dslreports logo
 
    All Forums Hot Topics Gallery
spc
Search similar:


uniqs
1196
lutful
... of ideas
Premium Member
join:2005-06-16
Ottawa, ON

lutful

Premium Member

[Insulation] Attic insulation and venting topic

Click for full size
During the discussion of snow on attic, several other issues were brought up, such as venting, condensation, ice dam, and insulation. Let's discuss them in this thread without getting into personal attacks.

I annotated a typical attic diagram from the main page of international home inspector association.

Majority of such attic diagrams do not show the incoming air being moved around, and heated up, within the attic space. Some even imply linear flow of cold outdoor air hugging the roof line. I think that is causing serious misunderstandings, possible leading to problems.

My comments, some posed as rhetorical questions, are in script font.

1. Incoming Air

Incoming air is ALWAYS slightly warmer than outdoor air, because the insulated walls allow some heat to escape.

2. Air Flow

On calm cold nights, there should be no air flow, but there is some. Why?

a) The insulated ceiling does allow some heat to escape. It also radiates as a black body.
b) The incoming air is slightly warmer.

3. Moisture in Attic

Where does it come from?
a) Some of the moisture is leaked from indoors, because no vapor barrier is perfect. The normal attic airflow moves it towards the ridge vent.

b) The rest comes with the incoming air, which is also slightly warmer, and hugging the cold roof at the intake! Common cause for many homes near water.

4. Ice Dam

a) Due to insufficient attic insulation, many air leaks, and high indoor temp. The melting occurs higher up the roof, and trickles down to form ice dams.

b) Due to solar heating, when temp is close to freezing.

c) Due to high wind and temp just below freezing. The high flow of warmed intake air can form ice dams on opposite side of the house.

PSWired
join:2006-03-26
Annapolis, MD

PSWired

Member

Maybe I'm missing the point of this thread, having not paid attention to the previous recent thread. At any rate, your assumptions seem to be valid other than the one about the intake air temperature at the eaves. Other than in the calmest outdoor conditions, I think this temperature can be assumed to be at ambient. Any heating of outdoor air due to radiated or conducted heat from the building walls is unlikely to be concentrated enough to affect the air temperature entering the eave vents.
Expand your moderator at work
lutful
... of ideas
Premium Member
join:2005-06-16
Ottawa, ON

1 edit

lutful to PSWired

Premium Member

to PSWired

Re: [Insulation] Attic insulation and venting topic

said by PSWired:

Maybe I'm missing the point of this thread ... Any heating of outdoor air due to radiated or conducted heat from the building walls is unlikely to be concentrated enough to affect the air temperature entering the eave vents.

Thank you for summarizing the most serious misunderstanding.

Insulated walls do warm the incoming air, and that small increase contribute to the airflow from soffit vent to ridge vent. It is the biggest driver on very calm nights.

There is similar warming of air above the attic insulation layer, and that was a very contentious point from previous thread. Curiously that diagram in the original post show warming inside the attic, but does not show warming of incoming air.

PSWired
join:2006-03-26
Annapolis, MD

1 recommendation

PSWired

Member

While I of course agree that any vertical surface with a higher temperature than the surrounding environment will induce convective flow parallel to the surface, I would need to see some sort of data to believe that this phenomenon could introduce any sort of appreciable flow into an attic through an eave vent. Even in the calmest of outdoor conditions, the heat loss through ceiling insulation into the attic and solar gain from the roof material should be the primary driving factors for convective flow through the venting system.
H_T_R_N (banned)
join:2011-12-06
Valencia, PA

H_T_R_N (banned) to lutful

Member

to lutful
said by lutful:

There is similar warming of air above the attic insulation layer, and that was a very contentious point from previous thread.

No, no it was not. The part you were pushing was that the snow layer would provide enough insulation to materially affect the temperature of the conditioned space, not that heat loss would provide a lifting effect on the air currently in the unconditioned space. There was not one person who disagreed that some amount of heat escapes through and into the unconditioned space.

In a desperate attempt to show proof you scoured the web and pulled a very bad thermo image from a page that was describing an energy audit with FORCED compression of the conditioned space to exaggerate the heat loss so leaks would be more detectable.

