Rumi

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Any Architects, Or Residential Contractors?
I expect my situation is one for a very knowledgable person regarding household airflow needs.

I'd like to insulate on top of our already finished third floor ceiling.

I have already gotten free styrofoam panels. I brought home from work over 200 "dropped ceiling" panels(2'X4') that I want to use for the insulation.
They are a full 1" thick and I will need to cut them to width, but hey, they were free!

The trusses are the older, actual 2X4 size.
The ceiling is made of the older lathe & horsehair mortar with the mortar squeezing and protruding upward into the 2X4's air space.

Can I put 1" thick solid stryrofoam panels on top of the lathe and mortar in that 4" high air space in-between the roof trusses of my finished attic ceiling and the solid 1X6 wood roof planks on the (top/outside) of the trusses?

My concern is- how much air space is required in these truss areas for heat and moisture movement?
There are a couple smaller vents on the roof below the crown line.

Can I place one, (or maybe two) styrofoam panels stacked in these air spaces for insulation purposes?
I'm thinking I would duct-tape the butt joints.

So I just need to know the limitations involved, or if its a bad idea.

I really need a professional, knowledgable answer.

Thanks guys, I appreciate your help.

- Jim

nealtw

You will want at least 1 1/2 Inches for air flow. Not sure how you get a nice tight fit if your sliding them into place, but free is always good.

BridgeMan

You've managed to confuse me. I understand the trusses being made of full-sized 2 x 4s, but you didn't mention how deep they are. Only to later say you had only 4" of space between the trusses for the insulation.

Could you possibly be confusing the term "trusses" with "rafters"?

joecaption

And it sounds like you plan on insulating the peaks of the rafters and not over the ceiling below. The only time the peaks get insulated is you it's going to be living space and even then you would have to add something like 2 X 2's to everyone so a foam baffle could be stapled in place before insulating.
If it's an open unused attic then there should be at least R-30 and up to R50 insulation in the attic depending on where you live. That would be a stack of those panels almost 12" high.
I know if I had this home inspected to buy it the fist thing I would have to do is get all of that stuff out and replace with fiberglass batts.

nealtw

I think the ceiling is the 2x4 rafters in a finished attic.

Rumi

Guys, I apologise for any confusion.

The ceiling from standing inside the room has a center, horizontal section that is 9' wide which meets roughly 45 degree sides going down at an angle for 64" inches to meet the verticle walls that then run 47" inches down to the floor.The horizontal ceiling section and the full house length crawl spaces behind the 47 inch verticle walls I'll insulate with fiberglass batts.

* Its the ceiling sections that are at the 45 degree angle that I'm unsure what kind of air flow is needed, if any for heat and moisture to move, so I really need your help.

I don't see any alternative for insulating these angled ceiling/roof sections, and still leave room for air/heat/moisture movement.

After reading about vapor barriers and mold, I'm going to have to think about that.
I don't want to cause a situation that causes mildew and mold by putting these panels between the rafters. Foam panels have a somewhat porous nature.

Oh yes, I live in northeast Ohio, as far as heat/ moisture levels are concerned.

neal - you're right of course. I just figure that some insulation in those ceiling sections has to be better than none.

Bridgeman - yes, they are indeed rafters, on 24 inch centers.

joecaption - I apologise for the misleading description, I won't be insulating all the way to the top of the rafters, as I've just described above. But thank-you for your information about how to do that.

It looks like the third floor was finished at original construction of the home in 1923, and has never had any roof vents until I had some installed a few years ago with new shingles; which leads to a different question for a different thread.

nealtw

Your right, anything is better than nothing. The oil base paint on the old plaster will give you some barier for vapour from the inside and if you leave some space above the insulation, you would be fine. Getting a tight fight between the sides of the foam and the rafters will be the challange.

Rumi

Thanks for that thought Neil. Oil-based paint, who'd a thunk it? I've got something going my way with that! And I can always put a new coat on come summer to further the effect.

Getting a tight fit when I have to slide the panels between the rafters to get them in place... I take what I can get. I'll cut them to width a hair on the large side and slide them slow and carefully. They're older and maybe prone to cracking, time will tell.
I'm thinking about doubling up the paneling, a great benefit structurally, and doubling the insulation value at the same time.

evstarr

So insulating the knee walls and the flat ceiling is fine. But you do need to allow continuous airflow through the portion of your room that is also the roof structure. If they are full 2x4 you can probably get away with two inches of foam in that space. Perhaps a better choice would be a foil faced foam with the radiant barrier towards the living space? (not sure about vapor barrier needed for this application)

On to the materials. Duct tape will fail within a couple of seasons in all likelihood.
I'm not aware of a lot of drop ceiling tiles that are actually closed cell foam. I'd be worried that they will trap moisture and become a source of mold in that application.

Above all else, whatever materials you use, you must ensure that the underside of the roof can breathe properly. If you don't. You'll have ice dam problems in winter and your shingles will fail prematurely from summer heat.

Good luck

Rumi

Thanks for your input evstarr. You brought up a couple things I need to think about.

Like, what type of tape will hold up long term?