Batt Insulation - Not all are poor!

Gregory La Vardera posted this excellent primer over at Green Building Adviser on the differences between fibreglass and mineral wool batts.

As Gregory points out, ROXUL Mineral Wool batts are not associated with the typical failings of a fibreglass batt installation. This is due to the density of the product and the ease of cutting and trimming. The product also sheds water and is fireproof.

My only critique of his article is is statement "I don't need my insulation to make an air seal, because I used that good ol' housewrap on the outside. Nope, nothing wrong with housewrap — but it provides no help with the air sealing you need at your vapor retarder. The air seal in this case wants to be on the warm side of the wall, to prevent interior moisture from entering the wall cavity and condensing during the winter heating season."

This is actually incorrect, an air barrier ANYWHERE in the assembly will block air flow through the assembly.  I will talk more about this in an upcoming blog entry.  For now, I did not want to detract away from the rest of the posters review of ROXUL mineral wool insulation.

How It's Made - Roxul Stone Wool Insulation

The popular 'How It's Made' TV series visits the ROXUL factory in this 5 minute video http://youtu.be/clN-wB8Vl_k

You may also be interested in this video of ROXUL's "Test The Best" demo presented at building stores across the country. http://youtu.be/7rbRYs0XEAM

ROXUL Mineral Wool Insulation - Highly Vapour Open

Dr. John Straube of Building Science Corp dispels the misconceptions of mineral wool insulation and identifies some of the many benefits from choosing ROXUL in this 3.5 minute video.

http://youtu.be/Fc6sVrVjRks

Of particular importance is his comments regarding the vapour permeance of a mineral wool insulation in comparison with rigid or spray foam insulation and why this is so important.

"Some insulation products that have built-in vapour resistance can impede drying and this can become an important concern during design. The resiliency of a wall to built-in construction moisture or accidental flaws in water control needs to consider how that insulation will allow drying outward."

"There are many types of foam insulation but all of them are characterized by limiting vapour flow through them."

"If the design is not taking into account resistant properties of foam, you can trap moisture in a wall or roof assembly, and of course trapped moisture leads to damage such as mould growth, corrosion or decay."

"One of the unique features of stone wool is that it is very open to vapour flow" "This means there are some tremendous advantages if you are trying to dry a wall or roof out or in, because water vapour came move almost unimpeded through the actual insulation product"

It's All About The Air Barrier!

Building Science Corporation has just released their report (available here ) that confirms that if you give insulation a chance by air sealing the assembly, all insulation will perform the same from a thermal transfer stand point.


The report highlights the increasing level of importance attributed to getting the details right as you increase the performance of an assembly by adding more insulation. Loosing 10% of your nominal value when you have a R15 assembly is a lot less critical than when you have a R50 assembly.

In order to complete this study, BSC had to design and build a hot box that significantly improved upon past designs.  

Key improvements include the ability to: 

• test higher R value enclosure assemblies (which have lower heat fluxes),
• expose enclosure wall samples to realistic temperature differences while maintaining the interior temperature at normal room temperatures, and
• measure the impact of imposed airflow at a given pressure difference across the specimen in both directions

This new and improved hot box was able to measure and confirm the effects of thermal bridging, the performance of insulation at various temperature gradients (for instance some polyisocyanurate insulations used in the TM Research Project exhibited a sharp increase in thermal conductivity - and decrease in Rvalue/in. - as temperatures approach and go below freezing), and the effects of air movement through part or all of the assemblies.

An interesting outcome of the air leakage measurements:

"Air leakage always increases the total heat flow through the building enclosure. However, air interacts with the materials in an assembly as it travels through. This interaction changes the temperature field in the assembly and through an assembly. The Thermal Metric wall test results provide strong evidence of the interaction between conductive and convective heat flows. This interaction results in heat exchange between the air and the materials inside the wall assembly and the total measured heat flow will be less than predicted by the commonly used discrete air leakage model"

Also of interest was the observation that "All of the reference test wall assemblies were subjected to significant temperature differences: up to 50°C or 90°F in the winter tests and up to 40°C or 72°F in the summer tests. Natural convective looping was not noted in any of the wall assemblies."

