Hybrid insulation and thermal mass

Heya,

I am getting ready to build a 16x16 structure in an hybrid fashion.  I am wondering if anyone else has done anything like this before.... and with success!  

Imagine a 'normal' 2x6 stick construction all the way around.  Infill with formaldehyde free fiber glass insulation.  Without any vapor barrier, adding wood lap siding to the exterior.  Then on the inside, a 4" (properly secured to the stick framing) layer of earthen plaster with a final layer of Natural American Clay.

The concept is to have thermal mass on the inside of the insulation to absorb the wood stove heat in the Winter versus burning me out. Also to have R19 insulation but without vapor barrier so that the earthen layer can breath moisture in and out and won't get too trapped (slight worried about the lap siding but I might not caulk it to allow vapor movement.

This room with be connected to a 12x16 room with Hvac.  So any excessive moisture coming inside during the hot humid months of central Virginia will be moved around with fans and removed by a ductless HVAC in the 12x16 room.

So do you all think this will work?

Randy Richey wrote:Heya,

I am getting ready to build a 16x16 structure in an hybrid fashion.  I am wondering if anyone else has done anything like this before.... and with success!  

Imagine a 'normal' 2x6 stick construction all the way around.  Infill with formaldehyde free fiber glass insulation.  Without any vapor barrier, adding wood lap siding to the exterior.  Then on the inside, a 4" (properly secured to the stick framing) layer of earthen plaster with a final layer of Natural American Clay.

The concept is to have thermal mass on the inside of the insulation to absorb the wood stove heat in the Winter versus burning me out. Also to have R19 insulation but without vapor barrier so that the earthen layer can breath moisture in and out and won't get too trapped (slight worried about the lap siding but I might not caulk it to allow vapor movement.

This room with be connected to a 12x16 room with Hvac.  So any excessive moisture coming inside during the hot humid months of central Virginia will be moved around with fans and removed by a ductless HVAC in the 12x16 room.

So do you all think this will work?

I'm doing something similar to a 1970s ranch remodel that I'm working on. I'm building a pizza oven in the corner of the kitchen along an exterior wall. The wall is 2X4 construction skinned with 3/4IN CDX plywood which was standard construction back then. I took out the FG roll insulation and used it elsewhere in the house. A friend had given me some 2 IN thick sheets (4x8) of foam faced with foil radiant barrier. It's not the sort of stuff that I would buy but it was given to me so I may as well reuse it. The remainder of the wall cavity (3 1/2 wood stud minus 2 IN foam will leave a gap of 1-1/2 that I'm going to fill with clay rich cob for thermal mass. Then I'm going to plaster the wall with roman cement (Lime+pozzolan+sand) for even more mass. The idea is similar to yours; create a thermal mass to absorb the heat from the pizza oven but since I'm retrofitting previous construction, I'm also using some modern materials. The biggest drawback will be the time required to allow the cob to dry but I'm in a very dry climate so it shouldn't be too bad. How are you going to build your 4IN earth plaster wall? Are you going to build it up in layers? Four inches is thick enough to make a rammed earth wall if you had backing on the stud side.

Sounds good. Cob Cottage Company in oregon is doing hybrid bale/cob buildings with the same principals of insulation wrapped mass walls. They have great success with this method and I imagine it would work just as well for a stick frame with fiber glass. They have found that cob works well to absorb any moisture that might accumulate in the insulation. I am currently building one of these hybrid homes on an earthbag foundation. Check out the link in my signature if you want. We haven't started the walls yet though. Good luck and post photos for us!

Since there will be neither house wrap nor a rain screen, perhaps rock wool would be a better insulation choice.

Thanks for the replies!

I did at one point want to do straw bale construction but I can't find any straw bales that were not grown with pesticides and insecticides - I am highly chemical sensitive.  I was then set on doing hempcrete (cob using hemp instead of straw) but they pushed back the vote to legalize growing it here in Virginia until next year with the earliest legal time to grow is 1 July 2018.  Hempcrete would be perfect because of the R value (about 1.5/inch) and it has thermal mass - best of both worlds.

Unlike Andrew, I am in a very humid climate, so I do have to watch vapor flow.  William, you might be right about rock wool, and by itself it seems toxic free but they apply, so I understand, fire retardant on it and that wouldn't work for me. Dan, saw your site - looking nice!  I took a few courses in straw bale construction with lime stucco on the exterior and how this system allows the walls to breath.  I have 48 acres mostly wooded and a sawmill, so other than work, lap siding is economical for me and I do take great care on selecting which trees to use.  I will be putting tung oil on the inside of the lap siding but I am still worried how, in the long term, it will hold up to the moisture.  

It is always fun trying to figure things out!