Now with this post you are stating what everyone in the other thread was saying, heat in the unconditioned area will travel up and out the ridge vent aided by the soffit vents, providing no measurable affect on the conditioned space.

garys_2k
Premium Member
join:2004-05-07
Farmington, MI

garys_2k to lutful

Premium Member

to lutful
said by lutful:

Insulated walls do warm the incoming air, and that small increase contribute to the airflow from soffit vent to ridge vent. It is the biggest driver on very calm nights.

Says who?
lutful
... of ideas
Premium Member
join:2005-06-16
Ottawa, ON

lutful

Premium Member

said by garys_2k:

said by lutful:

Insulated walls do warm the incoming air, and that small increase contribute to the airflow from soffit vent to ridge vent. It is the biggest driver on very calm nights.

Says who?

It could even be YOU ... if you sincerely explore following rhetorical questions.

Assume an experimental home has 2000 sq ft R20 walls and 1000 sq ft R40 ceiling, with "adequate" passive venting.

Assume it is a perfectly calm night with -25F outdoor temperature, and you want to keep indoor temp at +75F.

Q1: Do you need to run the furnace at all that night?
Q2: If answer is yes, how do the thousands of BTUs disappear?

Q3: Could some disappearing BTUs warm air molecules near walls?
Q4: If answer is yes, could that warmer air be drawn through soffit vent?

Q5: Could some disappearing BTUs warm air molecules in attic?
Q6: If answer is yes, could that warmer air rise towards ridge vent?

Q7: Could any other mechanism create airflow through attic that night?
rody_44
Premium Member
join:2004-02-20
Quakertown, PA

rody_44 to lutful

Premium Member

to lutful
I have nothing to contribute to the thread. Well maybe that i think we are splitting hairs. In otherwords i believe you are correct but dont really feel its something that the average person is going to be able to notice.

lutful
... of ideas
Premium Member
join:2005-06-16
Ottawa, ON

lutful

Premium Member

said by rody_44:

i believe you are correct but dont really feel its something that the average person is going to be able to notice.

After the basics are covered, we will explore attic upgrades that can actually lower energy consumption by 25 to 50% for a typical home. I am planning such a mod for our 25-year old suburban home.

garys_2k
Premium Member
join:2004-05-07
Farmington, MI

garys_2k to lutful

Premium Member

to lutful
said by lutful:

It could even be YOU ...

OK, never mind, I thought you had a reasonably good source. I'm not into hand waving.
lutful
... of ideas
Premium Member
join:2005-06-16
Ottawa, ON

lutful

Premium Member

I articulated the key concepts in the form of those questions, so any sincere person who tries to answer them will quickly arrive at the correct answers.

From AtticSim introduction: »web.ornl.gov/adm/partner ··· 0043.pdf

AtticSim is a Fortran 95-based computer tool for predicting the thermal performance of residential attics. It mathematically describes:

the conduction through the gables, eaves, roof deck, and ceiling;
the convection at the exterior and interior surfaces;
the radiosity heat exchange between surfaces within the attic enclosure;
the heat transfer to the ventilation air stream;
and the latent heat effects due to sorption and desorption of moisture at the wood surfaces.

Solar reflectance, thermal emittance, and water vapor permeance of the various surfaces are inputs. The model can account for different insulation R-values and/or radiant barriers attached to the various attic surfaces.

Unique among such programs, it also has an algorithm for predicting the effect of air-conditioning ducts placed in the attic.
lutful

lutful to garys_2k

Premium Member

to garys_2k
»www.buildingscience.com/ ··· -venting

... warm air next to the heated siding can rise, enter the vent, melt snow, and cause ice dams.

Even with their suggestion, the incoming air will still be 0.5F to 2F warmer than true outdoor temp for any practical soffit vent placement. This FACT can be easily checked using two identical digital thermometers.
rody_44
Premium Member
join:2004-02-20
Quakertown, PA

1 edit

rody_44

Premium Member

I live in a cape cod. Very little of this vented space. Like 12 inches at most of the areas. I have the crawl spaces and the peak of the roof. Snow is on there now but im using the same amount of pellets with or without the snow being on the roof. If you read the original post you will see where i said i never get snow on the roof. I usually dont but as of now i do have snow on the roof but really am not using anymore pellets than i was when no snow was on the roof. Its a cold ass fing winter. My house is 50s construction. Not much as far as insulation goes. But it is there even if it has fallen to the ceiling of the rooms.
H_T_R_N (banned)
join:2011-12-06
Valencia, PA

H_T_R_N (banned) to lutful

Member

to lutful
said by lutful:

I articulated the key concepts in the form of those questions, so any sincere person who tries to answer them will quickly arrive at the correct answers.

