Inside the attic space, air is your source of moisture. Venting means having air flow from the lowest part of the roof to the highest part of the roof. As the air flows from the soffit vents to the ridge vent, or wherever your highest vent is, it dries off the moisture that collects on your rafters from the air. The more humid your air, the more moisture you will have up there. Just the simple movement of air keeps everything dry up there.
When you close off your vents, you are not allowing air to flow anymore, so this now becomes a conditioned space, just like the rest of the interior of your house. Your HVAC system is now heating and cooling your attic space. The debate is over how much it costs to do this versus how much you gain by insulating the attic area.
In my opinion, if the house has an attic, it's more cost effective to blow in 2 feet of insulation into the attic. If the house has an open ceiling where there is just the space of the rafters between the ceiling and the roof, then spray foam is by far the best way to go, regardless of the cost.
I'm not aware of any reason other then cost for using open cell foam. It allows air into it, and air holds moisture. This moisture can build up over time resulting in mold and/or decreased insulating abilities and failure of the foam. Closed cell is just that, tiny little air bubbles that are air and water tight. It's what floating docks are made of. You can spray it directly to the ceiling and never have to worry about any moisture issues because no air will every contact what you have sprayed it to.
With new technologies and those trying to take advantage of home owners, you have to be very careful on what works and has a proven track record and what people want to believe works, but is really just a waste of money.
Keep it simple is very important with insulation. Do not over think it or over engineer it.
There are two things to remember with insulation. One is the R value of what you are installing. This is really just a scale of how efficient what you are using will work. One of my pet peeves is going all out on the R value of a wall by going thicker or using foam and other products, but then also having a bunch of windows with an R2 rating. Maybe if you spend a lot of money, you can get an R4 window. Even worse is putting in a skylight. Nothing loses more energy in a house then a skylight. I hate them and consider them the biggest source of lost energy every invented. Spending more money on windows and doors rarely shows any sign of return unless the windows are pure junk and really old. Companies list all sorts of gimmicks that claim to make their windows better, but in the end, it's just about the gas between the panes and how well it's installed.
The second thing that affects insulation is wind. Wind will go right through insulation and have a greater affect of inside temperatures then any amount of insulation. If wind gets through the exterior walls, it will change the temps inside the walls. In my opinion, wind barriers in the form of house wraps and tape are more important then insulation!!!!
I've seen numerous homes where they paid huge sums of money to have their windows and doors replaced and then they saw their energy bills increase. In every case, I remove the trim around the windows and find that there isn't anything there. Sometimes I can see right through the wall.
Be careful of what somebody is trying to sell you. It's your money and it's better to wait until you are 100% sure what you want then to do it hoping that it will be alright. It rarely works that way, and once the sales person makes their sale, they are long gone. Usually into a different field of work.
Eddie
