outdoor temps in winter are around 40F, but obviously a fair number of colder days and nights. House is on a slab. There are two HVAC ducts going to the new south-side addition (R-8 flexible insulated ducts) but we are considering switching to mini-splits for HVAC so could possibly remove those-- no other ducts in the attic space. Bath and kitchen vents go through the first floor ceiling and out through the soffits on the north side. No super-obvious signs of past mold issues, but we have made some changes in recent years-- adding the addition a couple years ago, as well as adding some thin wafer-style recessed lighting on the first floor-- so it's possible that problems are now brewing that we're unaware of...)

We are in zone 4A (DC area) with a 75 year old 1.5 story Cape Cod with small triangular side attics behind kneewalls that appear to be unvented or poorly vented (the roof geometry does not allow venting up to the top attic/ridge in most parts of the house.) Roofing is aluminum "Rustic Shingles" (with I believe tar paper and then plywood underneath, but not 100% sure-- was put on in the 90s.) There is old batt insulation on the roof slopes (maybe original, Red-Top mineral wool) and kneewalls (seems more recent, fiberglass), and loose-fill insulation beneath the side-attic floor, but the temperatures in there are typically fairly close to outside temperatures (within 10-15 degrees) rather than inside temperatures. The house is quite leaky (blower door test around 4200 CFM50, and I think the ACH50 is about 16?)

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Indoor air upstairs is usually in the 50s or low 60s RH in spring, summer, and fall, but definitely some stretches of high 60s/low 70s, generally when the outside RH is high for awhile (which is not infrequent.) In winter it's usually in the 25-45 range (other than short spikes after showers), but with stretches over 50 (again, usually when outside RH is high.) I've had a temperature/humidity gauge on the floor in the north-facing side attic for about 5 months, and it shows RH ranging from 30%-60% (generally in the ballpark of 45%-55%) with a few stretches up near or above 70% (especially one May week where the attic temps dropped from low 80s to high 50s as the weather shifted from hot to cool.) The gauge shows dewpoints generally around the low 30s in February except for the coldest days/nights (eyeballing it I don't think the dewpoint was ever above the nighttime low), and all over the place since then (anywhere between 30 and 60, with a couple excursions up to 65 in the hottest weather.) It also shows the Vapor Pressure Deficit, which has stayed above 0.5 kPa with just two brief exceptions, and averaged more like 0.7-0.8 kPa in Feb/Mar and 1.0-1.5 kPa in April/May-- I gather this is a more direct measure of condensation likelihood, the higher the better?

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How risky is this? How do we know if there are existing problems with condensation/rot/mold, given that we can't see the underside of the roof deck beneath the insulation? (There's one spot with no insulation and the roof there looks fine, but might it be worse under the insulation?) And if this is problematic, is there any way to fix the situation besides closed cell spray foam (which we really, really want to avoid) or re-roofing to add insulation above the roof deck?

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And is there any way to improve insulation/air sealing in those side attics, or would any changes (other than closed cell spray foam or re-roofing) be too risky and we should just leave it be? For example, could we add dense-pack cellulose to the roof slopes behind a smart vapor barrier and use that to bring the side attics into the building envelope (while also air-sealing the kneewalls and floors to minimize how much indoor air enters the side attics)? Or could we air-seal and better insulate the kneewalls and attic floors, pull down the roof slope insulation, and keep the side attics outside of the building envelope (figuring that the cold and the leakiness in the side attics would keep them dry enough)? Any other options that don't involve spray foam? These improvements would allow us to tap into some big state HVAC rebates so we'd love to do them, but not if it's too risky.

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Thanks so much for any advice or insight you can share!

We are in zone 4A (DC area) with a 75 year old 1.5 story Cape Cod with side attics that appear to be unvented or poorly vented (the roof geometry does not allow venting up to the top attic/ridge in most parts of the house.) Roofing is aluminum "Rustic Shingles" (with I believe tar paper and then plywood underneath, but not 100% sure-- was put on in the 90s.) There is old batt insulation on the roof slopes (maybe original, Red-Top mineral wool) and kneewalls (seems more recent, fiberglass), and loose-fill insulation beneath the side-attic floor, but the temperatures in there are typically fairly close to outside temperatures (within 10-15 degrees) rather than inside temperatures. The house is quite leaky (blower door test around 4200 CFM50, and I think the ACH50 is about 16?)

Indoor air upstairs is usually in the 50s or low 60s RH in spring, summer, and fall, but definitely some stretches of high 60s/low 70s, generally when the outside RH is high for awhile (which is not infrequent.) In winter it's usually in the 25-45 range (other than short spikes after showers), but with stretches over 50 (again, usually when outside RH is high.) I've had a temperature/humidity gauge on the floor in the north-facing side attic for about 5 months, and it shows RH ranging from 30%-60% (generally in the ballpark of 45%-55%) with a few stretches up near or above 70% (especially one May week where the attic temps dropped from low 80s to high 50s as the weather shifted from hot to cool.) The gauge shows dewpoints generally around the low 30s in February except for the coldest days/nights (eyeballing it I don't think the dewpoint was ever above the nighttime low), and all over the place since then (anywhere between 30 and 60, with a couple excursions up to 65 in the hottest weather.) It also shows the Vapor Pressure Deficit, which has stayed above 0.5 kPa with just two brief exceptions, and averaged more like 0.7-0.8 kPa in Feb/Mar and 1.0-1.5 kPa in April/May-- I gather this is a more direct measure of condensation likelihood, the higher the better?

​

How risky is this? How do we know if there are existing problems with condensation/rot/mold, given that we can't see the underside of the roof deck beneath the insulation? (There's one spot with no insulation and the roof there looks fine, but might it be worse under the insulation?) And if this is problematic, is there any way to fix the situation besides closed cell spray foam (which we really, really want to avoid) or re-roofing to add insulation above the roof deck?

And is there any way to improve insulation/air sealing in those side attics, or would any changes (other than closed cell spray foam or re-roofing) be too risky and we should just leave it be? For example, could we add dense-pack cellulose to the roof slopes behind a smart vapor barrier and use that to bring the side attics into the building envelope (while also air-sealing the kneewalls and floors to minimize how much indoor air enters the side attics)? Or could we air-seal and better insulate the kneewalls and attic floors, pull down the roof slope insulation, and keep the side attics outside of the building envelope (figuring that the cold and the leakiness in the side attics would keep them dry enough)? Any other options that don't involve spray foam? These improvements would allow us to tap into some big state HVAC rebates so we'd love to do them, but not if it's too risky.

Thanks so much for any advice or insight you can share!