One of the most important confirmations for me was that "All wall assemblies experienced a loss in thermal performance due to air movement through the assembly. This is true for all of the assemblies tested regardless of the type of insulation material used (e.g. cellulose, fiberglass, ocSPF, ccSPF, XPS)".  With the failure of an effective air barrier that I typically see on spray foam jobs, this research helps to confirm that without an effective AB, even the might spray foam has diminishing thermal resistance. The report also confirmed "spray foam insulation only seal areas where the spray foam is installed; significant leakage paths often remain at wood to wood connections"  This confirms my conviction that spray foam is highly over rated.  If it does not guarantee an air tight assembly (see 3.6.4 on pg 102 of the report to see air barrier failures that occurred even in these lab conditions), why use it when there are cheaper products of lower environmental impact available?  I see attempting to use insulation as an air barrier as being just as foolish as attempting to use a poly vapour barrier to double as an air barrier.  Both products are hard pressed to represent an effective and durable air barrier.

Finally, it was interesting to note that they were able to measure the various resistance to air flow that different insulation represent. In 1.1 the report confirms that wet sprayed cellulose has more resistance to air movement over various fibreglass bat products.  It is too bad that they did not test mineral wool as it would be interesting to see how it compares to WPC.  Maybe next time!

Thermal Bridging and Dew Points

Anyone who has heard me speak for more than 10 minutes has probably heard me talking about dew-point potential.  It is at the heart of good building science principles.  Move the dew-point to somewhere outside of your assemblies, and your assemblies will have a built-in safety regardless of any air movement through them.

But for many, this dew-point conversation is esoteric and abstract, they can not usually visualize a dew-point occurring (that they are aware of, the condensation on the outside of a glass or can of cold liquid is a demonstration of dew-points) and so cannot appreciate the importance of reducing its potential in building enclosure construction.

Well by happenstance, I had an excellent example present itself to me at my home this afternoon.  I was coming back from my neighbour’s house after ‘supervising’ the dismantling of some scaffolding, when I observed condensation on the inside of the window near my front door.  Now, I generally have single pane heavy aluminum frame windows throughout my home, so condensation around the frame margins of the windows is not an unusual occurrence.  However this window had a wood frame (site built) and did not usually suffer from condensation.

Why the difference?  Insulation and air films!

Condensation spotted on the inside of the window.

Inside of window is condensation free except behind some mail propped up against window.

Condensation is definitely on the inside face of the window.

General window surface temperature 56.5° F

Surface temperature drops to below 51°F behind the mail.

Temperature in nearby region is 68.1°F

Temperature behind mail 56.4°F with an RH of 64%

So, what does this all mean? 

I have to admit, that this took a lot longer than it should have to figure out.  On the first round of measuring, I had RH levels well below 50% and temps of 66.3°F ‘near’ the window (my office is at 44% and 72°F on the other side of the house and benefiting from all the heat being pumped out from the computer equipment). At these windows temps and RH, the dew-point should have been around 47°F which was clearly well below my initial recorded window surface temp of 53°F behind the mail. 

So why the condensation? 

It then occurred to me that this was a delicate microclimate and by removing the mail for too long to do the testing and photos,  I had inadvertently changed the local conditions and disturbed the air film that would have been against the surface of the window.  So, I put the mail back to let the conditions come to equilibrium again and reran the measurements to get the figures in the photos above.  As you can see, the temperature within a couple of ‘ft’ to the window was now over 68F and the humidity that was building up behind the mail was over 64%.  This works out to a dew-point around 56°F and with a window surface temp of just under 51°F, I was well below the dew-point and had condensation.

The mail was creating enough of a barrier to heat loss (acting as insulation), that it was lowering window surface temperatures over 5F and at the same time was creating a micro-climate where the humidity was ‘trapped’ by still air and building up well above the rest of the house.  

This demonstration highlights how delicate the balance can actually be and why heat-bridging in assemblies is so critical.  Whenever you have isolated conditions that allow a small area of your assembly to cool in relation to the assembly as a whole, or that allow the build-up of humidity, you have the recipe for  liquid water formation and when hidden inside an assembly, that can and often does lead to disaster!