Insulation in your house is like your winter coat.
Insulation is a blanket around the house that slows the loss of heat to the outdoors. Alternately, it slows the penetration of heat from the outdoors to your living space. It is easy to see how insulation adds to your indoor comfort, and reduces the energy cost to heat or cool your home.
Your winter coat is not very effective if the wind blows through it. It needs a layer of wind resistant material to be effective. The insulation in your house is similar. It is not very effective if air is allowed to freely circulate in and out of the house. The importance of sealing your home against air infiltration can not be over emphasized, if you are concerned with comfort and energy costs.
A coat or any clothing is not very effective against the cold if it becomes wet. The same is true for insulation. As well as reducing the effectiveness of the insulation, moisture encourages the growth of mold and rot within wall and attic cavities. Once moisture has entered the wall cavity it is reluctant to leave, so it is important that even tiny holes in the moisture barrier be sealed.
Do you remember your science from school? In my case I can barely remember school, it was so long ago. Anyway, we know that heat always moves towards cold (or the absence of heat), and that there will be a greater (faster) transfer of energy when the temperature differential is higher. If there is a science teacher out there, please correct me, if I have not explained this properly. The other pertinent fact is that warmer air rises. This means that the warmest air in your house is at the ceiling, and that this is where the greatest heat transfer can occur. In cooling season, the attic space of your house is going to be warmer than the outside ambient temperatures, no matter how effective the ventilation.
It follows that, added insulation in your attic will effect the largest energy savings.
It is also the easiest and least expensive area to increase insulation. There are no doors, and usually no windows, to reduce the overall effectiveness of the insulation. There is a lot of space to add insulation, with the possible exception of the area near the eaves of a gable roof. The use of rafter trusses with heels can alleviate this problem.
It is difficult to get an overall high insulation value in the walls because of windows and doors. No matter how much you spend on windows, you are not going to get an R value much above 4 with 3 being about the norm. Doors are not much better, and need constant maintenance to prevent air leaks. Windows and doors will probably make up from 10% to 20% of your wall area, and in some designs much more. Windows can be an attractive design feature, but can be costly in more ways than one.
There are several types of insulation available.
Fiberglass or mineral wool insulation is commonly available as batts. They are used in wall cavities for ease of installation, and because they are not subject to settling. Each type has about the same insulating value at just over R3 per inch. In other words, A 2 x 4 wall can be insulated to about R12 using this method. A 2 x 6 wall will be about R20. Batts are also often used in ceilings as well, but in my opinion loose fill insulation is the more effective for this application, with less likelihood of gaps. If it is still available in your area, do not use the type that has a kraft paper facing. It only serves to reduce fire resistance and hide gaps that may occur.
Loose fill insulation is either of fiberglass, rock wool or cellulose. Vermiculite was once widely used, but is not popular today, because of possible asbestos contamination. Loose fill insulation is easily installed in open attics by blowing it in. Insulation suppliers will usually supply the equipment for this at a low rental fee, or possibly even for free. Contractors specializing in this type of insulation, and with truck mounted blowers can be hired, if you do not want to get itchy. Truss members can add difficulty, but are not a serious complication. Application may not be possible in some types of roof. Fiberglass has an an R value of about 2.2 per inch, while rock wool and cellulose are at about 3.1. Cellulose may have the advantage of providing some air barrier qualities. Cellulose insulation is a environmentally friendly product in that it is usually made from recycled newspaper that has been treated with a fire retardant and rodent and insect repellents.
Foamed in place, or spray applied insulation comes in three types, wet spray cellulose, open cell polyurethane, and closed cell polyurethane. R values per inch are respectively 3 to 3.7, 3.6 and 5.5 to 6. The polyurethanes can act as air barriers, and the closed cell type is also a moisture retarder. These are not a do it yourself application, and can be more costly. Polyurethane foam insulation is excellent for providing extra insulation in narrow cavities. They are effective for difficult to insulate areas such rim joists. Available in aerosol cans and formulated for specific uses, foam insulation is excellent for sealing around windows, doors, and other wall perforations.
The more common type of rigid board insulation is polystyrene (often called Styrofoam which is actually a brand name). Other less common types are rigid fiberglass, or rigid mineral fiber insulation. Polyisocyanurate (WHEW, say you can pronounce that and be honest) is a foil faced board. Expanded polystyrene has an R rating of 3.6 to 4.4 per inch while the higher density extruded type is 4.5 to 5. The polyiso sheet is R10.8 for a 1.5 inch thick sheet or R7.2 for 1 inch.
Board insulation can be used as sheathing, and under siding on walls. It is effective at breaking thermal bridges, occurring in house framing with cavity insulation. Self adhesive aerogel insulation strips can be applied to the framing under the drywall to reduce thermal bridging, as well. I am not sure how easy this is to find, as it is a relatively new product. A 2 x 6 stud has an R value of about 7 which is not too terrible. Framing will have to be braced, if using rigid insulation for sheathing. It does not provide the same protection against racking, as plywood or OSB. All insulation must be carefully installed to prevent gaps or compression.
For an air barrier on the exterior walls, a house wrap is applied under the siding. This is material that will allow moisture to escape outward, but prevent air and moisture from penetrating inward. It has replaced tar paper for this application, and is required by code in many jurisdictions. I cannot attest to it’s effectiveness, but demonstrations I have seen are pretty convincing. It must be meticulously sealed with special tape, and caulking at all joints and penetrations. My thinking is that the fasteners used should be somehow sealed over, as well, with tape or caulking, but how would you do that when you apply siding. I would appreciate some input on this. Always lap an upper application over the lower one.
To protect the insulation, a moisture barrier must be applied on the interior side,and also sealed well at joints, openings and penetrations.
It also acts as a barrier against air infiltration. A 6 mil poly sheet is required by most codes. A mil is a unit of length equal to one thousandth (10-3) of an inch (0.0254 millimeter).
Pay special attention to sealing electrical outlet boxes and plumbing penetrations. Electrical penetrations are a serious source of air infiltration. Don’t forget to caulk where wires go through wall plates. Plumbing penetrations occur in areas where humidity is higher, and as a result can be a serious source of moisture penetration. Keep all plumbing to interior walls if possible. Even then sealing is important to prevent condensation from forming on cold water pipes, and creating a pleasant environment for mold growth. Don’t forget the ceiling penetrations for plumbing vents.
I am not very concerned with fastener penetration in the interior. Primer sealer and paint over drywall, provide an extra layer of protection against moisture.