insulation; roof/ceiling vs. walls
Last Post 16 Mar 2021 10:31 AM by Ron031. 35 Replies.
Printer Friendly
Sort:
PrevPrev NextNext
You are not authorized to post a reply.
Page 1 of 212 > >>
Author Messages
bpnkrtnUser is Offline
New Member
New Member
Send Private Message
Posts:60

--
15 Nov 2011 05:04 PM
OK, I'm about to demonstrate my ignorance again.  I have read this forum for a while and have gathered information from several other sources.  Would this generally be a true statement;  "The roof/ceiling is the largest source of heat gain/load in a hot climate and the major source of heat loss in a cold climate."  As with ALL generalizations there are undoubtedly exceptions?

Thanks in advance for helping to educate the 'peanut gallery.'                                            
rbisys1User is Offline
Basic Member
Basic Member
Send Private Message
Posts:142

--
15 Nov 2011 08:05 PM
Greetings,

In regards to summer the large gain is due primarily to using the wrong insulation. Example.

Using FG in the attic of a house that is unshaded and has a dark roof will result in attic temps reaching as high as 150 Degs. The roof sys radiates infra red energy to the FG which absorbs it at about 95% eff. This along with the 99% + air volume allows the energy to transfer to the ceiling which can easily reach 110 deg on a 95 deg + day. If you have a 75 deg floor temp the ceiling will radiate about 37 btu/hr/sf. The 110 deg is about 25 degs hotter than a radiant heat sys for winter. This is why ac costs are so high and poor people die in their homes. God bless the mineral wool companies.

Using cellulose or foam insulation will reduce this some what but neither of the three are as efficient as a reflective insulation sys (RI) which when properly installed will radiate only 2 btu/sf/hr to the drywall. This results in interior temps reaching only about 80 degs on a 95 deg day with similar conditions.

Winter is pretty much the reverse except that you have some moderating effects from the suns' energy on the roof deck. There is a winter space radiation factor that can increase the heat loss with the bulk insulation.

So it isn't so much the ambient conditions or the roof itself as choosing the right insulation.
ICFHybridUser is Offline
Veteran Member
Veteran Member
Send Private Message
Posts:3039

--
15 Nov 2011 08:55 PM
It's pretty much air leakage first, followed by windows and doors then foundations or ground loss. Walls and ceilings come lower down on the list.
Lee DodgeUser is Offline
Advanced Member
Advanced Member
Send Private Message
Posts:714

--
15 Nov 2011 09:51 PM
Rather than using some nonsense rule of thumb, why not use RESCheck, available for free, and let it compute the UA factors for the different parts of your house? The UA factor is just U, the thermal conductivity, times A, the area. Heat loss/gain is proportional to the UA factor. You can easily change insulation levels and identify where the gains and losses are. For my well insulated house, the UA proportions for the house are computed by summing all the UA factors to get the total for the house, and then dividing each of the UA factors by the sum to get:

38% losses in windows adjusted for insul. shades
25% losses in walls
16% losses in ceiling
15% losses in crawlspace not including floor
6% losses in doors adjusted for storm doors

To see the details on the house construction, read http://www.residentialenergylaboratory.com/rel_description_am.html

So in contrast to your suggested rule of thumb, the majority of the heat loss/gain is not through the R-60 ceiling for this house design. A shortcoming of this approach is that it does not include solar gains through the windows. For that, you need to use something like RESFEN, also available for free. In my case, the high SHGC windows are actually a net postive for (my desired) heat gain in a heating only climate, not a net loss as shown by above by using RESCheck alone.

In a well insulated house in your area (central Texas), heat gains through the windows might be the largest heat gain, and would need to be dealt with using overhangs, outdoor shading (trees or porches), indoor shading (pull down shades), etc. The easy thing about insulating the ceiling when a vented attic is used, is that you can put down a lot of insulation cheaply. Of course, now days, people probably want conditioned attics, and the high insulation values might be achieved at much greater expense.(?)
Lee Dodge,
<a href="http://www.ResidentialEnergyLaboratory.com">Residential Energy Laboratory,</a>
in a net-zero source energy modified production house
jonrUser is Offline
Senior Member
Senior Member
Send Private Message
Posts:5341

--
15 Nov 2011 10:14 PM
Lee, what percentage of your heat loss is due to air exchange/infiltration?
Lee DodgeUser is Offline
Advanced Member
Advanced Member
Send Private Message
Posts:714

--
15 Nov 2011 11:07 PM
jonr-

The RESCheck calculations that I referred to above do not include air infiltration/ventilation. To answer the question of the original poster (OP), I was showing simple conduction heat losses that did not include window heat gains or air infiltration/ventilation, but do show the relative conduction heat losses in an easily obtainable way.

