Plywood over attic insulation - ventilation concerns?

I'm looking to put plywood down to create more storage space in my attic, but can't seem to find a straight answer on ventilation concerns. I have an older home - 1940's - 2x6 joists with pink fiberglass insulation rolled in. In general, the insulation is fairly 'loose' and doesn't protrude over the top of the joists. I know I can easily get the plywood down over the insulation and the insulation would probably still be somewhat loose in there allowing for 'some' airflow, but are there any concerns as to ventilation/condensation in the insulation once its been sealed over with plywood?
Also, my home is older and does not have soffits and/or soffit vents. I have gable vents on either side and two roof turbine style vents. Are there any concerns with butting the plywood sheets up against the outside roof lines? Or, should I leave some area where the roof line meets the structure for some reason? My house is brick exterior with fascia right over the brick at the roof line. I've been told (not sure if this is reliable) that air needs to come up behind the fascia into the attic for proper ventilation? Of course this also means insects easily get up in there and that's part of the reason I would love to be able to plywood the entire attic area flush with the roofline (as well as for cosmetic reasons).
Thoughts on this from those of you who understand attic ventilation? Many thanks!
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Practice today is to have a vapor barrier between the ceiling and the insulation. Don't know what they did in the 40's, but then I don't think the pink fiberglass was in use back then either, so maybe it was added later. If you're not having any issues now, I tend to doubt adding the plywood floor is going to create problems.
But, are you sure they are 2 x 6's? If so, then it sounds like you have only 4.5" or so of attic insulation. That is inadequate for most or all of the US and could be a real $$$ out the window problem if you're in say MI. And even if they are 2 x 8's, having more than 6.5" of insulation is a good thing. How good depends on the climate. Bottom line is when you put in a floor, that is one of the tradeoffs you have.


It would seem likely that you'd wind up with some gap, no matter how far over you bring it, because you can only go so far until it hits the rafters.
My house is brick exterior with fascia right over the brick at the roof line. I've been told (not sure if this is reliable) that air needs to come up behind the fascia into the attic for proper ventilation? Of course this also means insects easily get up in there and that's part of the reason I would love to be able to plywood the entire attic area flush with the roofline (as well as for cosmetic reasons).
That is definitely the most desirable ventilation flow. It allows cool air to come in at the soffits and exit via vents near the top. Today, usually the preferred exit is a continous ridge vent. The vents on soffits normally have holes so small that insects like wasps can't get in. Those vents today are usually installed as continous ones the length of the soffits. But they can also be distributed seperate ones. If you can put them in, that would be a good thing. In which case you need to make sure that insulation is not blocking them where the roof meets the perimeter of the house. Very common to see new houses where the insulation guys proceed to block them up. There are plastic baffles available at HD, etc that you can staple to the underside of the roof, between rafters, where the roof meets the exterior wall that keeps a channel open for the soffits.


