Adding a layer to an already-insulated wall?

[EricQC]

I plan to renovate an open area of ~900 ft² (84m²) within a well-built horse barn. I'm located in Quebec, where we can get winter temperatures down to -40 (C and F) -- but typically more like -25C (-13F) and very dry -- and summer temperatures up to 35C (95F) with high humidity. I hope to install radiant floor heat in 3 zones to make the space usable in any season.

The exterior walls have the following (from the outside): steel siding; black cladding paper; ½" (13mm) plywood sheeting; 2x6 stud walls with 6" of glass fibre insulation; a plastic vapour barrier; then interior cladding of 2" T&G hemlock.

I have two questions related to avoiding moisture buildup:

1) For the finished interior, can I put drywall directly over the T&G without creating a moisture problem? The hemlock has been in place for >20 years, so is probably now as dry as it will get.

2. If I decide to upgrade the insulation by adding a layer (either new stud walls or foam panels), I would prefer not to have to remove the T&G. However, if I do add insulation inside, I'm concerned about causing moisture to build up within the hemlock. Is there an insulation solution that would not involve removing the T&G boards?

 

[Perry525]

The place to start is the fact that water vapor is put into a home by the people living there.
It comes from cooking, washing, drying, breathing, sweating, indoor plants, fish tanks, animals.
Water vapor is attracted to the nearest cold surface and your walls are cold.
You should start by installing an exhaust fan in each bathroom, kitchen, utility room. Exhaust fans fitted with heat exchangers are best as these will save 90% of your heat and if fitted with a humidistat will turn off once the humidity level set by you is reached.
You have two choices, fitting five to ten inches of polystyrene sheet over the outside of your walls, ceilings and floors or fitting the same amount on the inside.
Heating is getting very expensive and will continue to rise, insulation at the proposed level will last forever, will give you a very comfortable home and will save 90% of your heat and will pay for itself in three to four years.

 

[EricQC]

You have two choices, fitting five to ten inches of polystyrene sheet over the outside of your walls, ceilings and floors or fitting the same amount on the inside.

Thanks for the response Perry. I do intend to install heat recovery ventilation, but I'm not sure that will be enough to eliminate moisture problem in the current hemlock interior surface if I add insulation on the inside. (Adding on the outside isn't an option for design & cost reasons -- but it would still create a volume sealed between vapour barriers.)

Poly sheeting inside (probably XPS blue foam boards) is certainly an option, but 5-10" seems like overkill. With foam batts in a 2x6 wall, I probably have ~R19 now, pretty close to the R20 minimum recommended for above-grade walls in Canada. Adding 5" of XPS would give me more than R40 (and 10" would be >R60).

The easiest method would be to just screw gypsum board panels through 2" XPS boards directly over the existing hemlock T&G to get ~R30 -- but only if that would not create a moisture problem between the existing plastic vapour barrier and the new XPS board.

Although painted gypsum board does create a vapour barrier to some extent, I could avoid sealing the hemlock T&G between impermeable layers by building a new interior 2x4 stud wall inside (and use more fiberglass batt insulation to bring me up to ~R34), but this approach would be more complicated and would reduce the finished room dimensions.

Since I'll have solar HW heating for the radiant floor, I'm not overly concerned about the energy cost for heating.

My real concern is still about avoiding entrapped moisture in the wall between the vapour barriers. Am I missing something in my analysis or is my concern not justified?

 

[AU_Prospector]

Polystyrene yields R-5 per inch of thickness. This is also a vapor block and will not allow air movement if installed correctly. (covered completely with no gaps or gaps properly taped or sealed with foam)

Assuming your talking about a barn and not a home, you are not concerned about the look of it?

"!!!The easiest method would be to just screw gypsum board panels through 2" XPS boards directly over the existing hemlock T&G to get ~R30 -- but only if that would not create a moisture problem between the existing plastic vapour barrier and the new XPS board.!!!"

I believe your application above is acceptable. Do it now (winter) when the wood is at its driest. Use greenboard and do a good job taping it. Your layers are right on top of one another, right? Air will not be able to pass into the space between the XPS and the plastic sheet, so the wood should be fine.

Your ceiling insulation should not have a vapor barrier or you will completely trap moisture. You should also have ventilation installed just for those times when your interior moisture goes up dramatically. (busted pipe, horse kicks over water trough, etc... think 200 cfm bathroom fan or something like that).

 

[EricQC]

The building was originally built as a (luxury) horse barn, but no animals have been in it since 1989. The inside is still faced with the original hemlock boards but they are marked up in places where horses had been able to kick. Since hemlock gets very hard as it dries, removing it would be quite difficult. As well, covering it over retains its structural contribution.

My intent is to have heated living space with clean drywall walls and ceilings. The space will be used for a workshop and studio, but also as overflow accomodation for guests and for people staying with us to help on our organic farm. The kitchen (also used for beer brewing) and bathroom (shower & composting toilet) will be fully vented with an HRV system to manage humidity.

Good point about doing the walls in winter when they will be dryest, but I'm not so sure about the ceiling advice. If I don't have a vapor barrier under the glass batts, won't that cause problems where moisture does manage to leak through? Certainly all of the ceilings in our house have very-well-sealed vapor barriers between the interior drywall and the glass batt insulation in the attic.

 

[AU_Prospector]

Hmmm, I think unfaced insulation is for the attic. Both homes I have owned and also my mother's home is without vapor barrier in ceiling, but with vapor barrier in exterior walls. My current home is a combination of blown in fiberglass and fiberglass batts in the ceiling with no vapor barrier. Go to a home improvement store and look at the unfaced insulation, it should say 'attic' on it. You dont want to completely trap water vapor, the living space has to breathe.

In your ceiling, what would the water vapor leak through to? As long as it can get out to the outside without condensing on something, there would be no problem. ie to a well ventilated attic, etc.

 

[EricQC]

The attic is open, and includes very good ventilation to the exterior, so any moisture getting up into it will be vented away. Note that there is already insulation above the ceiling: from the inside, it has 1/4" chipboard with a plastic vapor barrier above it nailed to the trusses, and then filled with 4" of glass batts. I will need to replace the chipboard (it sags and looks ugly) and would likely replace the vapor barrier before putting on drywall. Of course, I could consider sandwiching some rigid foam panels between the drywall and the trusses I suppose...

 

[AU_Prospector]

I see. If it were me, I would remove the ceiling/attic vapor barrier when you remove the chipboard. Perhaps the vapor barrier was put there at a time before the space was intended to be inclosed as a people living space. If your going to have a kitchen/bath here I would remove the plastic, but that is just me. Otherwise I would think you might trap too much moisture, it might become musty.

Having a spacious and properly ventilated attic is key, the water vapor will dissipate to the outside and not at all be a problem.

Oh and the rigid foam is cost prohibitive. For about 35 cents per square foot tax included, you can get R-30 unfaced batts. Where as 2 inch thick polystyrene sheets (only R-10) run almost 1.00 per square foot tax included.
You get more bang for your buck with batts in the attic space. Layer them crosswise if you put in more than one layer of batts.

 

[nealtw]

Your concern about 2 vapour barriers is a good one. You could drill a few holes thru the hemlock in each bay and that would allow air movement between old and new. I would leave the ceiling in place and add drywall to it, the poly up there is required.