Insulating concrete block house
Hey everyone,
Looking for some thoughts from the building science gurus out there. I am looking at a renovation project that involves adding a second floor to an existing one story house. The existing outside wall are 8″ concrete block with stucco. The stucco is in great shape and we would like to keep it totally intact. However, we may need to comply with the new stretch energy code which says these walls will need a certain level of insulation added. If we do need to comply the insulation can really only go on the inside of the building. I am in New England, climate zone 5. The architect, homeowner, who is a mechanical engineer, and I, the GC have all thought about the best way to insulate from the inside but we are all concerned about the dew point and where that happens with our assembly. Does anyone have a tried and true method to insulate CMU walls from the inside? I’d love to hear some suggestions. Thanks
Replies
Solid wall insulation.
You can insulate a wall in two ways.
On the outside, on the inside.
Both methods have there good and bad points.
The best place to start is your life style.
If you are at home 24/7 then having the insulation on the outside, works by allowing the walls to absorbe heat, which then filters back into the home when the temperature drops indoors.
If you are out a lot, and you turn the heating down or off, then having the insulation on the inside, means the home warms more quickly.
Having written that!
Putting the insulation on the outside, means that heat enters the walls and is conducted away to the foundations, ground, roof, sky, etc. The heat loss is high.
Having the insulation on the inside works best! A perfectly insulated room, has insulation, covering the inside of the walls, ceiling and floor. You stick sheets of polystyrene to the walls, using drywall adhesive. Then stick drywall over the polystyrene sheets, using more drywall adhesive. Finish as required. A skim coat of wet plaster provides a hard, easily finished surface to decorate. The ceiling should have sheets of polystyrene screwed to the joists, with drywall screwed through the polystyrene into the joists, again finished with a wet plaster skim coat. The floor, a layer of polystyrene, with a fully floating t&g, glued oriented strand board finish. Plus underlay and carpet.
The choice of polystyrene sheet thicknes is down to you, three inch on the ceiling, one or two on the walls, one to four on the floor.
You will have, one warm, comfortable, room to live in. The polystyrene sheet has been around for 52 years since Dow brought Styrofoam to market, it is made of mostly air, with a very small amount of plastic, it has been in use for a long time and the original stuff is still perfect. Keep in mind that once done, this will last the life of your home, keeping you warm in winter and cool in summer.
Heat Insulate!
I will just comment on your proposal.
Block walls have a great ability to absorbe heat. This is why they are used in glass fronted office blocks. The problem is, that a block wall is not like a tub of hot water where the hot water rises to the surface or a room where the hot air adjacent to a radiator rises and transfers most of its heat to the walls and ceiling, before cooling and dropping to be reheated.
A block wall transfers heat by conduction in direct lines, this means that the heat entering the wall disperses, through the wall, up the wall and down the wall. I does not stay in, or even near the room. This means that you are throwing your expensive heat away, to places where it will never return to heat your room.
I understand that the traditional way of building and heating holds sway and most people do not understand thermodynamics, this will change as more people see and get to understand the advantages of Passive house building. A new Passive House Dentists office has just been completed and certified, the first in the world. The message, insulate, is being noticed.
Expanding on Perry's note you might want to see if you can make a Trombe wall work for you.
Internal insulation, condensation.
This only applies if you live in an area where you heat your home in winter.
Heat always moves to cold! Water vapor is programmed to move to cold. (It always tries to escape from heat) The inside of the average home is nearly always warmer and wetter than the outside air. That is why we have ventilation, that brings in colder drier air. So the movement of water vapor is always outward, in cold areas.
Water vapor does not have a problem with solid walls, it merely passes through and disappears into the sky.
Therefore, when (as in your case) you have a nice solid wall, that has been wet plastered, you can stick sheets of polystyrene to the walls (this is the perfect way to avoid heat bridges) preferably taking care to closely butt joint the polystyrtene sheets to avoid any cold gaps. Then you cover with dry wall. Result.... the water vapor in the room is faced by a wall that is at the same temperature as the air in the room. There is no dew point - ever - and therefore no condensation -ever!
Simple really!
Thanks Perry
Thanks again Perry. I appreciate the time and thought you are putting into this topic and I understand what you are saying. My fear, based on stories I've heard about interior foam on the inside of a block house, along with the hesitation of the architect, homeowner, and my insulation subcontractor, all have me still slightly concerned. The explanation the building science groups gave me as for why this was a bad approach are the following. The dew point, they said, would be where the warm indoor air meets the cold block on the backside of the foam. Then the water vapor dries to the outside, destroying the stucco by rotting out the wire lath and pushing off the paint. I know there shouldn't be air movement through the foam, but in our situation there would be because we can't use foam on the floor or ceiling to finish the envelope. FDo you think these building science experts have it all wrong, or maybe just erring on the side of caution to protect themselves? Thanks again Perry. I love talking about this stuff and it's fun especially when dealing with a quirky old space.
So where is the dew point in an uninsulated masonry/stucco structure?
