attic vapor barrier and subfloor gluing
[I originally posted this in the General Construction discussion,
but didn’t get a complete picture of how to proceed—perhaps
some of you insulation experts know the answers.]
This 1905 house is in northern Colorado and has no insulation
in the attic. I’ve sistered all of the original 2×4 (16″ center)
joists with 2×8’s, so the spacing is not standard. Thus, I’m
planning to use cellulose fiber instead of fixed size batts. The
ceiling below is plaster over wood lath. A plastic vapor barrier
will be between the lath and the insulation. The attic will be
unheated.
I’ve read that gluing the subfloor (with construction adhesive)
to the joists results in a stronger floor, but I don’t know how
this works with a layer of plastic between. Do I:
1) cut plastic for each channel and glue to the edge of the
surrounding joists (if so, what do I use to glue the plastic
to the joists?),
2) lay plastic over the joists and put adhesive both under and
over the plastic, or
3) forget about gluing down the subfloor?
Also, I’m not real sure what kind of plastic to use. I’ve got
a huge roll of 3 mil polyethylene (Dow Chemical Heavy Duty
Polyfilm).
4) How thick should the plastic be?
5) Can I use multiple layers of my 3-mil plastic, or will I
get condensation between layers?
6) How does one seal between different sections of plastic?
Replies
I'm not an insulation specialist, but I do live in Minnesota so I know about keeping homes warm.
The vapor barrier is important but with an older home and an unheated attic it is not necessary as long as the attic is well vented. This requires both eve and ridge venting to keep the cellose insulation dry. Alyhough if your set on a vapor barrier, I would not crawl around in a attic gluing strips of poly between the joists. Another choose is to use fiberglass insulation with a kraft facing (vapor barrier) ceiling side down. Additional cellulose can be added to get the right depth. Another choose is using a vapor barrier paint on the plaster ceilings. I've heard that oil paints, stain killing paints can do this but I haven't tryed it myself.
I'm not sure why you have added the 2x8's. All I've explained could be acheived without the added joists. Are you planning on adding an attic floor? for what purpose?
The attic is a huge space, 43' x 43'. It has a hipped roof that is about 20' high in the center. It even has two dormers with windows. I'm planning to use the space for storage after putting in a subfloor, though friends have suggested holding square dances....
The original 2x4 joists were barely able to hold up the plaster. In fact, several of the joists that had to run an additional three feet into a bay window had failed. We jacked up the ceiling below in this area over 4" and secured it with the sistered 2x8's. I can now walk around up there without hearing the plaster below crack (and I spent a lot of time up there while we rewired the place).
The sistered 2x8's have left the spaces uneven, so the fiberglass would have to be cut to fit. I think I want plastic in any case to keep the insulation dust out of the house.
Thanks for the explanation. I now have a better understanding what your up against.
Using plastic, use only one ply. I use a 6mil poly. I would not suggest gluing the subfloor to the poly. Just use nail the subfloor down. I would not floor the whole area and leave plenty of space between boards to aid in the venting issue I noted earlier. Drape the poly up and down the joists and if you have to cut it, make sure you patch it. You might try "great stuff" foam at the seams, etc.
You might consider foaming the whole attic. No dust and it's closed cell so the moisture, vapor barrier solutions is not an issue. I believe this might break your budget though.
Good luck, sounds like a fun project.
Brokenhead,
For attic insulation, the vapor barrier is supposed to be face down so that heat rising from the room below the attic is stopped from enteriing into the attic and penetrating the subsequent insulation. If you were installing kraft faced insulation in the attic, the kraft paper would be positioned face down and the insulation would remain exposed on top. At this point you would then be installing your attic subfloor, which you could easily affix construction adhesive to the top edges of your 2X8 joists before laying down the ply subfloor and nailing off.
Since you are planning to use blown-in insulation between the joist bays due to the spacing irregularity ( which is a good idea)...if you wanted a moisture barrier, you would install strips of poly sheeting down into the bays first. Then blow the insulation down on top of the poly...then install your subfloor. This poly moisture barrier in no way affects your subfloor glue-down issue...for there is no such issue to worry about...the poly is not in contact between joist and subfloor.
You said the attic was unheated and was to remain that way. In regards to this I, would as earlier suggested, just skip the moisture barrier, blow in the insul, and then attach the subfloor overtop. You want to glue the subfloor?...sure, no problem. Any moisture that may pentrate the attic area will eventually find its way out through proper vent channels.
Installing poly on top of the attic joists ( as you were indicating earlier) and then installing the subfloor would create a situation where uprising moisture from the room below would become trapped in the layers of insulation and remain there...possibly causing a mold situation over the long haul.
Because the blown-in insul just lays in place, there will not be dust filtering down through the old plaster ceiling ( least not unless this ceiling is cracked all to hell already.) Dust is not gonna rise up through the new attic subfloor (once installed) either; so your fears and reasons for wanting to install a poly moisture barrier are ( in my opinion) base-less.
Good luck on your project.
Davo
As Davo pointed out, the poly is not neccesary. Thanks also for commenting on the possibility of trapped moisture accumulating in the joist if the vapor barrier covers them. This could cause serious concerns down the road. Remember to properly vent this attic space to prevents mosisture buildup in the insulation which renders it useless. Being in colorado, you would also want to prevent heat build up in the attic, which would cause snow melt and possible ice damming. Good luck.
Happy insulating
I have an older house than yours also with plaster ceiling and celulose insulation. The only vapour barrier is the plaster and paint. There have been no problems as long as the attic is ventilated. Do not glue down the plywood because it can become a vapour barrier as well trapping moisture.
I faced the same issues some years ago while planning to insulate the floor of an addition with unheated crawl space underneath. I ruled out poly because the glued tongue-and-groove plywood is the vapour barrier. Our winters are almost as cold as yours.