how do you install a vapour barrier on ceiling (from attic side)?

If you live in an area where freezing occurs, a vapor barrier is critical. Without a proper vapor barrier you will experience a serious condensation problem. Be certain that the barrier is installed on the warm side of the insulation. Stapling is the usual method of attaching the plastic to the framing members. An R-38 is a desirable amount of insulation for ceiling or roof. Keeps you warm in winter and cool in summer. 45 years in home construction and repair.

You know i've thought about this problem many times over the last several years. I really don't think this will be a problem, if it is, the summer time would be the best time for a problem, and then not much of one. Moisture in the home is due to many things such as showers, cooking, and even gas heat puts off moisture. I don't think the moisture level in most homes is a big problem,especially if you have electric heat. All homes seem to dry out in winter from electric forced air heat, and in the summer your central air unit actually removes the moisture as it condenses on the a/c coil and collects through a drain. You may get some moisture in the attic on damp, spring days however you do have a vapor barrier on top of your insulation. Today's interior primers/paints are much better than years ago. If you have an extra day or 2 and a buddy, you might want to turn the insulation over, but I really don't think this will be a problem, in fact many new homes have insulation blown in the attic without any type of vapor barrier installed first, many new homes have this.

To do the best job you should clear away the existing insulation and install the vapor barrier so that both the ceiling and joists are covered. Tape it down to prevent moisture from migrating around the edges. If your attic is adequately ventilated it isn't necessary to go the these extremes your idea would be fine. Go to your home improvement store and ask them what kind of mask is recommended when handling insulation.

I have never seen a vapor barrier on the ceiling joists. It simply isn't necessary. Your attic probably is ventilated. If it isn't, skip the barrier and ventilate the attic.

Attic Vapor Barrier

Yes, that would work fine and Yes you need to clear out the old insulation first.

However, I wouldn't recommend blow in insulation.

If you want a higher R factor and an insulation that will stay in place from now one I recommend you use rolls of insulation and put it in place. It will have an adequate vapor barrier built into it and it will last. Blow in insulation blows around the attic and often out of the attic after 10 years it will have to be replaced and after 5 years there will be less protection than when you first placed it.

If you are looking for easy to do then put in the vapor barrier you were thinking about and hire a truck to add expanding foam insulation. It can be sprayed in as easily as blown in insulation, but it will remain for the life of the house. It won't have as get an R factor as the rolls of insulation, but it will have more R value than the blown in insulation and it will remain for a while. Furthermore the worker spraying in the insulation can go to the sides of your house and blow in insulation between the studs giving you much more R value in your house.

R value is the Resistance value and that is the measurement used to calculate how effective insulation will be the higher the R factor the better it will be. The technical replacing your insulation will have to drill hole in the stud to access the spaces in between, but due to the nature of wood and construction design that won't compromise the strength of the structure. The advantage of going in from the top is that you won't have to replace the sheet rock and repaint the walls, you only have to drill a saw large enough to fit in the nozzle.

Call insulation contractors and ask them about the cost of spray in insulation. If you are an active person then you can drill your own holes in the stud and save some time and in construction time = money. But, before you do that clear it with your spray in insulation contractor.

The added advantage of the spray in foam is that once it dries it acts as its own moisture barrier so you don't need to put in the plastic vapor barrier.

If your heart is set on using blown in insulation then I would highly recommend that you go back after the job is over and place an additional layer of insulation on top of the studs to keep the blown in insulation in place. This can be a thinner plastic like garbage bags, it only has to be strong enough to hold the insulation in place. Still the blown in insulation has a tendency to settle reducing its R factor over time. There is nothing that you can do about that except to keep adding more insulation.

Both rolled insulation and blown in insulation don't degrade over time and will last much longer; on the order of at least 25 years compared to 10 years for blown in insulation. I don't see why it can't last for the life of the house (or structure).

Putting in a second plastic barrier will also give you more dead air between the studs and dead air makes the best insulation thus increasing the R value of the final insulation. Of course if you do it with rolled insulation or spray in you won't have to put an additional layer of plastic on top.

At least check out the price of spray in foam insulation, it may be comparable to blown in and it will last far longer.