Insulation and an Air Barrier

My plans called for fiberglass batt insulation in the stud cavities outside the air tight barrier (ie OSB) and inside the exterior EPS foam sheathing.  The problem I ran into is that the walls are 2x8.  From my investigation of materials available through local suppliers, there are very few fiberglass options for a 2x8 wall (7 1/4" thick).  Several companies make batt insulation with depths of 8.5 (R28) and 7.5 (R22).   Another option was to gang up batts of different thicknesses and R-values but this seemed complicated and installation could take twice as long. In either case the insulation will have to be compressed into the wall cavity with the interior OSB.  Yet another option is dense packed blown in fiberglass.  At about R29.8 it is the best option for conformity inside the wall cavity and provides the highest R-Value.  This would be my best option if budget allowed for it.

Before making a final choice about which products to use, I decided to further investigate the documented effects of compressing fiberglass insulation.  I have heard people say that you shouldn't compress fiberglass insulation.  However, this depends heavily on installation details.  As you compress fiberglass, the R-value actually increases.  Lets consider a 2x6 wall cavity. If your wall is R20 (5.5" standard density batt) and you compress, you end up with an air void in the 2x6 wall cavity.  Air voids provide routes for air movement and convection and decrease the thermal performance of the wall.  Under the right relative humidity conditions, condensation can become an issue if the cavity is exposed to temperatures below the dew point.  Extended exposure of wood surfaces to water under the right conditions can lead to mold and eventually rot.

So can you compress a deep fiberglass batt into a shallow stud wall cavity?  You sure can, and you will end up with an insulated R-Value that would be higher than a standard batt required for that cavity.  The formulas are pretty simple.

Final R-Value = Original R-Value + Change in R-Value 


Change in R-Value = 1/2 x (Percentage change in thickness) x (Original R-Value)

where the "Percentage change in thickness" is 

100% x ( Compressed Thickness - Original Thickness)/(Original Thickness)

So for R-31 (9.5" thick) batt compressed into a 7 1/4" stud space, the percentage change in thickness is

100% x (7.5"-9.5")/9.5" = -21%.

This translates into a change in R-Value equal to

1/2x(-21%)*31 = -3.3.

So the final R-value would be 27.7 in the cavity.  Compared to R22 for a standard batt, this seemed like a good option.  But there were some details to keep in mind and things to work around.  

1 . R-31 only comes in 24" but the stud cavities are actually 22 1/2" because of the 24" on-center spacing of the studs.  So every R-31 batt would have to be cut to the proper width.  RESNET recommends insulation 1" bigger than the cavity (from what I recall).  This is interesting because most manufacturers provide insulation at widths that are only 1/2" wider than the stud cavity.

2.  I omitted blocking from the walls in order to provide a full stud space for insulation.  This lead to a minor problem:  As the studs started to dry, they also started to warp.  The cavities were 22 1/2" at the plates but up to 23" wide elsewhere in the cavity.  Cutting insulation at non-uniform widths would be extremely time consuming.

My work around was to use the whole batt in the cavity without cutting it to width. At 1.5" larger than the cavity in places, the batt would only be wider than the RESNET specification by 1/2".   The effect of this is compression along the width of the cavity.  Since the R-Value changes linearly with compression, I would expect the same formula above applies to the width of the cavity also.  So using the above formula and my calculated R-value under compression (ie R-27.7) the insulation would have a variability from R27.1 - R26.8 assuming a perfect installation.

Several people suggested that this much compression would lead to issues with installation of the interior OSB air barrier leading to "bulging wall" between the studs.  We mocked up a batt installation and covered it with a sheet of OSB.  We did need two people to install the mock up.  One person to hold and compress the the insulation with the sheet of OSB while applying pressure and the other person nailing.  The trick was to start nailing (pneumatic nailer works best) at the bottom plate.  We then pushed the sheet in (compressing the insulation behind) about 16" up the wall and nailed all studs again.  We kept doing this until the full sheet was nailed.  After installation was complete we ran a strait edge horizontally across the OSB and there was no bulging from the insulation.  The bending stress on the OSB across a 2' span associated with the back pressure from the insulation appears to be non-existant.  Compressing fiberglass with OSB works fine.  However I can guarantee  that anything more than R-31 is very hard to compress.  Don't bother trying it!