From AtticSim introduction: »web.ornl.gov/adm/partner ··· 0043.pdf

AtticSim is a Fortran 95-based computer tool for predicting the thermal performance of residential attics. It mathematically describes:

the conduction through the gables, eaves, roof deck, and ceiling;
the convection at the exterior and interior surfaces;
the radiosity heat exchange between surfaces within the attic enclosure;
the heat transfer to the ventilation air stream;
and the latent heat effects due to sorption and desorption of moisture at the wood surfaces.

Solar reflectance, thermal emittance, and water vapor permeance of the various surfaces are inputs. The model can account for different insulation R-values and/or radiant barriers attached to the various attic surfaces.

Unique among such programs, it also has an algorithm for predicting the effect of air-conditioning ducts placed in the attic.

NONE of this addresses your contention that a layer of snow on the roof will impact the thermal requirements of the conditioned space. You can continue to ignore that if you like. With this latest set of posts you are making a point no one was arguing against but you; heat escapes and travels up and out the ridge vent. (or at least should) It would be nice if you could tie the two together sooner rather than later.

garys_2k
Premium Member
join:2004-05-07
Farmington, MI

garys_2k to lutful

Premium Member

to lutful
said by lutful:

Even with their suggestion, the incoming air will still be 0.5F to 2F warmer than true outdoor temp for any practical soffit vent placement. This FACT can be easily checked using two identical digital thermometers.

It can? Show us the data. As my former boss always said, "Without data you're just another opinion."
lutful
... of ideas
Premium Member
join:2005-06-16
Ottawa, ON

lutful

Premium Member

said by garys_2k:

said by lutful:

Even with their suggestion, the incoming air will still be 0.5F to 2F warmer than true outdoor temp for any practical soffit vent placement. This FACT can be easily checked using two identical digital thermometers.

It can? Show us the data. As my former boss always said, "Without data you're just another opinion."

Since the 1990s, I have actually done such measurement on dozens of homes, hundreds of times, to compare with AtticSim predictions. Other experts have done it thousands of times.

You can personally try it a few times before this winter is over. You could also try to run AtticSim with help from an engineering student.
lutful

lutful to H_T_R_N

Premium Member

to H_T_R_N
said by H_T_R_N:

NONE of this addresses your contention that a layer of snow on the roof will impact the thermal requirements of the conditioned space. You can continue to ignore that if you like.

I did not ignore it - I highlighted several inputs used by AtticSim. If you carefully consider them, you will realize why a layer of snow on the roof MUST impact the result. So will a layer of rain, dirt, or bird poop.

You can ask any engineer to help you run a few simulations with varying amount of snow on a roof with properly insulated and vented attic. You can sit down and specify all the parameters.

John Galt6
Forward, March
Premium Member
join:2004-09-30
Happy Camp

John Galt6 to lutful

Premium Member

to lutful
For those who have an interest in this kind of thing (monitoring and logging), I'd suggest this low-cost solution:

RECEIVER:

»www.tindie.com/products/ ··· eceiver/

2-CH TEMPERATURE MODULE:

»www.tindie.com/products/ ··· oralarm/

1-CH MINI TEMPERATURE SENSOR:

»www.tindie.com/products/ ··· -sensor/

MANUAL:

»drive.google.com/file/d/ ··· NVU/view

Calibrate the sensors by running them side-by-side throughout the anticipated temperature range for a few days to understand how they react.

The transmitters default to hourly data transmissions, but that is fully configurable, and is covered in the manual.

You can add many more sensors...they're listed on his site.

There is a nice Windows-based application available for $10, but you can get it for free if you buy the Starter Kit:

»www.tindie.com/products/ ··· r-kit-3/
H_T_R_N (banned)
join:2011-12-06
Valencia, PA

H_T_R_N (banned) to lutful

Member

to lutful
said by lutful:

If you carefully consider them, you will realize why a layer of snow on the roof MUST impact the result.

