My 1940 house's upstairs ceiling is composed of:

  • 1/2" of plaster
  • 1/2" of solid paper backing
  • 2x6" joists spaced 16" apart (2" rolled batt and 4" open-faced batt insulation)
  • 1/2 inch plywood
  • Open attic space (A-shape) which I can easily stand in (I wanted to use this as storage but previous owners neglected a broken basement window and mice got in. I may or may not have got them all but I'm not sure. I do not wish to use this as storage.)

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The floor does not actually run parallel to the ridge of my roof; it is perpendicular but I didn't know how to illustrate that.

My region is Upstate, NY. I think R60 is recommended.

I have a bunch of leftover blow-in cellulose insulation from insulating my walls and was wondering if I could just freely blow it into my attic? Is the R value of plywood beneficial/harmful? Could the plywood alone cause some sort of moisture trap?

I know you can get ceiling sag if you blow in directly into exposed joists so it seems beneficial to just blow in on top of the plywood because I could do R80 if I wanted to.

I know I need to install rafter baffles so that I can insulate out to all edges of my attic. Should I foil-tape or caulk/silicone all the plywood seams?

I welcome all suggestions and especially appreciate references to good tutorial videos!

Side-note: I plan to do a complete tear down of the upstairs and remodel every room in the next 5-10 years.

  • @isherwood It's not gypsum board behind the mud. It's a solid half inch of a paper material.
    – MonkeyZeus
    Oct 31, 2019 at 16:28
  • Fair enough. Just trying to help with clarity. I've never heard of such a thing being used as plaster lath.
    – isherwood
    Oct 31, 2019 at 16:49
  • @isherwood My house has all sorts of character that I am discovering. If I get a picture of it some day then I'll try to remember to ping you here
    – MonkeyZeus
    Oct 31, 2019 at 16:56
  • You have a hole somewhere big enough to get the plywood sheets out? I know that I don't. It was a PITA, just getting some 2'X4' sheets up there so as to be able to move without disturbing rock wool. Oct 31, 2019 at 18:27
  • @WayfaringStranger Yes, I should be able to get them down the attic ladder. I assume that's how they got them installed in the first place. If not then I have a circular saw. Are you implying that I should remove the plywood sheets?
    – MonkeyZeus
    Oct 31, 2019 at 18:30

2 Answers 2


Yes, you can add insulation without removing the plywood.

There are several issues you’ll need to address: 1) ventilation, 2) vapor barrier, 3) recessed ceiling lights

1) Ventilation in the attic is critical. The code calls for “cross ventilation”. That means soffit vents, gable end vents, or both. (Ridge vents can supplement soffit vents or gable end vents, but can’t replace them.)

So, when you add the insulation make sure you don’t block the soffit vents (by using soffit baffles) and don’t block or restrict the gable vents.

2) Vapor will change to moisture at the dew point. Plywood is not a vapor barrier so it won’t trap vapor and moisture in the middle of the insulation, but the vapor and moisture needs to be vented out of the attic.

3) Recessed lights, if any, can build up heat if covered in insulation. Currently, there probably isn’t enough insulation to completely cover the lights. However, once the new insulation is installed it could cause a fire.

  • I updated my crude diagram to address some of your listed concerns. As for the the lighting fixtures, we have no ceiling lights on the 2nd level of the house. Any ceiling lights installed in the future would be underneath the plywood and will contend with the rolled batt insulation, not the cellulose; either way they would be properly housed.
    – MonkeyZeus
    Oct 30, 2019 at 19:20
  • 1
    @MonkeyZeus Note that recessed lights will build up much less heat if they are LEDs, rather than incandescent. Oct 31, 2019 at 15:30
  • @MartinBonnersupportsMonica I don't have much experience with the old incandescents aside from knowing the bulb itself gets very hot. I don't know if you've ever unscrewed an LED after using it for an hour but the base will burn your hand. I plan to properly secure all lighting fixtures regardless of bulb type especially since I don't plan to put an "LED only" warning label inside every fixture.
    – MonkeyZeus
    Oct 31, 2019 at 18:35

I'm assuming that the floor of the attic is insulated, and the roof is not insulated at all. Do I have the right picture?


A: Put insulation on top of the plywood. This is straight forward, and can be done with soffet baffles at the edges, and half a day's work with an insulation blower. You can choose between cellulose and fiberglass. The former is environmentally more friendly, and usually cheaper, and not nearly as itchy.

B: Remove the plywood, and put insulation on the old insulation.

C: Insulate the rafter space.

  • Only insulate the existing web.
  • Make the rafter space deeper by adding 2x to the edges of the rafter.
  • Use a higher R-value insulation between the rafters.
  • Fasten foam board to the interior edges of the rafters, then cover.

Tradeoffs on removing the plywood

You lose the potential storage space if you put insulation on top of the plywood. (You said you're not interested in doing this, but anyone else who finds this answer may be.) Removing the plywood will be a big pain. You also lose the support. More than one person has stepped through the ceiling of the room below when missing a joist.

While plywood isn't a vapour barrier, is will be a limited air circulation barrier. Normally when adding insulation, you want about twice as much on the cold side of such a barrier to keep the dewpoint on the correct (cold) side of the barrier. Blown in insulation is cheap.

Tradeoffs on insulating the rafters.

This is far more time consuming. You either are getting less insulation, having to make the rafters deeper, or working with foam, then protecting the foam from fire. This may or may not be necessary depending on local building code and the particular foam product. See http://msdssearch.dow.com/PublishedLiteratureDOWCOM/dh_0944/0901b80380944bea.pdf?filepath=styrofoam/pdfs/noreg/179-00011.pdf?filepath=styrofoam/pdfs/noreg/179-00011.pdf&fromPage=GetDoc

You could do this by putting 2x4 on edge on the existing rafters. then insulate. If the existing rafter is 2x8 (7.5") then adding 2x4 will give you 11 inches. Fiberglass at R3.5/inch gives you R38. If the floor is 2x6 and filled well it's about R20.

The flip side of this is that you have the space as a either storage, or hobby room, or potential spare bedroom, with the addition of a folding attic stair. This room is halfway between two insulation layers so it will tend to be warm in the summer, and cool in winter.

As even a roughly developed storage area it may increase the value of your house. You could consult a local realtor to find out.

You can also blow in insulation in the hollow of the 'A' so it could be R60 for that part of the roof.

Another possibility would be to use foam insulation. Insulate the 2x6 truss elements with bats, then put up 2" foam boards, then drywall. (Foam board MUST have a firebreak -- it releases cyanide when it burns) This has the advantage of insulting the thermal bridge that the 2x6 has in the roof.

  • Please see my edit. I don't have trusses. Also I don't care about the storage potential.
    – MonkeyZeus
    Oct 30, 2019 at 15:21
  • Sorry. I should have said 'rafter' I have edited my answer. Nov 1, 2019 at 14:44

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