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 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…)