That's a good question Dan. I am not exactly sure. I would think it is variable, but somewhere in the middle of the block wall. is it possible that there is not a point where condensation occurs because there is no "hard line" where warm meets cold. I could see the whole temperature change from inside to our being very gradual through the thickness of the wall. Without the foam to the inside the wall may dry to both sides lessening the chances of trapping moisture for a longer time? I don't really know. Still curious to see what others think.
The "dry to both sides" thing
The "dry to both sides" thing is a red herring, for the most part. As are most arguments about dew point -- most folks using the term barely know what dew point is or how to calculate it.
If you are in a primarily heating environment, and if the outer "shell" is reasonably good at shedding rainwater, a vapor barrier of some sort near the inner (warm) surface prevents moisture from transiting through the wall, and hence prevents condensation inside the wall (because dew point remains below temperature throughout the wall). OTOH, insulation (eg, fiberglass) without a vapor barrier tends to create a low temperature within the outer few inches of the wall which will often drop below the dewpoint at that location. (Add a few coats of paint to the exterior to create a relatively "good" vapor barrier there and you have a real mess.)
Of course, other concerns may dominate if you're in a coastal region (within a few miles) with driving rain, or some other special situation.
Water vapor.
The dew point, they said, would be where the warm indoor air meets the cold block on the backside of the foam.
Then the water vapor dries to the outside, destroying the stucco by rotting out the wire lath and pushing off the paint.
I know there shouldn't be air movement through the foam, but in our situation there would be because we can't use foam on the floor or ceiling to finish the envelope.
Do you think these building science experts have it all wrong, or maybe just erring on the side of caution to protect themselves?
Thanks again Perry. I love talking about this stuff and it's fun especially when dealing with a quirky old space.
Dew Point Calculation Chart (Fahrenheit)
%RH AMBIENT AIR TEMPERATURE IN FAHRENHEIT 20 30 40 50 60(70)80 90 100 110 120 90 18 28 37 47 57 67 77 87 97 107 117 85 17 26 36 45 55 65 75 84 95 104 113 80 16 25 34 44 54 63 73 82 93 102 110 75 15 24 33 42 52 62 71 80 91 100 108 70 13 22 31 40 50 60 68 78 8 96 105 65 12 20 29 38 47(57)66 76 85 93 103 60 11 19 27 36 45 55 64 73 83 92 101 55 9 17 25 34 43 53 61 70 80 89 98 50 6 15 23 31 40 50 59 67 77 86 94 45 4 13 21 29 37 47 56 64 73 82 91 40 1 11 18 26 35 43 52 61 69 78 87 35 -2 8 16 23 31 40 48 57 83 30 -6 4 13 20 28 36 44 52 61 69 77 At Sea Level (14.696 psiA) I have printed my Dew point chart above, from it you can see how water vapor in the air/condensation works. If you take a typical indoor air temperature like 70F (top line) then move down the far left column to a typical indoor relative humidity say 65%.where the two lines cross 57F is the dew point. This is the temperature where condensation forms on your bathroom mirror or windows. The windows are the coldest part of a room and condensation forms there first. The next coldest is the chimney breast. Given the proposed construction, polystyrene on wall, drywall, the wall surface temperature will always be above dew point. Another example, my lounge at this moment is 22C/72F my relative humidity is 68% there is no way that condensation is going to form on or in anything in this room, and this room has been much the same, slightly hotter at times, more humid, less humid - but never anywhere near a low temperature of 15C/58F. My floor is between 24 and 26C my walls are 22C, my ceiling 22 to 23C, the centre of the windows is also 22C. (Note:I have quad glazing - the room side of the windows stays at room temperature.) The outside temperature is 15.3C the outside humidity is 98%. The lowest outside temperature I have recorded is -18C humidity 35% this had no effect indoors. From these examples you can see that your clients fears are groundless. Keep the indoor temperature and humidity steady and there will never be any transfer of water vapor onto or into the walls. In any event, you can use an invisible rain proof, breathable wall coating that will let water vapor escape from the inside, and keep the rain out, or a paint of the same standard. See my potted history notes, on my first remodelling and how lack of floor insulation makes no difference. These days I always insulate floors, mainly for comfort, then to save money.Water vapor.
In 1839 Eduard Simon, discovered Polystyrene.
In 1941, Dow Chemical invented a Styrofoam process.
In 1961 or thereabouts Dow brought Styrofoam to market.
Shortly afterwards owners of potato stores and commercial deep freezes saw the benefits
of polystyrene and started to save big bucks on their storage power costs. Commercial buildings
continue to be built to a higher standard than most homes.
I read about polystyrene and saw the benefits polystyrene could bring to a home, I decided
to build or buy a house and insulate it using polystyrene.
In 1969 I bought my first house, a solid brick walled house with common clay tiled roof and
ventilated crawl space, I gutted it and remodelled/refurbished, covering the walls and ceilings
with sheets of one inch thick polystyrene, and finished with an inch of wet plaster. I fitted
hydronic baseboard and UFH. I lived there for 28 years.
