Remodeling existing block house on lake
I am remodeling a 70 year old block house on a concrete slab in zone 4A. I am removing the old roof and interior wood frame partitions and windows, all that will remain is concrete exterior block walls and slab. I am building a carport roof that will completely shade the existing house and an attached carport/ shop area, this roof will be completely independent of existing house being 6-9 above the house. This is to provide cooling shade and storm protection from falling trees. The current house has no insulation or heating and cooling. It’s a lake shack. In the summer the temperature is slightly cooler than outside due to some shading, the attic and fans. I have never experienced any mold problems at all, the interior walls are painted block and the interior walls are pine plywood. The relative humidity is the exact same inside as outside- very high, it’s 150’ from large lake and in NC. The NC energy code will require that I install insulation and hvac. I DO NOT WANT TO USE THE AC IN THE SUMMER, because if AC is on people won’t use the lake and I’m nostalgic. I want to take advantage of the existing block as a thermal mass and “outsulate” the house. I need a wall insulation system that will allow drying To the outside. I want to use clay plaster on inside over the the block, rock wool on the exterior with an air gap and galvanized steel corrugated sliding. The roof will be low slope SIPs with tpo membrane (or wood joists with dense pack cellulose). NO AC in summers, windows will be open all the time, heat will be on in the winter with wood stove and mini splits if needed.
The reason the house is mold free now is there no difference in humidity and only a small difference in temperature in summer (it’s more like a tent than a house) . I want to make it slightly cooler in summer without AC but don’t want to create any problems.
what insulation systems can I use to avoid moisture and mold issues?
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I get the aesthetic of wanting to have the windows wide open in the summer. If the climate allows for it that's a very pleasing way to live.
I would expect your big concerns are going to be minimizing solar gain and managing humidity. If the house is shady and breezy on the inside it should be comfortable. Each resident of the house is going to be releasing about five pints a day of moisture into the house, if the house isn't air conditioned and isn't well-ventilated that is going to make it feel clammy inside. The key to good natural ventilation is having cross-flow, operable windows or sliding doors on opposing walls. You block solar gain by having the house be well-insulated, particularly under the roof; by having low solar-gain windows; and by positioning windows so that they are either shaded by an overhang in the summer or don't face the sun.
I'm not crazy about the idea of insulating the concrete block on the outside to try to get it to provide passive cooling. In your climate it's going to be very difficult to provide cooling without condensation. Schemes like that often don't work, but if it did you'd probably find the cool walls were prone to condensation. I think you're better off insulating on the interior.
Code requires it, but I think it's a good idea also to have the ability to close the house up and run AC when it's hot or heat when it's cold.
I'm thinking a layer of foam board against the inside of the concrete block, then a stud wall filled with batt insulation. I don't think the block is going to be able to do much drying, you need to protect it on the interior from condensation, hence the foam.
insulating the inside the way you describe would cost me quite a bit of square footage in an already small house. i also do not believe the interior of the blocks walls will ever get so cool as to cause condensation. As I stated, I will have a complete heating and cooling system (and whole house dehumidifier) installed, I just won’t use it unless i have to. as things are now and hopefully will be, no one is inside except to sleep or cook. everyone spends all their time outside or on the porch. the house will be completely shaded with lots of windows allowing ventilation
I don't see that allowing the wall to "dry to the outside" gains you anything here. If the windows will be open, so humidity levels will be the same, you won't really have any moisture drive into (or out of) the wall, either. All the block will be doing is averaging out the temperature a bit between day and night, it will not provide any actual "cooling" of the structure.
If you really want something vapor open, you have two choices:
1- Rigid mineral wool board ("Comfortboard"), which is very vapor open, basically like a regular batt as far as vapor permeability goes, but it's expensive. This is by far the most vapor open rigid insulating material available.
2- fiber faced polyiso. This is much less vapor open than rigid mineral wool, but it's also cheaper. The main downside here is that you do NOT want that polyiso to get wet, so you need to be more careful about protecting the exposed edges from bulk water. I'd pay particular attention to the lower edge here, which should not be in contact with the ground.
Bill
I plan on using 3” of comfortboard against the block with a 1x4 on top of that creating an air gap and galvanized corrugated siding. it’s a small house so the extra expense of the comfortboard will not be so onerous. I have seen in other posts that a waterproof coating on the exterior of the block is suggested. I do not understand why as this would inhibit drying to the outside. in the winter the house will be heated which will drive moisture outward and my main concern is to not trap any moisture in the wall at all.
You want to put a vapor barrier/retarder on the interior side if you're worried about moisture in the winter. You want to keep the moisture from getting into the wall in the first place.
Note that concrete block walls do not have the problems wood framed walls do in terms of moisture, rot, and mold. Concrete doesn't care if it's wet. You only need to worry about mold growth on the surface, not within the wall. Note also that a "waterproof coating" can still be vapor permeable. Tyvek is an example of a product that is a water barrier but is still vapor open, for example.
Bill