Attic insulation - 2 questions
1. I have an old farmhouse - just gable vents. I'm about to add more blown in cellulose to part of the attic - do I still need to use ventilation baffles in the soffit area even though there is not soffit ventilation? I have heard conflicting opinions on this.
2. Half of the attic has plywood decking down on the floor and is used for storage - there is about 7" of fiberglass and cellulose below. To bump up the R-value in that area and air seal the ceiling w/out ripping up all of the current flooring and insulation, I was going to put down 2" rigid foam on top of the plywood, seal all seams, and then put down 1/4" plywood on top again. Is there any problem w/ creating a double vapor barrier with the rigid foam? I live in Virginia.
I suggest installing the baffels even though the soffits are not vented at this time.
Some day yo may wish to add some vents.
As to the foam/plywood sandwich---I don't like it --but I see no harm if it's used only for light storage.
I doubt if there is any harm --any moisture under it will vent out the sides of the joists.
Air Seal, Air Seal, and Air Seal.
Do that first before you add more insulation without fail.
You can dense pack that area of storage for proper insulation and dense packing stops air movement as well. After that, you could ad a layer of rigid foam (do not tape of seal the seams) and then a layer of plywood over that (effectively creating a sip panel).
Use a minimum of 2" of Poly Iso. More is better in this case so if you can put down 2 layers at 2" that would equal about an R-25+ and combined with the lack of thermal bridging and what is under the floor...you should be good to go.
+1 on potentially adding baffles if you decide to vent the soffits. Once you air seal, your necessity for ventilation goes down even further and if your attic is moldy already, it will be that much dryer after you air seal the floor.
We had an energy audit w/ blower door so I am definitely up to speed on the air sealing - it will be messy but worth it in the portion of the attic not used for storage. I'm going to pull back the old insulation a section at a time, air seal, put the old stuff back down and blow in more cellulose.
As far as not taping the seams of the rigid foam - is that to allow any potential moisture migration up through, or is it just not necessary if dense packed below? Do you think I could cause potential problems with my initial idea using the rigid foam?
Also, it is not really required from the standpoint that they do not need to be air tight.
If you dense pack under the storage area, it is a not issue anyway.
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