So, to answer your question requires more effort, but this effort is not required to answer the question of the OP. The total heat losses from RESCheck were computed to be 28.1 MMBtu. I have estimated the heat losses due to air infiltration (0.15 ACH_natural from blower door test, although the rater came up with 0.11 ACH_natural and I got 0.15 ACH_natural) + forced ventilation using 71% efficient HRV of 10.8 MMBtu. This would mean air infiltration/ventilation represents 28% of total heat loss. This heat loss can vary a little with setting on HRV.

Beyond your question, passive solar gain is estimated to be 12.9 MMBtu. Domestic hot water with solar was very roughly estimated at 3.2 MMBtu. Appliance heat losses to house when they could be used for heating were estimated at 6.0 MMBtu. These sum to 23.2 MMBtiu. Actual natural gas usage for first year was 22.5 MMBtu. This agreement tends to contradict the comments by ribsys that energy losses cannot be calculated from R-values.

To meet net-zero goal, the 23.2 MMBtu gas usage was more than offset by excess electrical generation from PV panels.
Lee Dodge,
<a href="http://www.ResidentialEnergyLaboratory.com">Residential Energy Laboratory,</a>
in a net-zero source energy modified production house
Lee DodgeUser is Offline
Advanced Member
Advanced Member
Send Private Message
Posts:714

--
16 Nov 2011 12:13 PM
ICFHybrid and jonr-

Maybe I owe you guys an apology. ICFHybrid, I did not see your response before I sent in my first note. Indeed air infiltration is usually a significant factor in heat loss/gain. Air infiltration and heat gains through windows are more difficult to account for than straight conduction heat losses/gains. As soon as someone says they are more difficult to account for, too many people throw up their arms and say "math, yuck, just give me a rule of thumb so I don't have to do any work. Why don't you smarter guys just figure it out for me while I finish my beer." So my approach was to say, let us look at this in the simplest way possible, and look at the different pieces of the house and see which parts are most significant, at least from a conduction standpoint. And while we are at it, we can also look at the effect of varying insulation levels on the overall picture. REScheck is an easy approach where it does all the math, and allows the relative contributions to be evaluated through the UA factors.

Air infiltration requires a blower-door test, or at least a very educated guess to get started, and then some math (that yucky stuff again) to turn that into heat loss/gain. Actually the math is very simple for those that are interested. The natural air changes per hour (ACH_natural) value is mulltiplied by the house conditioned air volume times the heating degree days times 24 to convert into heating degree hours times the specific heat of air (call it 1.0 kJ/kg-degC) times the air density (call it 1.2 kg/m^3). The blower door test provides ACH at 50 Pa, but the natural ACH can be estimated as 1/20 of that (there are more accurate fudge factors given by Max Sherman, if desired). Then do the same exact thing with cooling degree days substituted for heating degree days to get cooling requirements. I will try to add some example calculations on my web site in the future.

Anyway, I did not mean to minimize the importance of air infiltration, since it is always significant. It is just harder to quantify it, and can be harder to design for it and/or fix it.
Lee Dodge,
<a href="http://www.ResidentialEnergyLaboratory.com">Residential Energy Laboratory,</a>
in a net-zero source energy modified production house
rbisys1User is Offline
Basic Member
Basic Member
Send Private Message
Posts:142

--
16 Nov 2011 12:47 PM
Greetings,

QUOTE>This agreement tends to contradict the comments by ribsys that energy losses cannot be calculated from R-values.

You can use "R" factors if you choose. My contention, based on 30 yrs experience, is that "R" factors are not constant. There are too many outside conditions that affect the reliability. You can't use the manufacturers "R" factors because the tests are not "in situ" tests.

