That probably depends on just how cold it gets in that attic space and how much ventilation there is above the plywood. The plywood won't be air tight so there will be at least some migration of moisture up through it which should be carried away by even modest ventilation. If the attic space though gets really cold then you may find moisture is condensing on the under or warm side of that plywood which will wet the insulation and if enough collects may begin wetting the ceiling or allowing mold or mildew. It will also depend on just how much of the area you cover.If just a four foot strip down the middle it is unlikely to be enough to cause a problem but if the entire space then it could well be.n
Another concern is that the plywood does not compress the insulation as this will reduce the efficiency. Often people will put 6 inches of insulation between the ceiling joists particularly if it is fiber glass bats or they will run a second layer of thicker insulation across the top of the joists. Is it your intension to create a floored space to allow for storage? It it is just to get around up there I have made sort of duck boards to put down over ceiling joists to crawl or walk along on and which I can pick up when not in use. Usually the ceiling is the most effective place to put insulation and usually more is better though there is a diminishing return curve. ----- Original Message ----- From: Spiro To: [email protected] Sent: Tuesday, October 20, 2009 12:58 PM Subject: Re: [BlindHandyMan] additional about spray insilation question. considering this evaporation Dale is it then unwise to tack down something like 3/8 ply down over the insulation between the two by's? thanks I've obviously not done anything yet as it was way too hot up there, and so dusty I couldn't breath. On Mon, 14 Sep 2009, Dale Leavens wrote: > A certain amount depends on the climate where you are applying the insulation. > > The reason you ventilate the roof space is to keep it cool in hot weather and to evaporate moisture which leaks up into the attic space in heating weather. > > Usually you wouldn't want to be ventilating a part of the living space allowing heat to escape in heating weather or cooling to escape during cooling season. > > If you insulate under the roof deck you will retain all of the radiant heat from the sun right there. The insulation wont allow any of that heat to dissipate. That may help to keep the living space cooler to some point at least but the shingles or what ever else the roof is made of will become outrageously hot in the sun. > > Think of the roof as weather protection only. > > Well, that is the usual situation. > > In fact my roof is insulated sort of. I put an addition onto a house which is a story and three-quarters, that is, the walls front and back slope and become the ceiling. I did not wish to use the same balloon style construction for the addition and didn't much like having that top bit of the wall only the four inches thick of the roof rafters so, while keeping the similar design inside I built a second roof that runs over the entire first roof and the new addition. I then insulated over the original roof and the dummy ceiling part of the extended roof with 18 inches of glass fiber. this also allowed me to run a continuous poly vapor barrier right over the original roof and under the insulation and second roof. I have insulated under the original roof as well, over the warm side of the vapor barrier. The attic space then is part of the heated environment and the old and false roof actually perform the function of walls. I had a hell of a time explaining to local builders how t > his works but believe me it works and works well. > > There is a about 8 inches of free air space under the actual roof deck. > > Probably the most important factor in energy efficiency is to have an air tight envelope and control the ventilation, to keep conditioned air in and unconditioned air out. You then must control ventilation to keep the air quality satisfactory then you must add insulation to keep heat transference from conduction and radiation under control. The direct movement of warm and cold air though is easily the most important factor in comfort and energy saving. > > > > ----- Original Message ----- > From: Shane Hecker > To: [email protected] > Sent: Sunday, September 13, 2009 11:17 PM > Subject: Re: [BlindHandyMan] additional about spray insilation question. > > > I could see that if the vents were blocked. But if they aren't, wouldn't they work as before? As far as the ceiling goes, I thought when you sprayed foam under the roof deck, you turned the attic into a part of your living space. I'm a bit confused as everything I've seen suggests you can do this. They say if the roof is ventilated, don't block the vents, but that it still works. But it's also said the best way to do it is to not ventilate the roof and spray the entire underside with foam. > > Shane > > ----- Original Message ----- > From: Dale Leavens > To: [email protected] > Sent: Saturday, September 12, 2009 8:29 PM > Subject: Re: [BlindHandyMan] additional about spray insilation question. > > He wouldn't spray the foam under the roof deck, that would negate the point of ventilating the space and further, he would lose the air sealing effect he would gain from covering the ceiling. > > ----- Original Message ----- > From: Shane Hecker > To: [email protected] > Sent: Saturday, September 12, 2009 8:25 PM > Subject: Re: [BlindHandyMan] additional about spray insilation question. > > So long as you don't block the vents you will be fine. Do you have recessed lighting? If so, be careful with those fixtures (especially if they are older) as they can get hot. Do you plan to spray foam on the floor or under the roof deck? Either one will work, but if you spray under the roof you will turn your attic into a storage space, which will be only a few degrees warmer or cooler than your living space. > > Shane > > ----- Original Message ----- > From: Spiro > To: [email protected] > Sent: Saturday, September 12, 2009 4:05 PM > Subject: [BlindHandyMan] additional about spray insilation question. > > My attic has opening to allow blow through. > Could I have this spray job done to save me long hours of dragging either > pink or stryofoam up into that dusty oven? How will the flow of air work > if it gets sprayed up there? > > On Sat, 12 Sep 2009, aadorno wrote: > > > hi listers I am thinking about getting spray insilation for my house. the way this works is that they make a hole in the wall or floor and they spray in the insilation and after a bit it drys and expans in the space that it was sprayed in. > > my question is to the list is this. > > is any one on the list ever had this done to there walls. also is it safe? > > well that's it. > > thanks for your time, Angel. > > > > [Non-text portions of this message have been removed] > > > > > > [Non-text portions of this message have been removed] > > [Non-text portions of this message have been removed] > > [Non-text portions of this message have been removed] > > > > > > [Non-text portions of this message have been removed] > > [Non-text portions of this message have been removed]
