Stone house wall insulation

Questions and answers relating to houses built in the 1800s and before.

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Stone house wall insulation

Post by stonehousetom »

I need some advise, i bought an old stonehouse, 18 thick walls, plaster and lathe interior walls. There is not muuch space from the surface of the plaster to the stone( less than 11/2" ) Should i gut the home to put a 1/2 inch of foam board. Stone havy any r value? Thanks for your advise

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Post by CThomp »

I wouldn't worry about insulating 18" thick stone walls. If anything i'd make sure the interior cavity is sealed. Make sure heat can't escape. Insulate your attic/ceiling and be done with it.

What kind of stone? Just curious. It doesn't matter to much. You have an 18" solid rock wall between you and the elements.

If you insulate you attic/ceiling with a high r-value insulation with some variety of spray foam or something like that you bascially be living in a cave.

I think the Notre Dame Basillica in Montreal has 36" thick stone walls. So for a house you have half that. Just a comparison.

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Post by CThomp »

Just thinking more about this. I'd love to see pictures. I was in Quebec last January and every place I walked into the first thing I noticed was how massively thick the walls were. My wife thought I was insane of course. But all the houses had these mega thick stone walls. They've been there for like 200 years. Freakin castles. It's incredible.

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Post by angolito »

oh dear gussie.....don't "gut" the house for r-factor.

the walls must be amazing we would love pictures? i also have a "thing" about the depth of the windows and doorways in old stone homes.

back to the r-factor. your air space is providing all the insulation you could need in your construction, i would think. cthomp sounds right to me. i personally would never tear out viable old plaster for higher any-factor.

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Post by lrkrgrrl »

The thermal qualities of a big fat stone (or brick) wall are distinct from "r-value" which indicates insulation quality. Stone's virtue is that it provides thermal mass, which absorbs heat, from the sun and/or indoor heating sources, retains that energy, and radiates it back.

Insulate the attic, and leave the poor walls alone.

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Post by HB »

Warning, this is a bit of a ramble..... :oops:

I have stone house too, and I've been doing research on how to properly care for it and make it as energy efficient as I can for the past 6 years.

I understand that you're anxious to get a lot of work done, but you need to know that modern building methods and materials are not really the best thing for your house. In fact, some modern materials can significantly accelerate deterioration of the structure.

It;s important to look at how your home was built originally. When the stones were laid up to form the walls, they were fitted together reasonably well, but there was also a bedding mortar used to help the stones seat together so that they didn't all need to be cut perfectly to fit everytime. The face of the stones on the exterior were generally worked to a point that they present a reasonably flat finished looking surface, while the interior of those walls were left rough (very rough) since the plaster would cover them. After the walls were erected, they finished pointing was done with a lime and sand mixture on the outside of the house and the inerior walls were leveled and smoothed out using lime based plaster.

The pointing kept the water out of the stone wall, but if moisture did get in the lime pointing would act as a wick, helping to draw the moisture out of the wall, protecting the bedding mortar and preventing the growth of mold. This also prevented the moisture from rotting the wood trim that is inset into the walls. The plaster on the inside of the wall was also moisture permeable so that it would also allow moisture out of the wall. The cool thing about Lime based motar is that it is self healing. There is always some free lime in the mix that does not become recarbonated as the stuff cures, so if cracks form in the pointing, everytimne it rains, a little of the free lime is disolved by the rain water and carried into those cracks where it recrystallizes, filling the cracks and sealing them. The other nice thing about lime based mortar is that it's a bit flexible so it doesn't crack with every movement of the building.

A little known fact is that many stone and brick homes built in the 18th and early 19th centuries were finished with a lime wash over the whole structure. This film of lime acted as another barrier to moisture entering the walls and provided an additional supply of free lime to seal any cracks that developed in the pointing.

So you see, the mortar, stones and plaster that make up the walls of these old houses act together as a system to keep the walls dry and the wooden trim intact and rot free.

Enter one of our favorite modern building materials - cement. It's hard, cures in a matter of hours instead of days or weeks like lime based mortars, and it's virtually weaterproof.

As a result, a lot of people repoint their stone homes with cement based mortar.