The only reliable test is actual buildings, measuring the ceiling temp to a 75 deg floor, on exterior ambient conditions because that will tell you the "actual" btu gain. It incorporates "all" applicable factors.

In any case you can't show me a "in situ" bulk insulation sys that performs better than a reflective insulation sys., summer or winter. Been there, done that, too many times.

As far as nonsense, being tied into some faulty archaic sys is not my idea of professionalism. You can dazzle your clients with all the so called tech papers you want, but, it still comes down to, what is the ceiling/wall drywall temp which determines the energy gain. What hac sys you use and the amount of operating costs is another factor because of their variables. With RI it requires very little solar gain to provide a 24 hour flywheel.

Air infiltration, windows, etc, affect the overall performance too, but, that was not the question poised.
These factors will not affect the drywall temps.

Also, you have not insulated a house with RI so your comments are not valid as they are based on a lack of direct experience. Almost all the *erector contractors in my area don't even want to talk alternative because they are not intelligent enough to understand the significance of providing an energy efficient home, and more importantly, they don't care. They do not want to take the chance of losing a sale, because of not being the "same". Show me a ranch home with unshaded, dark roof, 95+ deg o/s temps where the indoor temp not exceed 80 degs with power OFF for six days with a bulk insulation ceiling sys.

* The people building homes today are "erector contractors", compared to true building contractors, which is just about an extinct species.
jonrUser is Offline
Senior Member
Senior Member
Send Private Message
Posts:5341

--
16 Nov 2011 01:10 PM
air infiltration/ventilation represents 28% of total heat loss


Thanks. In other words, air infiltration/ventilation heat loss is about tied for #1 with windows. Seems like there should be ways to reduce it. For example, no HRV when no one is home or zoned HRV (no ventilation of unoccupied rooms). Or reduced ventilation by making sure that there isn't anything harmful in the air (selection of building materials or filtration).


Lee DodgeUser is Offline
Advanced Member
Advanced Member
Send Private Message
Posts:714

--
16 Nov 2011 01:22 PM
rbisys1-

I have often seen you state that you have never seen anyone calculate heat losses from R-values and have the calculations match the measured values. I presented results that contradict your statement and do show exactly that -- good agreement between heat losses calculated from R-values and heat losses/gains over a one-year period as determined from natural gas quantities used for heating. I am not alone in showing that science as we know it does work.

I made no statements regarding radiant barriers. I have no particular interest in radiant barriers. I live in a net-zero energy home that does not use radiant barriers. It does use high R-value insulation products.

I do object to people using this forum solely to peddle their wares, with their constant response to any and all questions, "Buy my stuff and it will fix your problem. Ignore those posters that use science that disputes my folklore."
Lee Dodge,
<a href="http://www.ResidentialEnergyLaboratory.com">Residential Energy Laboratory,</a>
in a net-zero source energy modified production house
Lee DodgeUser is Offline
Advanced Member
Advanced Member
Send Private Message
Posts:714

--
16 Nov 2011 02:11 PM
jonr-

You have to be careful in looking at the percentages only, since the overall heat losses are a relatively small number. Total natural gas usage including space heating, hot water (supplementing the solar thermal), and cooking was 22.5 MMBtu for one year. I would be interested in hearing from others about their total heating and cooling energy, including heat supplied from firewood if applicable.

When comparing infiltration heat losses with the losses through the windows, there is a problem. The heat losses through the windows that I showed from REScheck were conduction only, and not including solar gains. When solar gains are included, RESFEN estimates net losses including conduction and infiltration of:
north 3.17 MMBtu (a loss)
east -1.41 MMBtu (a net gain)
south -4.28 MMBtu (a net gain)
west -1.42 MMBtu (a net gain)
so an overall net gain for the windows of 3.94 MMBtu. (This is for Alamosa, CO rather than Salida, CO, but we'll call it representative.) So the infiltration losses are greater than the net losses for the windows, since they become a net gain with passive solar. The 28% of total heat losses for infiltration were for total heat losses not including the offset by solar gains.

I have actually determined two significant air infiltration problems (the same problem twice) that are next on my list to address, and these are a design problem rather than a construction problem. If I can reduce air infiltration, then I will need to increase mechanical ventilation if I want to meet ASHRAE specs, but that uses the HRV to recover heat. On the other hand, I think the ASHRAE specs are based in part of removing moisture from the house, and I just added a humidifier to add moisture to the house (30% or 35% set-point at 67 F). The dew point outside today iis 3 deg F, while the dew point inside is 52 deg F.