So you have no actual proof of this, just a mathematical equation and some creative thinking. Lets assume there is 8 feet of snow on the roof, how much less energy would it take to heat the house then if there was none. Assuming a 1000 SF single floor home, with a 4/12 pitch roof with a ridge vent and soffit vents along both 10 foot eves, with a vapor barrier and R40 insulation? I'll make it easier, there are no penetrations into the attic space at all. No ceiling fixtures to speak of and no attic access, there is also no windows and only one door.
lutful
... of ideas
Premium Member
join:2005-06-16
Ottawa, ON

1 edit

lutful

Premium Member

Click for full size
said by H_T_R_N:

said by lutful:

If you carefully consider them, you will realize why a layer of snow on the roof MUST impact the result.

So you have no actual proof of this, just a mathematical equation and some creative thinking.

I was waiting for your comment before posting an actual chart.

AtticSim is used millions of times a year, by thousands of professionals. Google some of their reports to see how closely it matches real observations. FYI engineers also simulate snow load on roof, etc. Simulation is an established method.

*** Somewhat relevant non-technical article discusses energy reduction by piling snow outside (already insulated) walls.
»www.motherearthnews.com/ ··· goe.aspx
Imagine outer walls had ventilated space like attics, there will still be some reduction, and engineers will use simulation to figure it out, before building anything.
lutful

lutful to rody_44

Premium Member

to rody_44
said by rody_44:

Very little of this vented space. Like 12 inches at most of the areas. ... Snow is on there now but im using the same amount of pellets with or without the snow being on the roof.

If you very carefully recorded weather conditions, snow height, and total energy consumption to maintain a fixed indoor temp, there would be a clear pattern over a whole heating season.

Imagine the vented space gets smaller and smaller ... eventually you have two different layers of insulation, one being the snow. For such attic designs, layer of snow on roof helps even more to counter the radiation heat loss, and that will lower the attic moisture level.

I copied very relevant but "unexpected" facts from a definitive guide on moisture. »web.ornl.gov/sci/roofs+w ··· book.pdf

rody_44
Premium Member
join:2004-02-20
Quakertown, PA

2 edits

rody_44 to lutful

Premium Member

to lutful
I dont have functionong soffits on my house being a cape cod. i only have two gable vents. My garage was built like this also. The garage is gone. garage construction like house was cinder block with the old tongue and grove wood for the roof. Anyway, i decided to 3/4 inch celotex and vinyl side the garage and put a new plywood roof over the tongue and groove. long story short i had a fire and the celotex fumes built up in the garage which resulted in leveling the garage when the fire department kicked the door in. Fire martial said it was all because i never installed the gable vents. He said the house is fine being it has functioning gable vents and something about it cant happen in a house with drywall. In otherwords i had a massive flashout in the garage. Flames went a good 50 feet up the driveway. Fire department wouldnt go near it again and they had me signing papers thinking i placed a bomb in there.

Morale of the story being not everyone has much air movement. Dead serious about the garage.
H_T_R_N (banned)
join:2011-12-06
Valencia, PA

1 recommendation

H_T_R_N (banned) to lutful

Member

to lutful

Academia.edu is a social networking website for academics. So basicly a facebook for people that think they know better the the rest of us, but no one in a real position buys their shit.

I could put a paper up there showing how the phases of the moon affect the activities of certain types of men and the bodily changes they go through during the full moon. I would still not proved the case for werewolves.
How about an ACTUAL paper from the EPA or at least a paper someone can read without signing up for another social networking site.

I'm gonna take a stab and say thats your graph?
said by lutful:

I was waiting for your comment before posting an actual chart.

As only a true troll does.
lutful
... of ideas
Premium Member
join:2005-06-16
Ottawa, ON

lutful

Premium Member

said by H_T_R_N:

Academia.edu is a social networking website for academics. So basicly a facebook for people that think they know better the the rest of us, but no one in a real position buys their shit. ... I'm gonna take a stab and say thats your graph?

No. The graph is actually from a relevant masters thesis.

Hosseini, Mirata (2014) Cool Roofs Savings and Penalties in Cold Climates: the Effect of Snow Accumulation on roof. Masters thesis, Concordia University.

