[Equest-users] Questions - Foam Board R-value Variations by Temp

[Bishop, Bill via Equest-users]

Hi Chris,
Some quick thoughts:

1.)    It sounds like this question of effective R-value as a function of temperature needs to be addressed through more research. Perhaps NRCA could coordinate an effort with ASHRAE (but I wouldn't know how to pursue this).

2.)    You will have several inches of continuous insulation in Climate Zone 8 for roofs and above-grade walls, with minimum R-19 up to R-35 c.i. required per ASHRAE 90.1-2013. There will be a temperature gradient through these layers of insulation, so that only the outer layer would approach the outside air temperature. A math-savvy person with some time on their hands could take the temperature-dependent curve you mention and apply some calculus to estimate the effective U-factor for the assembly.

3.)    How to model? We often compare designs to an ASHRAE baseline. Standard 90.1 requirements are in terms of effective R-values and U-factors which will be modeled the same way in eQUEST regardless of the effective R-value per inch. We always have to make judgement calls regarding the effective (and long-term) R-values of insulation relative to the published literature available. So maybe polyiso isn't R-6/inch. Maybe it's R-5, R-4 or worse. In any case, we typically have many other variables that are unknown with equal, or greater impact on our energy numbers, such as infiltration rates. We can always perform sensitivity analyses of these variables to see how important they are to the energy performance of our particular designs.

4.)    For design/loads simulation purposes, you could use the worst estimated U-factor for your constructions for heating sizing, and the non-temperature-degraded value for cooling sizing. For annual energy simulation, better to estimate the assembly U-factors at a weighted-average outside temperature. I am not aware of a way to directly simulate temperature-dependent R-values in DOE-2/eQUEST.

5.)    There is also a question of thermal mass. If you have to add more layers of insulation to get a desired assembly U-factor, then you are also adding mass, which impacts the response factors of assemblies. With the relatively low densities of continuous insulation, the added thermal mass might not have much impact on the time-delayed heat flow through the wall. Again, this is something we can determine through sensitivity analyses of our designs should we wish. If NRCA and/or ASHRAE conduct new tests of insulation materials, it would be good to get data on specific heat values as well.
Regards,
~Bill

William Bishop, PE, BEMP, BEAP, CEM, LEED AP | Pathfinder Engineers & Architects LLP
Senior Energy Engineer

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From: Equest-users [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Chris Baker via Equest-users
Sent: Tuesday, August 30, 2016 10:33 PM
To: equest-users at lists.onebuilding.org
Subject: [Equest-users] Questions - Foam Board R-value Variations by Temp

One thing one of our suppliers has asked about is how the R-values get treated in equest, specifically, for foam board insulation and it opened up some reflection and in-depth thought on how this might be addressed in equest moving forward.

I decided to post this more to incite discussion (my knowledge on this is limited).

It appears that NRCA has been publishing R-value testing data separate from the actual NRCA roofing manual as they get newer data and more accurate data.

I was emailed a 2016 PDF file called "Insulation Design Predictability in Alaska" containing testing data and R-value variations by temperature for different foam board media.  If you email me I can forward it.

The result -  that has been the trend over the last few years - is that rated R-values have been going down for polyeurethane board and polyiso board.

The example for polyisocyanurate from 2014 was telling.  At 75 degF a 2" polyiso board had an average (mean) R-value of 5.55 out of a total of 7 samples.
The sampling for the same 2" polyiso at 25 degF had an average R-value of 4.049 (some say the newer data is 3.9).

One question is how we might be able to address this in equest in the future as we push for more accurate representations of energy usage and energy modeling in equest.

A published R-value on a cut sheet is not necessarily the performing R-value of the material in an Alaskan winter where we can get -40F or -50F for up to a week before warming up to -10 or -20 (sometimes colder).  And Alaska is definitely not alone in this.

But it doesn't appear that even NRCA has testing data for any foam board under a temperature of 25 deg F (which is warm for Alaska in winter).

The question some people have in Alaska is how this will be addressed in terms of energy efficiency calculations - which, obviously, isn't easy to answer.

There's a definite bell curve associated with the testing data that shows a steep dropoff of R-value with temperature - particularly temps colder than 25F - for both polyeurethane board and polyiso.

It would appear that the only way to achieve an accurate representation in cold climates would be to conduct independent testing and establish an overall annual average R-value (or at least a calculated number).

But even this would not necessarily be accurate. An equest model with a single (averaged) input R-value would represent slightly elevated cooling needed in summer and heating in winter that would likely be reflected as "too efficient" than what you would see in reality.

Given the lack of data we ended up going with cutsheet standard values for our previous model in equest but I, obviously, now know that 2" polyeurethane board is not necessarily one R-value or another.

When we model a wall or roof buildup in equest we are doing so to represent the thermal performance of the unit throughout a model year - with the foam being just one piece of the puzzle.

What isn't clear is how we might be able to move forward with more accurate data.  Our foam supplier that manufactured all the material for our LEED project was asking me if there was some way to address this moving forward.  I can't say...

To my knowledge, it still remains to be addressed since we don't have a lot of accurate testing data applying to these materials in places like Climate Zone 8 or 8A here in Alaska.

So without accurate testing data we really can't even calculate the R-value accurately other than to continue the bell-curve of the current data into the lower temps.  But then you would be seeing a 2" polyiso foam board with R2 when you get into your below zero temps which is - wow...  Something is wrong here.

The manufacturer said this was due to the nature of how the foam changes with temp - specifically, if the cells within the foam can shrink with cold temps.

It is even possible that this stressing of the material over extended periods can cause it to pull in moisture and moisture-laden air (slight amounts) which could also play a roll in cold temps.  But to what extent this can occur is unknown.

I'm curious what others have seen or come across or how we might be able to reflect this more accurately in equest.

Thank you!

Chris Baker
CCI Alliance of Companies
Fort Wainwright, AK

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