So I do not consider that I have a bad air infiltration problem at 2.45 ACH at 50 Pa, but I think that I know how to reduce it. The percentage is significant because the other losses are low.
Lee Dodge,
<a href="http://www.ResidentialEnergyLaboratory.com">Residential Energy Laboratory,</a>
in a net-zero source energy modified production house
jonrUser is Offline
Senior Member
Senior Member
Send Private Message
Posts:5341

--
16 Nov 2011 03:38 PM
I agree, once you start thinking about payback, then you want to use btus, not percentages. Factoring in solar gain, air exchange is clearly #1 for your house. Other than humidity (in some cases - depends on people, plants, etc) I've seen few details regarding why the ASHRAE specs need to be followed exactly. Ie, what would happen to me if I turned off the HRV? If vapors from the carpet are harmful, then I should probably get rid of it completely vs just diluting it some.
Dana1User is Offline
Senior Member
Senior Member
Send Private Message
Posts:6991

--
16 Nov 2011 04:17 PM
"The percentage is significant because the other losses are low."

And that's the reason why the original question...

Would this generally be a true statement;  "The roof/ceiling is the largest source of heat gain/load in a hot climate and the major source of heat loss in a cold climate."

...has no definitive answer.  It all really depends on the specifics of the design & construction, which are literally all over the place in terms of air tightness, roofing color & pitch, actual R/U values relative to code mins, etc. 

In general it's safe to say that in any climate the cost of going high-R on simple roof designs can be low by-design, and that it's more expensive do that with walls or under slabs. The crossover point where the net present value goes neutral for a high R roof is high enough that in a well-considered design the roof is NOT the highest source of heat gain or loss.

That said, most homes aren't designed & built in such a well=considered manner.

In typical existing homes in US cooling dominated climates it's generally true that most of the heat gain is through the roof, compounded by the fact that it's common to put AC ducts in attics above the thermal insulation (nearly doubling the cooling load for many homes.)  In more-humid cooling climates air-infiltration can be more than half the cooling load, primarily as latent load. The infiltration has a real but relatively modest effect on the sensible load, but the latent loads from infiltration can be gia-normous, and often in excess of the sensible heat gain through the roof in a home with the ducts inside of conditioned space.

In typical existing homes in US heating dominated climates conducted heat through the roof is usually lower than the heat losses from air-infiltration (the latter of which is often close to half the heating load.  As a retrofit treatment it's sometimes easy to reduce measured heat loads by 20-30% in some homes simply by air sealing the bigest holes: Band joists & foundation sills, attic floors, plumbing/flue/electrical chases that run from basement to attic, etc.

If all homes in the US were as tight at Lee's 2.5ACH/50 the resulting heating & cooling energy savings would be huge, independent of the rest.  Typical new construction in the US is more than 4-5x that leaky, and in older construction it's often 6-10x that.  Min spec tightness for a Canadian R2000 house is 1.5ACH/50, and has proven to be a relatively cheap & easy thing to implement on new construction.  Retrofit sealing to 3ACH/50 or lower can be very difficult & expensive to do on an existing house, but quite reasonable on a new house that...
 
A: Has a continuous primary air-barrier defined from the slab, up the walls, over the roof/ceilng down to the slab during the design phase...

B: Has a competent air-sealing Czar monitoring and rectifying air sealing during construction, and...

C. Is blower-door tested & remediated at some point during the construction after the shell is up and closed in (not after the finish paint has been applied.)

The IRC 2009 has a max-spec of 7ACH/50 for new construction, but it's not clear if it's been verified or enforced anywhere in the US yet. (Some areas are making noises like they will.) This level of tightness is achievable as a post-testing retrofit with typical frame construction (new or old), but it's cheap and easy to hit without need of post-remediation if built by a trained construction crew & management.  It's all but assured with SIP or ICF construction, barring egregious practices at the roof/ceiling, doors/windows. But 7ACH/50 is still a pretty breezy house considering how easy it is to build tighter than that. (Still it's a start.)  The fact that it's easy to inspect insulation with an eyeball and a ruler, and takes special equipment and training to do a blower door test makes me believe that enforcement or even testing of that part of the code isn't going to be rampant any time soon, even if higher min-R values see some enforcement.