Installation of the batts was tricky at first.  Simply pushing them in to the cavity lead to nonuniform filling.  Repeatedly, the batts did not fill the corners of the wall cavity at the exterior side.  To correct this we had to start the batt install by angling it into the corner while compressing it in width to fit into the opposite side.  We would use our hands to slightly pull away the insulation and visually inspect the corners.  Tucking the insulation into the corners push the insulation back into the corner if necessary.  At the front face of the batt we roll the corner in place with two hands until the edge of the batt was flush to the stud it was bulging in the middle.

From my experience thus far, fitting full batts into a standard cavity is pretty easy once you get the hang of it.  Fitting batts into smaller cavities is much harder.  once fitted, it is hard to get the batt to bulge in the center.  The trick to making it puffy is to make sure that the batt is tight fitting but only about 1/2" bigger than the stud cavity in both height and width.  The batt then slips in the cavity with ease and the center still bulges outside the stud cavity which will make for more uniform compression once the OSB is installed.

From the beginning we knew there would be scraps of insulation that would have to be thrown away. I hate throwing away what appears to be good material but trying to fit scraps into a stud cavity is asking for trouble.  Using a single piece cut to fit gives a better install:  It fits tight against the studs and provides for more uniform filling through the cavity.  Overall it will lead to better thermal performance.

Having several people to install insulation makes the job move along quickly.  However, it also introduces inter-user variability in the installation.  We inspected each others installation and watched one another's installation method to narrow down the best way to achieve a good installation and came up with a set of rules for installation.  Our reasoning was that it would decrease inter-user variability and provide for a more consistent installation.

After insulating was completed we started installing the first component of our air tight barrier ie OSB.   We just followed the procedure that we mapped out with our mock up from earlier when testing the insulation compression.  Since we wanted to keep joins to a minimum, we worked with full sheets.  Around windows, we typically placed the sheet vertically in front of the buck then scribed a line around the buck opening onto the OSB and cut the opening out with a circular saw.  If the sheet covered the window, we ripped the sheet to the width of the nearest 24 oc stud centered under the window opening so we could reach inside the buck opening to scribe the window opening. With this methodology we will be sealing two less joins per window when compared to installing individual pieces above and below the window bucks.  In some cases this may not work but I figured that more solid OSB means less potential for air leakage and is therefore the better choice.  It also saves on expensive tape as well as time for caulking and taping.

Some penetrations in through the air barrier were easier to implement up front rather than waiting till later.  The ERV vent was one of those penetrations.    The vent was the DVS-100 from reversomatic.  It is a dual vent that can be fitted into the wall in a single piece.   See pics below. The vent was fitted into the wall cavity from the exterior.  Extension pipes were added to the vent and the vent was spray foamed around.  The spray foam was finally trimmed so fibreglass would fit nicely around the pipes.  We marked out where the penetrations would be on a sheet of OSB and drilled the holes necessary for the vent pipe penetrations.  In order to get nice tight insulation around the vent pipes, I had to figure out how to drill holes through fibreglass.  Using a template placed on the batt at the right location, I stepped onto the templates with all of my weight.  A drill with a hole saw could then be used to drill the hole locations to give nice round holes for fitting around the vents.

Anyway, more to come, within the next week we'll see much more happening.  I expect that we'll start the interior air sealing details and exterior cladding has already started....

Photo 1-1 After some practice we were able to achieve insulation fitting tightly in the corner of a stud cavity.

Photo 1-2 Starting to erect the OSB air barrier

Photo 1-3 OSB air barrier compressing the the fibreglass insulation.  Despite what people told me, it works fine!

Photo 1-4 Rather than adding corner studs we simply added a corner drywall corner and screwed the OSB to it. Corners are bad enough as it is for transmission of heat, eliminating a stud here and there does make a difference.

Photo 1-5.  ERV vent.  DVS-100 by Reversomatic.  Its a combination vent.  A hole was cut in the exterior foam the vent was inserted.

Photo 1-6 Holes drilled in OSB for the ERV vent.

Photo 1-7 How to drill fibreglass!  make a template of a hole. lay it on the fibreglass

Photo 1-8 Stand on the template and drill the fibreglass insulation.

Photo 1-9 Once the holes in the fibreglass insulation are drilled the insulation can be neatly filled around the ERV vent!

Photo 1-10. Installation of OSB around the ERV vent pipes.



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