On this retrofit project, the home has vaulted ceilings, so plenty of room to add insulation at the interior ceiling and re-finish.  There's existing 7.5" (poorly) insulated ceiling joist bays with 1/2" drywall finish, so one option is to simply add say 4" of EPS to the assembly, screw & glue to the drywall, and then finish that with perhaps cedar panels.

 

Question is, is this a durable assembly, one that won't have moisture or other problems?  Ideally we'd add it on top of the roof sheathing, but the roof is pretty new so theres no interest in re-roofing.  In the Pacific northwest a mostly heating climate we dry from the inside out, is this assembly going to put the dew point in the wrong place or something? Anyone ever done insulation added from the inside retrofits?

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I have done both interior applied and exterior applied. Both of us will agree that exterior is superior, with inherently higher integrity. But reality dictates, so will not argue that point. When you say EPS, I presume you mean expanded polystyrene, the "styrofoam cup" material, versus extruded (ExPS), the pink or blue board, more of a closed-cell product? I would favor the latter. Regardless, your system seems fine to me, a tightening up of the inside and whatever may get through the first layers will disperse through the more permeable layers.

My caveat in the interior application, if not obvious, is the continuity factor. The ultimate success of adding an isolated plane of high-integrity continuous insulation in a structure is dependent on what you do at the edges, the margins. How do you prevent "leak around", both thermally and of air?

The best I could do (with an interior wall application, 1.5" polyiso screwed to studs which had blown-in FG in good condition, under strapping and under blueboard with veneer plaster): I used expanding foam above and below to seal off any stud cavities (balloon framing, 1873 house). Also, foam the ends of the joist bays with closed-cell foam.

In your case, or any case, I would think of the geometry of that added assembly, where it meets the conventional framing system, basically "think like heat would" or "think like air would", to grasp those paths of least resistance. Then go after them. My over-riding concern is, if this assembly is a 'one-shot then you stop' approach, it may not yield the benefits you hope. If it is Phase One of an ongoing project, that is another thing, but you still have to contend with intersecting partition walls and those intersections are usually concealed in vaulted or cathedral ceilings. Best you can do there is to use expanding urethane foam in the upper stud cavities. Imperfect, but the best you could do, I submit. Hope this helps in some small way.

If the insulation is on the inside in a heating climate it needs to be an effective boundary against vapor transmission to keep moisture away from a potential condensing surface.  Especially if there is a tongue and groove ceiling finish.  A good method would be to install two layers of foam board with the seams offset in both directions and taped.  Penetrations for electrical boxes should be treated carefully.

On the other hand, one wonders whether it wouldn't be easier, more cost and labor effective, and do a better job to take down the existing ceiling and insulation, foam the cavity,and put a ceiling back up.  For a better job, add your foam board with taped seams, and install new sheetrock or whatever on furring.  This has the added advantage of bringing the existing marginal cavity insulation up to snuff.

 

 

I would lean more towards Ed's last solution with a caveat, I am assuming this roof is vented - if so I would trim down some XPS panels as firring strips 1.5" x 1.5" placed against the roof sheathing against the joist (2 in each bay - maybe 3 if 24" OC), install XPS panels foaming the seams, between panels & the edges inside the cavity with 2 layers of 1.5" panels & a final 2" panel. You know have 5.5" of foam, still have ventilation and can now easily install either drywall or cedar. Trying to install drywall or cedar with 6.5" connectors (that is what your system requires) can be a nightmare (especially on a finished ceiling)

 

Now if there is no venting - rip, remove & Closed Cell Foam

 

Either which way you are looking at a better structure with a better finish --- I would also recomend checking everything with an IR camera / blower door to verify everything is done properly / sealed tight

In 2006 I general contracted a house that we now live in. I was going to foam on top of the 5/8 sheet rock (needed for fire code) that was installed on the vaulted and drop ceilings. Because of cost and schedule I decided to instead use 2" thick Dow Thermax (R13 per Dow web site Vs R8 to10 for EPS) on top of the vapor barrier(which was put up before changing plans from blown foam to foam board) and then sheet rock onto the foam using 4 inch screws. I was a little worried about hitting the lower cord with 99% of the screws but I did not see one miss when I checked the attic. Since the vault is a 4/12 pitch most of the screws are in tension and not shear through the 2" of foam. I was a little hesitant to do this because it initially put the vapor barrier on the cold side. Since the thermax  I used has a 1 mil aluminum layer on both sides and I used 2 inch aluminum tape to seal all the seams and aluminum is significantly less permeable to vapor than poly sheet, I went for it. The foam on the inside also had the advantage of insulating the lower 2x4 cord on the scissors truss for a little better overall R value. Because I wanted access to the attic to check for condensation on the cold side vapor barrier, I did no further insulation, either blown glass or batt being left for the future. The roof system has gone through  three brutal Minnesota winters with a high potential of condensation on the cold side vapor barrier. Because I have easy access, I checked the whole attic including the perimeter several times. I have seen a few drops exactly where I  expected, on the foam board seams and at the perimeter. In no case was it enough to cause wetness under those areas and infact dries out seasonally with the vented attic and I believe during certain periods in winter. This seems to prove that using Thermax with the .001 mil aluminum and taped joints is a good vapor barrier. After installing the blown glass insulation, I expect the cold side of the thermax to remain above the dew point even during the coldest periods.


    The Dow Thermax sheeting is fire rated and depending on local codes sheet rock may not be needed. I did install 1/2" plywood screwed to the trusses through the foam and sheet rock. The plywood really being the only way to fasten the cedar to the ceiling. Using paneling the plywood's not necessary.

   Be sure to do the building science for your climate.

Leo K. PE

Nice project, Leo.  If your Dow rep is as stellar as mine is, she can probably help you out in future with a piece of software they have that models your wall assembly and calculates where the dew point will fall at a given temperature.
I probably should have been more clear, but yes, the new applied interior skin would have an appropriate class of vapor retarder. That said, if the edges are not sealed, even the insulation will not matter. Heat and moisture will go where it is not.

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