I have a 1830 Post and beam house that has had all the beautiful beams covered with lath and plaster, then sheet rock over that. I plan to rip all the lath and plaster out and expose the beams in one of the rooms. I am trying to figure out how to get some insulation behind the sheet rock. The main beams are 8 inch wide but the cross members are 4 inches and are centered in the beams and I would like to have them exposed as well. I have 5 inches from where the face of the sheet rock will be to the outer sheathing. I have an unlimited supply of poly iso boards that are 4ft x 4ft and are 3 3/4 inch tapered to 3 1/4 in. They are aluminum faced. I thought about using liquid nails or another heavy duty adhesive and glue the Poly ISO to the sheet rock then cut the sheet rock/polyiso boards to fit. Do I need a vapor barrier between the poly iso and the sheet rock? How about between the poly iso and the outside sheathing? Would I be better off buying XPS and gluing the sheet rock to that with the same process? Can I push the XPS or the poly iso against the outer sheathing or does there need to be an air space between the two?

A foil facer IS a vapor barrier, so you already have vapor barriers facing both sides.

The real question is how to make it air-tight, and whether putting a vapor barrier there would cause other issues with the siding/sheathing/paint. You need to seal the edges with can-foam to reduce the possibility of transporting moisture from winding up in the sheathing/siding in winter.

Your exact location/climate is a factor too.

Tell is more about the sheathing & siding types, and stackup on the exterior! (Include any tarpaper, housewrap, rosin-paper etc in the description.) Also, how deep are the roof overhangs? Depending on the stackup you may need to provide a 1/4"-3/8" air gap between the iso and the sheathing vented top & bottom to the exterior, but in many cases that would not be necessary.)

BTW: Don't use Liquid Nails or any other standard construction adhesive, since a tear in the aluminum could lead to the solvents eating into the foam. There are formulations of construction adhesive with different solvents, designed for exactly this use- it's called "foam board construction adhesive". It's a bit more money, but it won't break the bank.

The wall build up goes like this from outside in. Paint Wood Siding 1/2 pine boards as sheathing 8x8 Beams or air space lath plaster wall paper Sheet rock wall paper paint You can see why I want to go down to the beams. They are beautiful beams and have a lot of character like bark and tenon joints. Right now I have one section of the wall removed to expose the beams. There is no air movement at all. Inside temp is at 71 and outside temp is -2 f. The house is located in upstate NY. I planned on sealing the edges with can foam but I get nervous spraying that stuff on anything because once its there its there and its a pain to remove. I'm not convinced that it lasts that long either. I sprayed some outside last year and this year it looks all yellowed and chaulky. Im sure its breaking down. I will look for that adheasive. I figured someone would have a better idea than liquid nails. (Another product that feels too much like a short cut)

Would it be better if I got the vent trays that are used in vaulted ceilings. They create an a small air space between the sheathing and the insulation.

Next year i plan to remove the wood siding and I will put up 1/2 inch pink board over the entire outside to create a complete thermal break and then I will house wrap it and cover it with vinyl or possibly wood again.

Posted By Biglurr54 on 24 Jan 2013 07:52 AM
...I planned on sealing the edges with can foam but I get nervous spraying that stuff on anything because once its there its there and its a pain to remove. I'm not convinced that it lasts that long either. I sprayed some outside last year and this year it looks all yellowed and chaulky. Im sure its breaking down. ...

The surface layer is breaking down due to exposure to UV rays from the sun.  If it is covered and not exposed to sunlight, it won't break down like that. 

Posted By Biglurr54 on 24 Jan 2013 07:54 AM

...Next year i plan to remove the wood siding and I will put up 1/2 inch pink board over the entire outside to create a complete thermal break and then I will house wrap it and cover it with vinyl or possibly wood again.

If you are going to that much trouble, I would put up more than just a half inch of XPS foam (~R2.5).  I would put on at least an inch, and preferably polyiso (R6).

He's talking about 3.5" average thickness tapered foil faced iso on the inside, and if he put any thickness of foil faced iso on the exterior it would create a moisture trap. Up to an inch of XPS or closed cell spray polyurethane on the exterior would be OK, or even 3" of EPS, but under no circumstances would putting foil faced goods on the exterior be a good idea when there is foil-faced goods on the interior. Even 2" of XPS might be too vapor tight.

Half-inch pink board XPS (R2.5) is a barely measurable thermal break for 8" thick (~R8) posts & beams, but 1 inch XPS (R5) is reasonable, cutting the heat loss through the beam by nearly half (looking at the isotherm sectional view with 2-3" of the posts & beams exposed to the interior conditioned space, not the simple 2-D model of a flat wall of uniform thickness.) Even 1" unfaced EPS would be pretty good, since at the cold temperature extremes it'll be running close to R5 in this application & location, even if it's annual average R performs at about R4.

If it's only going to be a year before the siding gets pulled you needn't be concerned about vent space between the iso. You don't say what type of siding- there is a pretty big difference in moisture handling characteristics between ship-lap siding which stay's pretty flat to the sheathing, with no back ventilation and clapboard or shingles, which are inherently back-ventilated, at least a little, and has much better drying capacity toward the exterior. But if it's coming off within a year there's not enough time to accumulate any real damage even if it's current configuration doesn't dry very well. If you put even 1/4" of air space between the exterior foam and the siding when it goes back up, both the sheathing and the siding are much more resilient to moisture intrusion, since they can dry reasonable quickly.

When you strip the siding, be sure to add a properly lapped housewrap or 15# felt, and be sure that it interfaces with window & door flashing with the correct lapping too. If the windows are the original antiques (or antique-enough) there may be NO window flashing at all, but this would be the right time to add some, since once you've put the iso on the interior and EPS/XPS on the exterior any bulk water intrusions will dry far more slowly than it did during the prior uninsulated-no-vapor-retarder condition.

The windows were replaces around 1900. They are sill wavy glass. There isnt a good flashing around them. I will add flashing when the siding comes off. The siding that is on there now is clapboards and that is what will go back up in a year. Hopefully i willbe able to find a decent looking vinyl that will keep the original look of the wood. That should provide enough airspace between the EPS and the siding.

Standard grades of vinyl siding are inherently back-ventilated, and don't require specific rainscreen venting.