Here is the official link. »spectrum.library.concord ··· /979065/
H_T_R_N (banned)
join:2011-12-06
Valencia, PA

H_T_R_N (banned)

Member

said by lutful:

said by H_T_R_N:

Academia.edu is a social networking website for academics. So basicly a facebook for people that think they know better the the rest of us, but no one in a real position buys their shit. ... I'm gonna take a stab and say thats your graph?

No. The graph is actually from a relevant masters thesis.

Hosseini, Mirata (2014) Cool Roofs Savings and Penalties in Cold Climates: the Effect of Snow Accumulation on roof. Masters thesis, Concordia University.

Here is the official link. »spectrum.library.concord ··· /979065/

The LINK DISCRIPTION STATES:
Cool Roofs Savings and Penalties in Cold Climates
The VERY FIRST LINE FROM THE LINK READS:
Utilizing a cool roof is an efficient way to reduce a building's usage of cooling energy, although it may increase the usage of heating energy in winter.
HOW, PLEASE HOW is this relevant to the heating requirements of a home that has a properly insulated and ventilated roof structure? Not a cool roof, not a non ventilated roof, or an igloo, a correctly ventilated roof?
You are continually taking the things you read out of context, in your search for proof of you theory. You can not just search for a line or a chart and use it as a reference without actually reading what it is simulating or describing. Like the image of an energy audit, just looking at the picture it shows a mess, until you read that its a house under 10 times more pressure than any house normally would so to exaggerate the heat loss in an attempt to locate areas of loss.

lutful
... of ideas
Premium Member
join:2005-06-16
Ottawa, ON

lutful to rody_44

Premium Member

to rody_44
said by rody_44:

I dont have functionong soffits on my house being a cape cod. ... Morale of the story being not everyone has much air movement.

A temperature and humidity sensing blower can be installed in your attic.

Lots of research going on now with multi-layer insulated roofs, which free up the whole attic in older homes, and allow energy conserving cathedral ceilings in new homes. I will post some links and diagrams.
lutful

lutful to H_T_R_N

Premium Member

to H_T_R_N
said by H_T_R_N:

HOW, PLEASE HOW is this relevant to the heating requirements of a home that has a properly insulated and ventilated roof structure?

Look at the chart again: there are bars for "dark roof" in addition to "cool roof" and he considered "effect of snow accumulation on roof" for both types in several cities.

I suggest that you find a local engineer who can run thermal simulations with home parameters that you personally define, and various height of snow on the roof. Your initial parameters will probably give 5-10% reduction. Try to come up with a practical attic design where a foot of snow on the roof has less than 1% impact. It is quite difficult.
H_T_R_N (banned)
join:2011-12-06
Valencia, PA

H_T_R_N (banned)

Member

I'm done.

garys_2k
Premium Member
join:2004-05-07
Farmington, MI

1 edit

garys_2k to lutful

Premium Member

to lutful
said by lutful:

Hosseini, Mirata (2014) Cool Roofs Savings and Penalties in Cold Climates: the Effect of Snow Accumulation on roof. Masters thesis, Concordia University.

Here is the official link. »spectrum.library.concord ··· /979065/

Thanks for the link! Too bad the paper is behind a paywall that I won't breach, but from the abstract:

In the very cold climate of Anchorage, AK, the simulated annual heating energy consumptions of the prototype old retail building with a dark (warm) versus a cool roof (without considering the snow) are 123548 and 125848 MJ/100 m2, respectively (showing a 2300 MJ/100 m2 penalty for the cool roof). These numbers reduce slightly to 123216 and 124409 MJ/100 m2, respectively (showing 1193 MJ/100 m2 penalty for the cool roof), when "late-winter packed" snow is considered.

GREAT! NUMBERS! Let's pick them apart a bit... Since we're only looking at the "cool roof" instance ("conventional attics") the important comparisons for us is 125,848 MJ/100m2 for the no snow, and 124,409 MJ/100m2 with snow. That is a 1.24% decrease in energy use.

Now, what of this? If the owner of this building spends $1200 per year on heat, he will save $13.72 with snow on the roof compared to no snow. Not even enough to buy lunch.

So, your source verified my advice: Having snow on the roof makes NO MEANINGFUL DIFFERENCE in heating costs if the attic is properly ventilated. If someone does experience a significant savings because of snow on their roof then it indicates a problem. Thanks for the help!