What’s In Your Walls?

by Dave on June 25, 2009

First seen in Coastal Sussex Weekly, May 28, 2009:

There is a part of your home that you only see for a few days while it is being built and then, hopefully, never again.  That is insulation – the stuff that made us itch like crazy when we were little and our parents made us go into the attic and get something down.  This is an important product considering that 70% of our residential energy consumption comes from heating and cooling.  Air leaking in or out can really waste energy by making our systems run more often.  Insulation comes in a few different forms – from the common to the advanced and from inexpensive to expensive.

Batt insulation is what most of us are familiar with – this is the fluffy yellow or pink stuff that makes us sneeze when near it.  Commonly, this has been fiberglass (reference the scratchy comment earlier) but there are new players in the market now – recycled cotton, wool and rock wool.  These new products generally perform better than fiberglass. The major downside to batt insulation is, no matter how diligent an installer is, irregular cavity shapes will cause voids – these voids lead to a heat loss and less efficient wall systems.  As an aside, the recycled cotton (sometimes called “blue jean insulation” because of the content) has no toxic substances within and low toxicity to produce making it overall healthier for you and for the environment.

A more energy efficient system is blown-in fiberglass or cellulose.  As the name implies, this insulation is blown into wall cavities and that alone will help the insulation reach those hard to insulate spots and fill irregular wall cavities.  These insulation systems can be installed wet or dry – wet application needs to be allowed to completely dry out before it is sealed off to prevent moisture from being trapped in your walls.  The key difference in fiberglass and cellulose is moisture absorption.  Fiberglass does not absorb moisture and cellulose does.  Either can be a benefit – but the entire building envelope needs to be considered before that choice is made.

Topping out the cost market is spray-in-place foam.  The initial cost is high – really high – but it is a great air barrier.  There are two types of foam – open cell and closed cell.  The main difference is their permeance – the ability to allow water vapor to pass through them.  Open cell has a higher perm rating than closed cell.  Again, this depends on the application and whether you would want to have high or low perm rates.  To be truly effective, spray foam must be coupled with insulated sheathing.

The key to insulation is their R-Values.  R-Values are the measurement of thermal efficiency or, more simply, how much heat is allowed to pass through the insulation.  Batts can achieve R2.9 to 4.3 per inch if properly installed, dense packed blown-in insulation can achieve R4 per inch, open cell is around R3.6 per inch and closed cell (after the blowing agent dissipates) is around R5 per inch.  As you can clearly tell, this is a game of numbers – but more importantly, the application methods are what really “seal” your wall.  The blown-in and spay insulations are better at filling the small voids and really getting the “true” R-Value they claim to achieve.  This comes down to talking to the insulation contractor about the products that they offer with the information you have read above.  From there, having a good discussion about cost versus R-Value weighed against the products inherent pro’s and con’s will allow you to come to the right decision.

Another important factor to consider – and there is some debate about this one – is wall thickness.  A conventional framing system typically uses 2×4 walls (3.5” thick) but a lot of contractors are promoting 2×6 walls (5.5” thick) as a moderate upgrade.  There are a variety of reasons for this but one of the most significant is the additional 2” of insulation it allows.

Insulation is just one part of your homes defense against heat loss/heat gain.  Several other items come into play when building the R-Value of your wall – not to mention your roof.  In this column, we will eventually get to all those topics but briefly, some of them will include sealing around windows and doors, sealing at the sill and top plate, house wrap, OSB versus Plywood versus the Zip System, exterior coverings (vinyl siding, fiber cement siding, brick, stone, or stucco), and foundation types.

Matthew Peterson is the Principal of Element Design Group in Lewes. He can be reached at matthew@elementdg.com.

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