Guest Post by J C Sherrill III
How heat moves, Insulation, Radiant and Vapor Barriers
In relation to Indoor air quality part II
Everyone may be an expert and know a little something when it comes to insulation. We have all heard the saying “It’s so easy a monkey can do it.”
Information can be right or wrong. When you grasp the knowledge of how heat moves, how insulation, radiant and vapor barriers work and have it in your head you will have the ability to think through the job and DIY, You will have the confidence to get it right the first time. With insulation you do not have an easy option to “Fix it” after the wall covering is up.
How Heat moves; The 3 ways:
Radiant: Is like sitting the sun
Radiant barriers act like a reflective shade. It can stop the flow in both directions but this is not the only way heat moves. Radiation can also flow out of your body towards a colder area. Sit by a cold window at night without a lot of clothing in the winter and you can feel this effect.
Conduction: Heat the end of a metal pipe and hold the other end.
Convection: Warm air
Heat moves to cold and the greater the temperature difference the faster the change
Let’s say I go to Dunkin Donuts to get you a cup of coffee. You like it hot, but also want cream. Do I add the cream now or later?
Think about this: The greater the temperature, difference the faster the change. Keep reading and thinking. The answer will be at the end.
Insulation and Vapor Barriers A perspective from a heating contractor.
My information and opinions are based on experience in the field, an understanding of the properties of air and newer building science. Another reference outside of my 2 cents where I have gained a lot of knowledge on good building science is www.advancedenergy.org take a look at what these ”Experts” have to say on proper building science.
Go to buildings scroll down to the Knowledge Library and look at what they offer for free. This is a wealth of quality energy saving “get it right the first time” building science information at their site.
When I discuss walls within this text, my meaning is the envelope of the home that include the Walls, floors and ceilingsVapor barriers VB are in place to prevent moisture from migrating in the wall. Think of rain, it comes from the air. Air can carry a lot of moisture. Think of a Cold glass of iced tea in the summer. Ever seen moisture forming outside the glass? Cold surface + warm humid air. The dew point gets reached; the moisture in the air condenses on the glass
and rains down the outside of the glass. This is a not good thing when this vapor condenses inside of a wall.
Moisture in the air is measured in Relative Humidity. RH is measured as a % present at a given temperature. Warm air can hold more moisture than cooler air. Imagine air as being like a sponge. Now dip it deep in a tub of water, it is saturated. This is like warm air. The sponge can hold a lot of moisture. Now lower the temperature (squeeze the
sponge) the air cannot hold as much moisture at a cooler temperature. RH is the % of saturation the air can hold compared to a given temperature. Cooler air is denser. Air molecules are heavier than moisture molecules
since they pack tighter together. Dew point is the temperature where the air is fully saturated. It is also where condensation will start to form.
Ever witnessed dew on grass? The air was warm and full of moisture during the day. It got cooler overnight, dropped below the dew point and the moisture condensed from the air. You do not want this to ever happen inside of a wall.
Houses do not need to breathe. People need to breathe / Run a fan or open a window for ventilation.
Loose house = more infiltration = more moisture due to vapor migration.
Don’t beat me up on this one. A vapor barrier should never be plastic. A plastic VB will trap water in a wall. If you have a “Bulk water leak” from a roof, window, door trim, flashing failure, pipe leak, etc. Even an air leak from Infiltration can cause moisture to become trapped inside your wall. When this happens your wall is in trouble. This can affect your homes indoor air quality. If you have made this mistake and have plastic as a VB try and keep the inside and outside of the wall bulk water free. By using proper flashing, door and window caulking and fix
any spills or plumbing leaks ASAP.
Keep your home and wood dry and you will never need to be worried about mold, mildew and wood rot.
I have witnessed sagging bags of water, hanging under a house where the home owner installed insulation on the underside of his floor and then stapled 6 mil plastic on the bottom of the floor joist. These water filled tubes on 16 inch centers, made a nasty, moldy, soggy mess of the floor system. He is a commercial insulation contractor and got it
wrong. With a little knowledge, you can DIY with confidence.
A VB should go to the warm side. Usually towards the inside unless you live somewhere like South Florida, etc. Heat moves to cold you are trying to block the air/ vapor movement at the source.