Anybody building to code-min isn't very serious about it- the long term present value of building tighter and higher R than code-min is pretty good if done on a most-cost-effective basis. (Going to R70 on a roof with an all closed-cell foam solution won't look nearly as good in an NPV analysis as going R70 with blown cellulose.  Going <3ACH/50 with acoustic sealants, spot foam, and tape usually fares lot better from an NPV point of view than a 2" cc foam whole house overcoat too.)
rbisys1User is Offline
Basic Member
Basic Member
Send Private Message
Posts:142

--
16 Nov 2011 07:56 PM
Greetings,

QUOTE>I have often seen you state that you have never seen anyone calculate heat losses from R-values and have the calculations match the measured values.

Must have been a rbisys from another dimension.

I wish I could find an old report where the Naty'l Gas Assc, or similar, did a in situ survey of millions of homes, probably 60 yrs + ago, and found that rock wool got about 1/2 the "R" value published by the manufacturers. Just thought I'd throw that in.

QUOTE> I do object to people using this forum solely to peddle their wares, with their constant response to any and all questions, "Buy my stuff and it will fix your problem. Ignore those posters that use science that disputes my folklore."

Don't you just love " freedom of speech"?

Regarding air infiltration, I'm concerned about this zero infiltration philosophy. Zero infiltration leads to indoor air pollution and can cause a build up of chemicals used in insulation and other building and house hold items, especially mold. One of the nice features about RI is that it does NOT contribute to the problem. And yet no one on this forum directs attention to this problem or how to provide adequate air changes. Personally, short of gale winds, I don't get too concerned with some air infiltration as it helps the air change factor. Since the RI is reducing the energy use load by a considerable amount the home owner winds up with a positive net gain on energy savings.

QUOTE> The infiltration has a real but relatively modest effect on the sensible load, but the latent loads from infiltration can be gia-normous, and often in excess of the sensible heat gain through the roof in a home with the ducts inside of conditioned space.

And the condensation factor in fiber type insulations compound this on very hot humid days because the additional insulation moisture contributes to the interior moisture. This high humidity contributes to the "warm feeling" and the home owner turns down the thermostat to compensate. And of course it doesn't help that the energy from the ceiling provides a radiant heat factor which also promps a further thermostat turn down. Since RI does not contribute to the humidity problem the home owner can "raise" the thermostat setting and still be comfortable.
jonrUser is Offline
Senior Member
Senior Member
Send Private Message
Posts:5341

--
17 Nov 2011 08:43 AM
And yet no one on this forum directs attention to this problem or how to provide adequate air changes...I don't get too concerned with some air infiltration as it helps


Really? I recommend that you do more research on HRVs (starting with above posts).
rbisys1User is Offline
Basic Member
Basic Member
Send Private Message
Posts:142

--
17 Nov 2011 10:41 AM
Greetings,

Maybe I missed something, but, I'm talking about air change rates not air infiltration. Some of these houses are so tight the interior air quality is poor and that allows the pollution problems with most insulation to have a greater detrimental effect.

I'm familiar with home air exchange units, I have recommend them on a few occasions.
Lee DodgeUser is Offline
Advanced Member
Advanced Member
Send Private Message
Posts:714

--
17 Nov 2011 11:08 AM
Obviously bpnkrtn is a mystery writer, giving brief clues in various posts that must be assembled to answer his questions. For example, Dana1 has paraphrased bpnkrtn's opening question in this thread as:
"Would this generally be a true statement; 'The roof/ceiling is the largest source of heat gain/load in a hot climate and the major source of heat loss in a cold climate.'"
and Dana1 responded with:
...has no definitive answer. It all really depends on the specifics of the design & construction, which are literally all over the place in terms of air tightness, roofing color & pitch, actual R/U values relative to code mins, etc. "

He goes on to say, "In typical existing homes in US cooling dominated climates it's generally true that most of the heat gain is through the roof..." And Dana1 then discusses existing housing that is too general to come to any conclusion (except that existing homes leak a lot of air, and that is a problem).