What a lot of people don't realize is that it' s rigid so it cracks - water gets in through the cracks in the wall and then since the concrete has less than 30% of the moisture permeability of the original lime mortar it doesn't perform the same way, so the moisture sits in the walls and does bad things. If you couple this with the fact that we create a lot of steam in our homes by bathing and cooking, and this water vapor passes through the plaster and condenses on the back side of the concrete pointing since it can't pass through it, you can see that it compounds the moisture problem in the wall.

Now let's look at what you're proposing to do. You are considering ripping out all of the plaster on the interior of your home in order to put 1 inch of styrofoam insulation between you and your 18 inch thick stone walls. What will this really accomplish? It will cost you a lot of time and mess, it will be difficult to level out the new wall against that rough stone, and you'll probably wind up using spackle to try to level out the surface before you put in the insulation. The spackle will interfere with the way the wall works and potentially create a siuation were moisture can't escape from the wall as the original materials would've allowed. This could, in turn, cause a buildup of moisture in some parts of the home and create a mold problem. You would need to compensate for this in some way, either by using materials similar to what was originally used on the house or by adding a mechanical system that will help to properly regulate the moisture levels within the house. Not to mention that it will be difficult to get the insulation and sheetrock in place without building the wall out to a point where the trim is flush to the sheeetrock (Becuase the trim is most likely attached to the stone, not put on over the top of the plaster like they do in modern houses.)

Also, if you've read anything about the increase of mold problems that people are having in their homes, or about the increasing occurences of asthma in our population, you beigin to see that a lot of it can be tied back to the trend to seal our houses up so tightly.

The best thing you can do to insulate a stone house is to make sure that the pointing is in good shape and of a proper material. Then be sure to install quality storm windows on the house interior or exterior, rebuild your exitsting windows to add weather stripping and secure any loose glass and finally, be sure you have amply insulated your attic/roof.

The insulation in the attic will keep the heat from escaping out of the top of the house and the pointing and storm windows will greatly reduce any air infiltration.

Our stone house is 2500 square feet and I pay about the same to keep it at 70 degrees all winter as do some of my friends in new houses with 8 inches of insulation in their walls. The great thing about the stone is that it absorbs a lot of sunlight during the day and that helps keep the inside warm.

Best of luck to you.


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Post by CThomp »

I think that was the most educational thing i've ever read about a stone structure....ever.

Thank you.

Seriously...that was awesome.

When I insulated my walls I did my best not to overinsulate. Old houses have to breathe. All regular old fiber glass batts.

Dan P
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Post by Dan P »

I will have to agree that HB's commentary is outstanding. I would keep it as page 1 of my reserch matrial. I live in and am personally restoring an 18th c. limestone house. I have volumes of research material, been to many seminars and have learned a great deal about stone houses. It's abundantly clear, so has HB. Great job!
Currently living in 18th c. Virginia

Don M
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Post by Don M »

This fits with my personal experience; 2700 sq ft PA stone house! Don
1840 Limestone Farmhouse

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Post by KevinM »

I'm Kevin and am new to this forum. Allow me a moment to introduce myself and my house: I live in a former one room school house built circa 1860. Its a limestone rubble house that has been renovated many times over the years. Its located in Metcalfe, Ontario, Canada which is near Ottawa. We experience some cold weather here. I heat my house with a woodstove (Napolean 1900). To date, I've tackled some fairly major projects such as resetting/repointing the stone in one corner of the house and installing a post and beam support (the previous owner had cut the roof trusses to install some skylights).
Thats me and my house situation in a nutshell. Thanks for reading!

And now to my question...
My stone walls are at least 18 inches thick. The entire house needs repointing. Air is infiltrating. Last night I removed some drywall to assess the situation. It looks like I have 2x10 boards/slats fastened to studs, leaving a 1 1/2 space. Quite similar to the previous poster. Air is infiltrating where the wall meets the floor. I'm considering 2 options. One is filling this cavity with foam (icynene). The other is simply sealing the wall/floor joint with vapour barrier or foam. The attic I am tackling as a seperate job. Thoughts?

PS> HB, your post is excellent.
~ Kevin.

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