# insulating interior/partition walls in a high thermal mass home?



## K.B. (Sep 7, 2012)

We are getting ready to close in our interior walls now that the plumbing and electrical work is complete. One of the questions that we have been going back and forth on is whether or not to put any insulation in the interior/partition walls before closing them up. 

The house is oriented for passive solar heating/cooling with solid 8" masonry walls and a slab floor. The exterior walls are insulated to R21 (foundation has R15), the ceiling to R40.

My thought is to hold off on the insulation of the interior walls to see how the thermal performance works out, then blow it in later, if necessary. I want the heat to be able to redistribute freely between the rooms inside the home. The woodstove is in the center of the house and I don't want to end up with more cold spots than necessary due to layout. 

Any thoughts are appreciated.


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## SolarGary (Sep 8, 2005)

Hi,
What would you be aiming to accomplish with insulation in the interior walls?

It seems to me that it would reduce heat transfer from the spaces that are heated by the passive solar to the rooms not heated by passive solar. That is, it would make the rooms not directly heated by passive solar run cooler.

You might consider adding mass to the interior walls. This would decrease the daily swings in temperature that come with passively solar heated homes. Mass not located directly in the sun is less effective, but it still absorbs some heat during the day (from the solar heated air), and gives it up at night.

Are you planning to use insulated shades on the solar windows to reduce night time heat loss?

Gary


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## K.B. (Sep 7, 2012)

Hi Gary,

Yes - we will use insulated shades on the windows at night, as necessary, depending on the heating needs and thermal performance of the house. 

Some of the interior walls that we are considering insulating are the bath/shower walls. I like the idea of the bath and shower staying warmer, but we have windows in these rooms as well and I don't want to have them retain too much heat.

Sound dampening is another consideration. Insulation or other sound dampening material would be nice to include in the bedroom partition walls, but again, my concern is that the rooms may take on a "solar oven" effect if the heat can not easily transfer to other parts of the house.

Thank you for your thoughts!
KB


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## Gray Wolf (Jan 25, 2013)

We have insulated ALL walls in the last two houses we built for ourselves. This has allowed us to zone the house and only heat the rooms we want. 75 in one room and 55 next door.

It also keeps the house quieter in general and we use soundproofing insulation in bathroom and master bedroom walls.

We also have 12" of insulation in the floor/ceiling between the first floor and the second.


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## K.B. (Sep 7, 2012)

Thank you, Gray Wolf. Zone heating/cooling is definitely another factor to consider.


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## Solar Geek (Mar 14, 2014)

We have an aggressively passive solar home, built with ICF, 1/2 half basement, 1/2 poured 6" slab, due south orientation and 10KW PV and domestic solar hot water.

We used procelain tile over 6" poured concrete floors and had our overhangs adjusted by solar engineers so that in winter, we only need our woodstove and if it is sunny, only need it early in am or late at night. An overnight burn keeps us at 71F or higher. Winter sun penetrates into 3/4 of the entire house in December as they told us it would. (Of course that assumes there is SUN!)

To your point on insulation (this is our 2nd home build; 16th move), yes insulate if sound is an issue. If you went with solid (not carpeting) floors, sound will travel like crazy. In WI, apparently, you have to have your room doors cut an inch high off the floor so SOUND and luckily heat transference are aided by that air flow.

We then had transom windows (think outside awning windows) put above the doors in the upper level, worried the rooms would get cold but also that people would want privacy (go figure huh - just kidding!) The transoms came from a standard window store, stained the outside wood frame to match the doors and so it looks beautiful. Upper rooms are warm and cozy all night and cool in summer too.

Sound can be an issue in soalr homes without carpeting. Heat is less likely to move through a wall than an open door/window. We leave all doors open till bedtime and the rooms are toasty. We have the LOPI Liberty - rated for 2200+ sq ft but we have 3400 sqft -- to make it work we have a cathedral ceiling with wood carsiding for the whole main level. The heat does rise nicely and fans distribute it to each room. So far, 2 winters into this house, we only need to OPEN windows to vent heat in winter when we get it tooooo hot with the wood stove.

Best of luck on your new house!


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## K.B. (Sep 7, 2012)

Great info - thank you! It sounds like you have a wonderful house.

We had the slab polished and that will serve as the main flooring in a larger part of the house. Between the solid floors and all of the exterior wall insulation being on the outside of the masonry, we may get an echo if we're not careful... 

I am thinking of veneer plastering the interior walls rather than standard sheetrock. The plaster should provide some benefit with sound dampening, if we go that route.


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## Solar Geek (Mar 14, 2014)

K.B. said:


> Great info - thank you! It sounds like you have a wonderful house.
> 
> We had the slab polished and that will serve as the main flooring in a larger part of the house. Between the solid floors and all of the exterior wall insulation being on the outside of the masonry, we may get an echo if we're not careful...
> 
> I am thinking of veneer plastering the interior walls rather than standard sheetrock. The plaster should provide some benefit with sound dampening, if we go that route.


Your floor sounds beautiful. And our last home, a log home, we had polished and stained concrete floors in our walkout level. The log walls definitely kept the sound down. However it was still quite noisy on the main level where it was 100% log ceiling and walls, but porcelain tile.

Veneer plastering is a great idea. By the time we were done with the insulated concrete form home, and all the upgrades my husband wanted, there was no way we could afford to do that to our sheet rock. We did have a 20 foot high limestone wall put in behind the freestanding wood-burning stove to not only transfer heat but also to deaden the sound.

The other issue for us was leather furniture. About 100 years ago ha ha we decided that our lifestyle could only tolerate leather furniture as we don't like people having to clean themselves off before they come in and sit down. But leather furniture, unlike upholstered furniture, also reflects sound.

Between our stone wall, pleated shades, and the wood ceiling, it isn't as noisy in here as it could've been. And the air transfers and heat transfer are excellent. Good luck with all of your choices --they sound wonderful.


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## jwal10 (Jun 5, 2010)

Our off grid cabin is basically 2 rooms and a bath. We insulated and sound deadened our walls because it is so small and Sweetie and I are at both ends of the sleep cycle. I am a morning person, she night person. We like it because we can zone the living from bedroom/bath. We only have heat in living and leave bedroom door closed during the day. 1 hour before my bedtime we open the door about a foot. I close when I go to bed. Sweetie leaves it open about the foot when she comes to bed, around midnight when the wood stove has died down and on coals. During winter, I start a fire about 5 am, then let it go out after heating the water for Sweeties morning shower and tea, about 10:00 am. If it is sunny the living area will gain in temperature during the day. We have a concrete block wall behind the woodstove. We also have an air lock entry with a big south facing window that shares the masonry wall, we can vent the heat that rises in the masonry wall to the tray ceiling in the airlock entry. This lets us move that warm air into the living or bedroom/bathroom through an insulated flex tube to registers near the floor. We have 18" of blow in ,in the attic. I would never build another place without interior wall insulation for sound deadening and to make the best use of zone heating....James


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## K.B. (Sep 7, 2012)

Thank you, James and Solar Geek once again. Shared personal experiences are very helpful to factor in to some of these decisions.


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