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If you are considering covering the entire attic with plywood as you described there will be a temptation to load it up with lots of storage. 2x6 joists are not strong enough for anything more than a little light storage, they are designed only strong enough to support the ceiling on the underside and to support anyone working up in the attic. Before you cover the attic with plywood, you should sister all the joists with 2x8 joists at minimum. Plumbing vents and wiring may be a problem and interfere with adding the additional supports.
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StratfordOne:
Lemme explain attic ventilation first, and then I'd tell you what I'd do:
Some people down in the southern states figure attic ventilation is to allow the hot humid air out of the attic in the summer to make the house cooler and more comfortable, but hot humid air isn't going to damage your house, and so that's not why it's necessary.
Where I live (about 100 miles north of North Dakota's northern border) attic ventilation is mostly needed to eliminate the warm moist air that inevitably rises and finds it's way into the attic in the WINTER.
When that warm moist air gets into the attic, it condenses or freezes to the coldest surfaces in that attic; which is generally the underside of the roof and the roof rafters. That frost or condensation doesn't do any harm... until spring when it melts and drips onto the ceiling insulation, getting the ceiling insulation wet.
Insulation works by keeping air stagnant, so wet insulation takes forever to dry out, and having wet insulation in contact with wood ceiling joists is a prescription for wood rot.
And, on top of the wet insulation being in contact with the wood, you'll typically have a plastic vapour barrier under the ceiling joists that prevent the moisture from getting the plaster ceiling wet, and that lulls the homeowner into thinking there's no real problem until the damage is severe enough that the ceiling starts to sag. I myself have seen a photo of one rented house that was turned into a marijuana grow-op that had 3 inches of frost on the attic roof and rafters.
By providing some holes in the attic where air can get in and out of the attic, but not snow or rain, then the wind will ventilate the attic, thereby eliminating that moisture that could potentially cause an awful lot of damage.
THEORETICALLY, soffit vents on both sides of the attic to let cold air, and a ridge vent on the top of the roof works the best because it uses the inevitable heat loss from the house to drive a convective current that eliminates the humidity from the attic. However, what normally happens in reality is that days with no wind are comparativelyh rare and virtually any wind will break up that convective current that soffit/ridge vents rely on for theoretical 100% sweep efficiency. What normally happens is that there's some wind, and the wind blows cold air into the soffit vents on one side of the attic and the warmer moister air is pushed out the soffit vents on the other side of the attic. But, as long as the moisture is eliminated from that attic, that's all that matters, and a cross wind will do that as effectively as a convective air current.
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If it wuz me, instead of using plywood, I would use 1X6 construction grade lumber boards to make your floor, and leave a 1/8 inch gap between each board. That would allow for ventilation of the ceiling joists and the wood floor.
Now, the reason why 1X6 boards are commonly nailed down at a 45 degree angle to the joists instead of perpendicular to them is that by nailing the boards down at an angle you increase the rigidity of the house from twisting in a strong wind, and that reduces the liklihood of plaster cracks in the weak areas, like at the top corners of doorways.
Also, in most houses, access to the attic is through a small entryway, thereby precluding you're being able to get 4X8 sheets into the attic. Shorter boards nailed or screwed down in a staggered arrangement would solve that problem.
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And, to confirm you have enough attic ventilation now, go up into your attic on the coldest day (or night) of the winter. Look for frost or condensation on the underside of the roof or dirty spots on the ceiling insulation (because that dirt can be caused by water running down the rafters and dripping off a knot or something onto the insulation directly below).
If you don't see any frost or condensation, then your attic is well ventilated and you don't need any more ventilation up there.
Hope this helps.
--
nestork


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But certainly having inadequate attic ventilation in hot climates can shorten the life of shingles. And adequate ventilation does make the house cooler if you don't have AC and lessens the load if you do have AC.

Interesting theory, but I doubt it has any practical effect.

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On Tue, 23 Oct 2012 11:03:39 -0700 (PDT), stratfordone
can't seem to find a straight answer on ventilation concerns. I have an older home - 1940's - 2x6 joists with pink fiberglass insulation rolled in. In general, the insulation is fairly 'loose' and doesn't protrude over the top of the joists. I know I can easily get the plywood down over the insulation and the insulation would probably still be somewhat loose in there allowing for 'some' airflow, but are there any concerns as to ventilation/condensation in the insulation once its been sealed over with plywood?

gable vents on either side and two roof turbine style vents. Are there any concerns with butting the plywood sheets up against the outside roof lines? Or, should I leave some area where the roof line meets the structure for some reason? My house is brick exterior with fascia right over the brick at the roof line. I've been told (not sure if this is reliable) that air needs to come up behind the fascia into the attic for proper ventilation? Of course this also means insects easily get up in there and that's part of the reason I would love to be able to plywood the entire attic area flush with the roofline (as well as for cosmetic reasons).

You will still be ventilating the area above the insulation. IF you have a good moisture barrier below the insulation there will be no problem. If you do not have a good vapour barrier I would place 2X4s crosswize over the floor joists and put the plywood on top, leaving a good air space between the insulation and the "floor" - keeping an opening of a foot or so at each end to ventilateinto the ventilated attic space above.
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On Tue, 23 Oct 2012 11:03:39 -0700 (PDT), stratfordone

There are millions of homes with attic floors and insulation between them and the ceiling below. Your only concern might be where there is a recesseced light fixture in the ceiling, and heat needs to escape. Otherwise this is just a common attic floor and of no concern. I do agree that 2x6 joists are not real strong, so dont use 3/4" plywood, use half inch and dont overload it with heavy stuff. Most people just store xmas decorations and old clothing. That should not be a problem.
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In my 1929 house there were several kinds of insulation on the ceiling below the attic joists. We hired someone to remove it all and put foam insulation against the roof sheathing. The attic was MUCH cooler this summer, which must have helped with the A/C costs. This was done after an energy audit and together with a few other things qualified for a 50% rebate from NJ state.
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Best regards
Han
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in a older home the multiple layers of cieling paint, espically the oil based type act as a great vapor barrier......
i have a friend who used furrying strips so his attic floor was a little off the insulation but i doubt it matters
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