Dana1 obviously missed the clue given in an earlier thread when bpnkrtn asked about the importance of having insulation under a slab, which must imply his/her interest is in a new house, since that is not a practical addition to an existing house. (Another mystery is why bpnkrtn would be asking about insulating under a slab when he is within watermelon seed spittin' distance of Aquarena Springs, which everyone knows provides a constant year-round water temperature of 72 F.)

Similarly, jonr is focused on air infiltration, while the title of bpnkrtn's current post is "insulation; roof/ceiling vs. walls," so although we all know air infiltration is critical in both hot and cold climates, we better focus on ROOF versus WALLS.

So following these clues given by bpnkrtn, let's focus on new construction. Let us assume that bpnkrtn will at least build to current code, which is IECC 2009 in the area and requires R-30 ceiling, R-13 walls, and R-13 floors. When I put my generic 1600 sq ft house in Martindale overlooking the Martindale Dam so that I can check on those good-looking Texas State U. coeds sunning by the dam, and run REScheck, I get UA factors (thermal conductivity times area) of 51 for the ceiling, and 95 for the walls. This means the conduction heat losses through the ceiling and the wall are predicted to be in the ratio of 51:95 respectively. Now I don't believe REScheck properly accounts for the fact that attics in that area can get hot as he--, so that fact runs the ceiling heat loss up some. But I should not assume that I can build a energy-efficient house without also doing something about the wall thickness/insulation.

There was a recent discussion on this forum about not putting a power vent fan in an attic to cool the attic as it would suck air out of the conditioned area in the house. I concluded that those folks having that discussion had never ever crawled around in an attic in Texas in the summer. So my thoughts are shading by Pecan trees along the San Marcos River, a cool roof, put A/C equipment in a space other than the attic, power vent in the attic, expand walls to 2"x6" and fill with cellulose, put loose fill in attic up to R-40 or R-50, put remote solar PV out away from the trees, add wide overhangs and worry a lot about window placement and shades, and finally add a porch to check out those coeds and cut down on direct sunlight through the S and/or W windows. That's what I would do. You could also partially bury a house in that area to take advantage of the 72 F groundwater if you are into that sort of thing, but then it would be harder to keep an eye out on the scenery.
Lee Dodge,
<a href="http://www.ResidentialEnergyLaboratory.com">Residential Energy Laboratory,</a>
in a net-zero source energy modified production house
ICFHybridUser is Offline
Veteran Member
Veteran Member
Send Private Message
Posts:3039

--
17 Nov 2011 11:31 AM
Maybe I missed something, but, I'm talking about air change rates not air infiltration
What's the difference, really? The whole point is to control the air infiltration through tightness. Once it is controlled, you can choose how much exchange you want, not to mention being able to recover the heat from it.
Dana1User is Offline
Senior Member
Senior Member
Send Private Message
Posts:6991

--
17 Nov 2011 12:05 PM
"Dana1 obviously missed the clue given in an earlier thread when bpnkrtn asked about the importance of having insulation under a slab, which must imply his/her interest is in a new house, since that is not a practical addition to an existing house."
True dat!

On new construction on a custom design, MODEL the sucker (DOE2 or similar and/or optimize it cost-wise (BEopt)! Code mins are useless guidelines.
Dana1User is Offline
Senior Member
Senior Member
Send Private Message
Posts:6991

--
17 Nov 2011 12:21 PM
"There was a recent discussion on this forum about not putting a power vent fan in an attic to cool the attic as it would suck air out of the conditioned area in the house. I concluded that those folks having that discussion had never ever crawled around in an attic in Texas in the summer. "

All those grad students at Texas A & M back in the 1980s did the crawling around for me, and took actual power use measurements that resulted in the conclusions. Similar work has been done in Florida, more recently confirming the general conclusion: Power venting attics in the gulf states is more likely to increase total power use, and increases the average summertime moisture levels in the attic.
You are not authorized to post a reply.
Page 1 of 212 > >>


Active Forums 4.1
Membership Membership: Latest New User Latest: formsup New Today New Today: 0 New Yesterday New Yesterday: 1 User Count Overall: 34698
People Online People Online: Visitors Visitors: 76 Members Members: 0 Total Total: 76
Copyright 2011 by BuildCentral, Inc.   Terms Of Use  Privacy Statement