Exterior Wall Foam Insulation and Interior Mass question

[MartyM]

I'm not sure if I should start a new thread for this question or not. (MD split topic)

I have a tiny cabin  ( 8x 18 ft ) I am building right on the border of  3 and 4 climate zone.  2x4 framing r13 bats and r3.5 foil faced foam on the exterior. It looks like I am above minimums.  The way I understand this type of insulation system, any moisture entering the wall cavity will need to dry to the inside.  Correct? 

With such a tiny space, just opening the door to enter the cabin I will loose a relatively large percentage of my conditioned air.  My thinking is that to help with the temperature swings I will add as much thermal mass to the interior as I can, within reason of course. One of my ideas for adding mass is thicker drywall. That got me thinking that plaster may be a better solution to add mass to the walls.

Does any one know how well plaster breathes, and or holds moisture  in comparison to 5/8 drywall?   And while were at it any other mass adding ideas would be welcome.
   

[MountainDon]

Yes moisture dries to the inside in this case.

If this is an occasional use building, be careful of the interior thermal mass. That can work against you when you arrive to a cold cabin and have to heat it up. That said, we used 5/8 sheetrock in out cabin in climate zone 5 and warm up is not all that bad, just a few hours when it's zero. Be careful selecting primer and paint. Do not use one that advertises low permeability on the inside.  Not sure about the breathability of drywall vs real plastered walls.

[Don_P]

As compared to drywall, I don't know... gypsum vs lime. Historic lime renders do "breathe" much better than portland cement, this has been a topic among log and timber builders.  When we switched to portland cement it worked well for concrete and mortar for modern brick but when used for infill in timber frames or chinking in log buildings it trapped moisture and rotted a bunch of wood. Lime based plaster finishes do not cause these problems as the moisture can move through the limestone. Better fire resistance than drywall. That was roundabout but I think it'll work. Occassional use means you probably won't be producing a whole lot of moisture.

What you are creating is a flywheel, yes it will take longer to cool down, getting a large thermal mass up to temp is not fast. Weekend visitors to log homes often complain about how cold they are. By Monday morning the mass is warming up and the building slowly coasts back down over the next several days.

A cold impervious mass that is faced with rapidly rising temperature and humidity condenses moisture on its' below dew point temperature surface... the sweating garage floor in spring or my poor cast iron tool surfaces. More than once I've been called to talk about a leaking basement when it was condensation collecting on a cool floor. Plaster is not impervious and will adsorb a good amount of moisture, acting as a hygric buffer.

For occassional use I tend to think that lightweight fast reacting construction is more appropriate. When the average daily temperature is very close to our comfort zone, then a flywheel clips the highs and lows... say 100*F daytime and 50*F nite, a large mass helps to average those temps. Rather than AC during the day and heat at night, this has potential for no heating or cooling. Some folks want to extend that thought out to seasonal or annual averages, they are incorrect unless they want to insulate and charge huge masses. The thoughts I've seen along those lines are generally pretty far from any current reality. Going into colder climes where the entire ambient temperature average calls for heating  or cooling the mass is helping with the door opening problem but is not lowering the overall heating load.

We built a very nice workshop/garage with living area above a couple of years ago. The downstairs slab is radiant, lots of mass. The thermostat in the garage is wall mounted. When the doors are opened the air exchanges and the boiler kicks on as the wall mounted thermostat calls for heat. The doors close and the mass almost immediately rewarms the room. The heater shuts down but has been trying as hard as it can to heat the slab, the room overshoots the target and becomes uncomfortably warm until the flywheel can wind back down. This is a control logic problem, just pointing out the law of unintended consequences.

[MartyM]

Thanks Guys.

The fly wheel effect is what I am after. I was thinking a 12 hour cycle would be ideal, but I'm wondering if that is even attainable in such a small scale?   As for getting a large thermal mass to a comfort zone.  I think large is my main problem.  For my build I am dealing with what is really a well built and well insulated small mobile home.   Most of the walls will be covered in wood. Just the top 1/3 will be drywall, so about 400sq ft of drywall or plaster.  Really not that much mass for thermal use.   I am leaning towards tile floors for the same reasons. But still that's only 144 sq ft.  I think I can squeeze some stone in around my small wood stove to help out as well.

For the first few years it will be occasional use but  I have hopes of adding a bedroom and living room  and living there year round.

[Woodswalker]

I used sheetrock in my small (16' x 20') cabin for thermal mass.  Heat with a small wood stove.  Good insulation and thermal mass work well to reduce temp swings.  You could do a double layer of sheetrock to get lots more mass, much easier than going with plaster.

[MountainDon]

And 5/8 is better (more mass) than the std 1/2".  I used 5/8 type X

[Squirl]

I don't know what kind of foundation you are going with, or how rigid you built your structure.  I have lived in and worked in many old houses with plaster.  It is a complete nightmare.  Drywall is a great product in my opinion.  I never had to repair the same cracks in drywall that I did in plaster. 

You can also add mass in many other ways.  Brick wood stove surround, stone counters, tile floor, etc.
After the first inch from the surface, it starts to lose it's return on investment.

Most books on passive solar heating recommend 5/8" drywall.  It holds a good amount of heat, isn't too thick, and they say that the binders for the gypsum (some type of wax?) also help to gain and release heat.

There is a lot of research into the day/night heat cycle and thermal mass in many passive solar books.