Insulating conditioned space in vented attic

Hi everyone, I’m looking for advice because we recently discovered that the insulation in our attic wasn’t installed properly by a contractor. Here’s some background information:
- The attic is conditioned, with a central air system installed. The air handler is located in the attic.
- The contractor installed R-21 faced fiberglass batt insulation directly against the roof sheathing (I believe it’s 2×4 construction).
- We haven’t installed drywall in the attic yet.
- We’re located in Connecticut.
- It’s an old house, and the attic was never previously insulated or used as living space. However, the tax card indicates that the attic is included in the living space square footage.
- The attic has soffits, eaves, and gable vents.
- We live on the second floor, and the attic is essentially like a third floor with a separate staircase.
- The HVAC contractor installed only one thermostat for both the second floor and the attic.
Since the weather has gotten colder and the attic is already being heated, I checked under the batt insulation and noticed a few damp areas. At first, I assumed this was due to air leaks since we haven’t installed drywall yet. However, after researching, I realized it might be dew point condensation. It seems we may need to remove the insulation and install baffles.
I understand we need at least R-49 insulation, but hiring a contractor for spray foam insulation is currently out of our budget. I’m planning to add rigid foam insulation to increase the R-value, it won’t be possible to get to R-49, and I have a few concerns:
- Baffles: Should I install standard plastic baffles? Could they create additional condensation issues since they don’t allow vapor to escape? To avoid this, I’m considering using Tyvek house wrap under the batt insulation and leaving a 1.5-inch gap.
- Vapor Barriers: I plan to install rigid foam board but noticed most brands sell faced foam insulation. Since the fiberglass insulation we already have is faced, would adding faced rigid foam boards on top create two vapor barriers? Could this trap moisture?
This is my first time tackling a project like this, so I’d really appreciate your input. I have a very basic drawing of how I’m planning to install the insulation – I hope this helps and please don’t judge. I’ve been researching for the past few days and keep finding conflicting opinions. Thanks in advance for your help!
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Replies
se_19881,
You are on the right track, and it's great you caught the error now.
Permeable baffles are a lot more forgiving than the plastic ones as they allow vapour to escape into the vent space above the batt insulation you are installing. Tyvek will work very well for that.
The foam is fine. It will act as both a vapour-retarder and air-barrier if you tape it. The facing on the batts against the foam will not trap moisture, just slow its movement.
You ABSOLUTELY NEED THOSE VENT BAFFLES!!! Installing batt insulation directly against the underside of the roof sheathing is a big problem here. Sounds like you already know you need vent chutes, but I wanted to be absolutely sure to point that out.
You could use the plastic vent chutes, they aren't a very tight fit and leak enough to usually deal with any moisture that sneaks up into the assembly. It's a bit more labor, but might be cheaper, to use 1/4" wafer board and 1x2s to build out the vent channels. To do this, you tack 1x2s along the inside edge of the rafters tight to the roof sheathing, which then act as 1.5" spacers off the sheathing -- perfect for a vent chute (a bit more than 1" code minimum, which is good). I use my 15 gauge finish nailer to do this, which makes it go fast. Next, rip strips of 1/4" waferboard, which is like extra-thin OSB, into pieces that will fit between the rafters. These don't need to be precision fit, just close enough to fit while still being nailable to those 1x2s. I find it's often easier to make these 4 feet long and not the full 8 foot length of the sheet -- sometimes 8 footers are tricky to install since the material gets pretty floppy. Tack those up using that finish nailer again.
Once you have those vent chutes defined, either with waferboard or baffles, you can put your batts back in place. Try to use batts that are a reasonably good fill for the entire depth of the rafters.
To add additional R value, you can furr out the rafters, or run strapping perpindicular to the rafters to let the next row of batts go across the rafters instead of parallel to them (this cuts down on thermal bridging). If you want to use rigid foam, I'd use polyiso, with seams taped. It's OK to use non-permeable rigid foam here, since it will minimize how much vapor can sneak up into the assembly, while the vents can carry any moisture away that does manage to get up there.
If you're trying to keep costs down, it's likely cheaper to using strapping and more batts (and you can use unfaced batts for this if you have polyiso underneath). My preference would probably be to use batts to get most of the required R value, since it's cheapest that way, then put 1/2" polyiso up last with seams taped to act as a vapor barrier, then drywall over that.
Bill