Insulation

Good quality insulation is one of the keys to an energy efficient home. One can use most any of the commonly available insulation types out there from fiberglass batts, cellulose, recycled blue jeans, blown in fiberglass, blown-in-blanket, to spray-foam insulation. The main thing is to ensure every nook and cranny is filled with insulation and there are no voids for air circulation. One of the major details to make insulation most effective is to ensure the cavity where the insulation is to reside is air tight. Any insulation that is porus (including open-cell spray foam) will suffer degration in performance if air is allowed to enter and exit the wall cavity. Spray-foam and blown in products typically do a much better job of totally filling the insulation cavity, but with good cavity air sealing and attention to detail, other insulation products can be used successfully as well.

The climate zone where you live will influence the amount of insulation that you may need to use. It may also require that you use 2x6 exterior wall framing and/or continuous insulation to achieve the necessary R-value required for your particular zone. To find out what zone you are in and what are the current insulation requirements visit: http://energycode.pnl.gov/EnergyCodeReqs/.

The 2012 IRC building code states, for our area in Mason Texas, the cavity insulation must be R-20 or R-13 with an additional R-5 of continuous rigid insulation. That means that if we choose not to have continuous insulation, then 2x6 exterior walls must be used. But we can use 2x4 walls with R-13 insulation and 1" (R-5) of rigid insulation on the exterior. Attic insluation is to be R-38. There is clause to allow you to calculate an equivalent insulation (note this is not Effective R-value as stated by some insulation vendors). For example if you wanted to put 1 1/2" continuous insulation on the roof deck and apply spray foam insulation to the underside of the roof deck, then one can calculate the required depth of spray foam to get the same net effect.

Sorry, a bit of a rant here. Beware of claims made by some insulation contractors stating they you don't need as much insulation as you think because there product has a very high "Effective R-value". There is no such thing! You must go by the published R-value and select the thickness accourdingly. Unfortuantely, spray foam contractors quite often overstate their products. For example, if you need R-38 and the spray-foam insulation you plan to use is R3.8 per inch, then you need ten inches! Insulation ratings are derived using standard test procedure and the R-value the manufacture is allowed to advertise is based upon this standard test Effective R-value is just salesmenship... End or rant.

Let’s talk about insulation in more detail for the various portions of the home’s building envelope:

Exterior wall Cavity-fill Insulation. The section about framing discussed the importance using the advanced framing techniques to maximize the amount of wall cavity volume in a wall to allow for more insulation. The difference in overall wall insulation (compensating for the wood in a wall) of a conventionally framed house and one using advanced framing is (24.9%). Using advanced framing as a good starting point for your wall. So, whatever insulation you use (spray foam, Blown-in-blanket, or traditional batts), advanced framing maximizes the overall R-value of the insulated wall system.

Next, maximize the insulation’s effectiveness within the wall cavity. It is critical that, whatever insulation is used, no insulation voids be present in the finished wall assembly. One year we had a spray foam contractor fill our 2x4 wall cavities with foam, and then proceed to shave off the excess foam with a long blade attached to a reciprocating saw. Since the saw had to be held back from the wall, the blade curved inward as it was pressed flush to the studs. This left a nice curve to the surface and left the insulation depth remaining between the studs about 2 inches (should have been 3.5”). Now, when the sheetrock was installed we had a nice cavity for air to circulate within the wall. A similar thing happens when folks think it necessary to staple to tabs of insulation batts to the sides of the studs instead of the stud face. It leaves an air pocket and that is a no-no!! Those voids significantly reduce the overall R-Factor in the wall (can be upwards of 30%). We now use a spray-foam contractor that leaves the spray foam surface totally flat between the studs or we use blown-in-blanket which provide a flush surface with no voids.

A great technique that is gaining popularity is to first apply a thin coat (~1/2") of 2-pound spray-foam (that's ~R3). It is a closed cell foam that stops any air or moisture from infiltrating from the outside. You can then fill the remainder of the cavity with the insulation of your choice to acheive the required insulation level. If you really want do do it right, you should also seal the top, bottom, window openings, etc of the drywall on the exterior walls. This ensures the insulation is residing in a dead air space which allows for full efficiency of the insulation.
Exterior wall Continuous Insulation. Just because you put R-13 in the wall cavity doesn’t mean that your finished wall is R-13. If you are using a 2x4 wood walls each stud is ~R-3, so the overall R-factor of the wall assembly is somewhat less. And that depends on the amount of framing lumber contained within the wall… The framing members within a wall are known as thermal shorts because of their low insulation values. Heat follows the easiest path from a warm surface to a cooler one, so the framing members are responsible for a significant amount of the heat flow. If you’ve ever seen a picture from a thermal imaging camera, you can easily see all the framing in the house because it is conducting heat more readily that the insulated wall cavities.