I look at insulation and the VB like having holes in the bottom of a boat. If you have 4 holes and only patch 3 you still have a leak. Keep the insulation and VB continuous. Do not leave voids in the wall around electrical boxes, wires or pipes. Keep the insulation its full thickness and firmly pressed against the floor wall or ceiling. If you use faced fiberglass staple the flap to the outer edge, face of the stud. I have seen pros staple the flap to the inside of the stud wall leaving an air void at the edge of every stud.
You should only have one VB. Two can allow water to become trapped in-between a wall.
Tar paper VS house wrap. I have seen house wrap dissolve inside a damp wall. You cannot use house wrap on a roof so I prefer tar paper. When installed on the exterior of a wall, from the bottom up, any moisture inside a wall can escape through the overlapping layers of the tar paper. Tar paper will protect your wall and will last.
Insulate or not? It is way too hard to do after the inside walls are installed. It is a cheap bang for the buck. It will also make your tiny home much cooler, warmer and quieter.
Insulation Pro VS. Cons of the most common Insulation found in the box stores
Fiberglass, Spray foam, Sheet foam, Cellulose:
Insulation is measure in R values. R = resistance to heat flow. This R value is only half of the story. Some insulation performs better than other per given thickness and cost. The other half is infiltration. Infiltration = air leaks = moisture migration. Before installing the insulation seal all penetrations around piping and wiring in the walls with a good grade of caulk.
Fiberglass: Pros low initial cost easy to install as a DIY job Cons: Furnace filters are made of
the same material. Air can flow right through. *
Spray Foam: Although not usually found in a box store Spray Foam is a great insulator. DIY Kits are available www.tigerfoam.com or you could hire a professional. Foams are resistant to moisture and are a very good vapor and air barriers. Spray foam adds strength to the structure since it acts like a glue. Con: Expensive cost and expensive shipping cost. * Do not install spray foam without a respirator, face, skin and eye protection.
Foam sheets: Easy to install resistant to moisture good vapor and air barrier good insulator. Foam sheets that are cut to fit are a good option. When all your foam sheeting is installed, use Foam in a can, to fill any cracks and voids. *
Cellulose: Made from recycled newspapers is a good low cost insulator. This product has the ability to hold and release some moisture inside a wall. This protects the wall and it has the ability to pack so dense it acts as a VB. I have used this in an attic. Pros have a mesh fabric that they staple up to hold it in place while they blow it in. They can
also install it wet in walls. When it dries it is like dense like paper Mache.*
*Mask skin and eye protection required.
It is critical that the insulation and vapor barrier be installed properly. It is such a menial task that when installed professionally it is usually such a get it done in a hurry job that voids and gaps are present. Properly installed insulation and vapor barriers pays you back over the life of your home in energy savings and prevents moisture from condensing inside of the wall.
You should chase the cost of insulating your home with which type insulation gives you the best R value bang for your dollar. Cost and type may vary from one area to another so you will need to do your homework. Installed properly any type of insulation will perform well in your tiny home.
First to be effective it shouldn’t touch a surface so it needs an air space to prevent conduction. Dead air space is an insulator but doesn’t have as great an R value as insulation for the same thickness. Should you have much infiltration inside of a wall it would render the air space useless as an insulator. Second a small layer of dust will render a Radiant barrier less effective. Roof color light or dark? A white roof is best for hot climates. Metal roofing with a light color will provide your home a Radiant barrier.
I found this from NC Advanced Energy: New construction radiant foil on bottom of plywood roofing testing: “Radiant Barrier roof sheathing appears to reduce summer/cooling loads by about 3 Percent of summer/cooling loads” For the added cost I am not sold on foil radiant barriers.
Insulation properly installed can help keep your home more comfortable, use less energy, make it last longer, quieter and improve the indoor air quality. Stay safe.
Question: Let’s say I go to Dunkin Donuts to get you a cup of coffee. You like it hot, but also want cream. Do I add the cream now or later? Answer: I would add the cream at Dunkin Donuts.
Visual example: Empty a cup of ice in your yard in the winter, how long does it last? Now do it the summer. The greater the temperature, difference the faster the change.
© 2011 J C Sherrill III Reproduction without permission prohibited.