Author Topic: Only Once  (Read 549 times)

Bud9051

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Only Once
« on: July 16, 2013, 04:53:16 PM »
When we add up the heat loss from a home we need to be sure we are only counting those BTUs once.

How often have you heard the statement that air leakage reduces the effectiveness of the insulation?  It's one of those things that's easy to see.  Turn on the blower door on a cold day and watch the cold areas grow as the infiltration chills the walls.  We are even supposed to look at the resulting patterns and reduce the grade of the insulation accordingly.  Well, would you be surprised to learn that infiltration and exfiltration do not degrade the performance of the insulation when that the air passes through?  If it is r-19 with no air passing through, it is still r-19 with the leaks.

Here's where the distinction comes in.  If the infiltration on a cold day was headed all the way into the house, it was going to get warmed up to room temperature at some point anyway and those BTUs will be accounted for when dealing with the somehow determined air leakage number.  To further degrade the wall assembly insulation value would be penalizing that home twice for the same heat loss.

There are examples like attic venting where incoming air blows through the insulation and then carries that stolen heat out the high vents, but since no other calculation is accounting for that loss, then degrading the insulation is probably appropriate.

Bud
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SLS Construction

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Re: Only Once
« Reply #1 on: July 16, 2013, 05:36:46 PM »
Sorry but I got to slightly disagree - a product gets an R rating based on it being installed per manufacturers spec & is based on that using specific controls. If you short circuit it, the product no longer is nor will function as listed. Is it wrong to assign "levels" which essentially degrade the effective R-Value based on voids, compressions & then add in air leakage figures, I don't think so. Now doubling up on the air leakage number by degrading the insulation - that can get a little flaky & agree would be wrong

Bud9051

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Re: Only Once
« Reply #2 on: July 16, 2013, 05:53:25 PM »
This is a tough one I agree because sometimes degrading is required, but other times I think the heat loss has already been accounted for.
<Is it wrong to assign "levels" which essentially degrade the effective R-Value based on voids, compressions & then add in air leakage figures, I don't think so.>

How do we identify those voids & compressions?  To be honest I've have only looked at walls in this regards while a BD is running.  You wouldn't see a lot without the depressurization.  Then, how would I distinguish between a void and a few extra nail holes or a gap in the sheathing.  Correct me if I'm looking at this wrong, but we identify the anomalies by the effects of the air leakage.  And, if at normal pressures that air is headed to the inside, well it either gets warmed up while passing through the wall or it gets warmed up once inside.  Same BTUs and they are accounted for with the air exchange heat loss.

Interesting.
Bud
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SLS Construction

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Re: Only Once
« Reply #3 on: July 16, 2013, 07:49:57 PM »
New construction is easy as we do a pre-drywall inspection
For older homes BPI, RESNET it automatically is assumed to be a Level 3 & based on what I have see that might be generous for some
You can always use Infrared but they only allow for Level2 for RESNET - I can't recall BPIs standard & personally ignore both as they are essentially worthless
Might there be a few already accounted for, yes but just how many & does it really matter if we are off by a 100 or even a 1000 btus in total? Just how perfect do we create a model which can never really account for all seasonal changes & how something is used?

Bud9051

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Re: Only Once
« Reply #4 on: July 17, 2013, 03:16:14 AM »
The example of degrading a wall assembly due to air leakage was just one we all have seen.  But in some threads I have heard it expressed that air leakage renders fiberglass insulation worthless.  Where the air leaks into a wall cavity and back out (as in my attic example) that heat is lost.  But where the ait leaks in and then continues into the home, no added loss at all. 

You are correct it would either be too small of a number or at least too difficult to quantify, but in terms of keeping our thinking correct, it may not be as bad as we (note I'm included) thought.  It's like the old, "you can't compress fiberglass" expression because it reduces its r-value.  It reduces its thickness, therefore less insulation, but, up to a point, compression increases the "R" per inch.

Am I defending fiberglass??

Bud
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SLS Construction

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Re: Only Once
« Reply #5 on: July 17, 2013, 08:02:25 AM »
Yeah but in a cooling climate that is heat entering we need to remove & during winter that leak in is also displacing heated air out
Fiberglass is a great product, when installed properly & fully enclosed on all 6 sides (and anyone in the field should be able to say that as it is true) - but when air can rush through it, it loses its insulative ability & essentially becomes an air filter

Bud9051

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Re: Only Once
« Reply #6 on: July 17, 2013, 12:02:31 PM »
<but when air can rush through it, it loses its insulative ability & essentially becomes an air filter<
There is no denying that the rate of heat transfer is increased, but do we account for it through air leakage or degradation of the insulation.

When we calculate the heat loss using the full assigned r-value and then add in the heat transferred by the infiltration we will have accounted for all of the heat.  If we de-rate the insulation, we would be counting some of that transfer twice.

By the way, I'm avoiding hot climates and other variations for now, tough enough wrapping my thoughts around this one example.

Bud

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Re: Only Once
« Reply #7 on: July 17, 2013, 02:08:18 PM »
No problems Bud & I would say both as you are losing heat due to degraded performance just through normal thermal transfer - when you add in air leakage which occurs due to stack effect, wind, etc.. (which is not always constant) you lose even more than normal at that time

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