The method to make a significant improvement in overall wall thermal efficiency is to use “continuous insulation”. That is typically the application of rigid foam board insulation to the exterior of the home, just prior to installation of siding, or brick. Since this insulation adds an additional layer over both the cavity insulation and the framing. The total R-value of the wall assembly is increased by the R-Value of the foam sheet. For example, a wall with an aggregate R-value of 10.5 and foamboard with a value of 10 will produce a wall with a factor of R-20.5. The IRC 2012 building code covers this technique to achieve the necessary insulation level for your particular climate zone.
Floor insulation. Mason Texas is located in climate zone 3 and we build our homes on a slab. Insulation under the slab or at the slab edge is not “required” for us. However, we perform an energy analysis of our homes prior to construction (actually during the design process so we can address any energy issues early). The analysis of the homes now show our largest source of heat leakage is our slab edge. It didn’t just get worse (same slab configuration as always), but now is significant because other energy consuming issues have been addressed. To address this on our next house, we will be using slab-edge insulation. Since we have termites in our locale, an insulation solution that provides a termite barrier will be chosen.

If you choose to use rigid foam insulation at or near ground level, pay specific attention to protection against termites. It can provide a launch pad for invasion into the wood structure of your home.

When insulation a floor over a crawlspace, a good way to do it is spray foam, but if you do, use closed cell foam (2 pound foam) to stop air and moisture infiltration as well as insulate. Also, ensure the form is impregnated with borate to prevent termites from attacking the foam.
Ceiling Insulation. We have chosen not to insulate the ceiling (attic floor), but instead use spray-foam to insulate the underside of the roof deck. In the south, HVAC systems are typically installed in the attic (sometimes the air handler is installed on the main floor, but distribution ducting snake their way thru the attic.) Attic temperatures can easily reach 140 degrees, so you can imagine how much energy is lost from the nice cool air inside the HVAC ducts as they snake through the hot attic. Also ducting and joints may not be 100% air tight so any air that leaks, is simple wasted.
Attic Insulation. If one insulates the underside of the roof deck, the attic space becomes part of the conditioned building envelope. That means the attic temperature and humidity approximates the interior conditions of the house. The HVAC ducting in the attic is therefore located within the buildings insulated envelope. Any heating to cooling lost thru ducts and joints, is leaked into the attic, but it is not that detrimental since it is actually cooling or heating within the insulated envelope of the house. Of course, one wants the ducting and joints to be installed, and sealed properly so that conditioned air is delivered to rooms where it is intended and at the volume the HVAC design called for!!
Roof deck insulation. Like the external walls, one may need to provide a layer of insulation on the top side of the roof decking. This can be very beneficial to create a thermal break for the roof framing members. It is even more important if some of the ceilings in the home are cathedral or have an exposed rafter design.

In the past, we have sprayed additional foam insulation to cover the rafters to provide the thermal break. In the area of the home with cathedral ceiling, a layer of foamboard was applied to the underside of the rafters as the thermal break. Installing the foam on the topside of the roof deck may eliminate the need for the extra spray foam covering the rafters (but you must have enough depth to satisfy the code requirements.
Ventilated Roof. We install metal roofs in our area for a variety of reasons. We, all too often, get damaging hail which trashes many composition roofs, but usually leaves the metal roofs unscathed. Because of this, insurance companies provide a generous break on homeowner policy premiums, so there is a payback for any additional expense of the metal roof.

When we install a metal roof, we install 2x4’s purlins on top of the roof deck to which the metal roof is fastened. This provides and air space between the roof and the roof deck. Because we typically use an R-panel style of metal, it has enough open area on the underside of the ridges to allow air flow from the eves to the ridge of the roof. We provide an air path thru the soffits up thru a space in the roof deck for air to reach the underside of the metal roof, then a vented ridge is installed at the top to allow air to easily escape.

Because of the reflectant surface of the metal roof underside (we use galvalume panels) and the ventilated air space directly under it, we now have a quite efficient radiant barrier to help reflect radiant heat before it has a chance to penetrate the house. University studies have demonstrated the effect of this roofing technique results in the equivalent of R-11 insulation in the summer and R-5 in the winter (we dont include this extra r-factor in our energy calculations because it isn't certified as such). In the hotter sections of the country, this is a quite cost effective method to reduce the heat load of the home.