This was advanced stuff in those days, most people didn't know about polystyrene and didn't care
about heating costs or comfort. The first oil crises didn't arrive until a year later.
This was one of six identical houses. My system worked - I know my house remained the most
comfortable to live in and the cheapest to heat of the six, until I relocated 8 years ago.
I never did get round to insulating the floors. Merely covering the floorboards with newspaper,
sheets of hardboard, under lay and thick carpet.
I hope this piece of history goes some way to resolving your clients worry about interstitial
condensation. There seems to be a general misunderstanding about the possible location of water
vapor within a solid wall. What happens is, outside water vapor is driven into a wall by the heat
of the sun, it naturally moves into the coldest part of the wall as far from the sun's heat as
possible, this point is located on the outside edge of the insulation, which is keeping the
home's heat from escaping - with uninsulated walls, the location depends on the coldest point
within the wall, which can be anywhere. Placing the insulation on the inside of the outside
walls means (providing its installed correctly)that there is no contact between the warm wet
air inside the room and the cold wall - therefore, no problem. Water vapor that moves into a
wall under the influence of the sun, moves out again as the outside temperature drops.
Interstitial condensation only seems to be a problem with homes made of sticks, that are not
correctly insulated. With solid walls, it is not a problem.
Rain: - where a wall has been rendered, and the render has cracked with the normal year by year
expansion and contraction and rain has got between the render and the solid wall and then frozen
and expanded. This will in time force the render away from the wall. If this repeated year after
year, pieces will break off.
Obviously, a dry wall will keep a home warmer, water is 4000 times better at conducting heat
than dry air, a damp wall will loose a lot of heat, therefore, keeping a wall dry is important,
using an invisible waterproof breathable coating or breathable paint that will shed rain is
worth while.
Someone wrote. "Half of what we think we know is wrong!" I would guess that few if any of these
building science experts, has ever built or remodelled or insulated a brick, block or solid stone
building of any description, nor have they long time lived in one, they offer opinions as fact.
The problem often is, articles are written by people who are paid to form opinions and to create
demand. I read the same article, published under different names, often word for word, written in
newspapers and magazines published round the world, extolling one or a series of ideas they want
people to buy. The same article pops up in Europe, the middle and far east as manufacturers try
to drum up business.
Builders and re-modellers have to fight an up hill battle, trying to overcome opinions formed by
clients, from reading opinionator articles on the net and in newspapers written to create worry
and demand.
Fascinating story. In our case, our 1960 house had 10" block/brick walls with 3/4" of laughable batt insulation between 1x2 furring strips, and plaster inside. The house is small, so we chose to add 2" polyisocyanurate to the outside with 2x4 furring strips and natural cedar siding. On the inside, we left inside walls that were in good shape alone, but where we gutted bad ones we are adding 3/4" of foam in place of the batt insulation.
Fascinating story. In our case, our 1960 house had 10" block/brick walls with 3/4" of laughable batt insulation between 1x2 furring strips, and plaster inside. The house is small, so we chose to add 2" polyisocyanurate to the outside with 2x4 furring strips and natural cedar siding. On the inside, we left inside walls that were in good shape alone, but where we gutted bad ones we are adding 3/4" of foam in place of the batt insulation.
Fascinating story. In our case, our 1960 house had 10" block/brick walls with 3/4" of laughable batt insulation between 1x2 furring strips, and plaster inside. The house is small, so we chose to add 2" polyisocyanurate to the outside with 2x4 furring strips and natural cedar siding. On the inside, we left inside walls that were in good shape alone, but where we gutted bad ones we are adding 3/4" of foam in place of the batt insulation.
(Gotta say that the story sorta loses its fascination after the 3rd retelling.)
The dewpoint chart doesn't really tell you how it works. In general, in a heating environment, there is a temperature gradient from warm to cold through the cross-section of the wall. The inside surface of the wall is essentially at "room temperature", while the outside surface is at the outside air temp. There is also a dewpoint gradient, and similarly the dewpoint at the inside surface is the inside dewpoint and at the outside surface it's at the outside dewpoint, with points in-between being at intermediate dewpoints.
So both temperature and drewpoint describe a jagged but always downward slope from inside to out. So long as the dewpoint, at any particular point in the wall, is not higher than the temperature, no condensation will occur.
The simplest way, in a heating environment, to prevent condensation is to have a relatively good vapor barrier near the inside surface.
Vapor barrier, insulation.
With a stick built house, a vapor barrier on the warm side of a wall makes sense, as that is where the water vapor is coming from in a cold location. However, it is important to have enough insulation on the cold side of the vapor barrier, to keep the room side of the vapor barrier above dew point. Otherwise, you will have condensation forming on the vapor barrier and running into the room.
A vapor barrier as such, is not required on a block, brick or similar wall, as the water vapor merely passes through the thousands of microscopic holes in the wall.