Conduction, convection and radiation

tasdbois · 12-18-09, 05:05 PM

Convection is usually not much of an issue in house insulation when everything is sealed but under rare circumstances. The thicker the better as far as conduction is concerned. But what about radiation?

I am wondering what kind of heat loss from the house in cold weather is due to radiation. I'm getting a house built, so I'll make sure it's well sealed and the r value is decent, but I'm wondering whether or not a radiant barrier would be worthwhile. BTW I live in a very cold climate.

Daox · 12-18-09, 05:14 PM

Convection can happen through/inside fiberglass as the temperature differential increases. I'd recommend staying away from it.

http://ecorenovator.org/forum/conser...uantified.html

I haven't seen amazing proof that radiant barrier is really all that great. According to the studies I've seen, its more effective as you have less and less conventional insulation. This makes sense since the savings from the radiant energy would be a larger and larger percentage of the energy saved. With high a high r-value enclosure, the benefits of radiant barrier appear to be quite small. Unfortunately, I don't have time to dig up the article I'm referring to, but I'll get it when I can.

tasdbois · 12-18-09, 07:24 PM

Thanks for the comment. I've read through a lot of stuff linked over at builditsolar.com and the only convection issue I could find was with loose fill fiberglass. I'm supposed to get batt insulation in the walls and cellulose in the attic, but I'll inquire about using cellulose in the walls too.

With 2 by 6 outer walls construction and batts insulation I get r21 walls with not much space to add more stuff to increase the r value in a cost effective way. I was just wondering if adding thin radiant barriers could help in a significant way. I've read that it should help, but just thought i'd ask here for your opinion.

GaryGary · 12-18-09, 08:38 PM

Hi,

The R21 or R19 quoted for FG insulated 2X6 walls is figured as though the wall had no studs or other thermal bridges. In reality what you get on a real wall is more like R14. One way to get around this is to use rigid foam board on the outside under the siding -- the foam board has no thermal bridges.

Adding 1 inch rigid foam board takes the R14 up to R18.4.

See the "Whole Wall Heat Loss Calculator" on this page:
Heat loss and energy simulation software

Another good wall is the Mooney wall:
Mooney Wall -- A low cost, high R value wall
The pictures show the Mooney straping applied to a 2X4 stud wall, but you could just as easily use it on a 2X6 stud wall.

I would seriously consider dense packed cellulose in the walls rather than FG. To me, there is no reason to believe that the same kind of convection losses you get in loose fill FG would not occur in bat FG. Its alos takes a whole lot of care to fit FG bats well enough to avoid gaps etc., and most insulation guys are not going to take the time to do it.

Gary

Xringer · 12-19-09, 07:53 AM

If I was "getting a house built" in a cold climate, I would really worry about windows.
Make sure they are the best you can get and limit the amount on the north side,
or those that can't see much sun during the day.

The north side of my home has an add-on Den. The sliding Anderson windows keep that room extra cold all winter!
I am now considering adding another layer of glass over them.
Storm Windows ? Rolling Storm Window by Harvey

The sliding Anderson windows come packed with buyers remorse.

They leak and double pane just isn't enough for shaded areas when it 10 deg outside..

12-18-09, 05:05 PM	#1
tasdbois Lurking Renovator Join Date: Oct 2008 Location: Mirabel, QC Posts: 28 Thanks: 0 Thanked 0 Times in 0 Posts	Conduction, convection and radiation Convection is usually not much of an issue in house insulation when everything is sealed but under rare circumstances. The thicker the better as far as conduction is concerned. But what about radiation? I am wondering what kind of heat loss from the house in cold weather is due to radiation. I'm getting a house built, so I'll make sure it's well sealed and the r value is decent, but I'm wondering whether or not a radiant barrier would be worthwhile. BTW I live in a very cold climate.

12-18-09, 05:14 PM	#2
Daox Administrator Join Date: Aug 2008 Location: Germantown, WI Posts: 5,525 Thanks: 1,162 Thanked 374 Times in 305 Posts	Convection can happen through/inside fiberglass as the temperature differential increases. I'd recommend staying away from it. http://ecorenovator.org/forum/conser...uantified.html I haven't seen amazing proof that radiant barrier is really all that great. According to the studies I've seen, its more effective as you have less and less conventional insulation. This makes sense since the savings from the radiant energy would be a larger and larger percentage of the energy saved. With high a high r-value enclosure, the benefits of radiant barrier appear to be quite small. Unfortunately, I don't have time to dig up the article I'm referring to, but I'll get it when I can. __________________ Current project - To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts. To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts. & To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.

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12-18-09, 07:24 PM	#3
tasdbois Lurking Renovator Join Date: Oct 2008 Location: Mirabel, QC Posts: 28 Thanks: 0 Thanked 0 Times in 0 Posts	Thanks for the comment. I've read through a lot of stuff linked over at builditsolar.com and the only convection issue I could find was with loose fill fiberglass. I'm supposed to get batt insulation in the walls and cellulose in the attic, but I'll inquire about using cellulose in the walls too. With 2 by 6 outer walls construction and batts insulation I get r21 walls with not much space to add more stuff to increase the r value in a cost effective way. I was just wondering if adding thin radiant barriers could help in a significant way. I've read that it should help, but just thought i'd ask here for your opinion.

12-18-09, 08:38 PM	#4
GaryGary Apprentice EcoRenovator Join Date: Nov 2008 Location: SW Montana Posts: 139 Thanks: 1 Thanked 21 Times in 15 Posts	Hi, The R21 or R19 quoted for FG insulated 2X6 walls is figured as though the wall had no studs or other thermal bridges. In reality what you get on a real wall is more like R14. One way to get around this is to use rigid foam board on the outside under the siding -- the foam board has no thermal bridges. Adding 1 inch rigid foam board takes the R14 up to R18.4. See the "Whole Wall Heat Loss Calculator" on this page: Heat loss and energy simulation software Another good wall is the Mooney wall: Mooney Wall -- A low cost, high R value wall The pictures show the Mooney straping applied to a 2X4 stud wall, but you could just as easily use it on a 2X6 stud wall. I would seriously consider dense packed cellulose in the walls rather than FG. To me, there is no reason to believe that the same kind of convection losses you get in loose fill FG would not occur in bat FG. Its alos takes a whole lot of care to fit FG bats well enough to avoid gaps etc., and most insulation guys are not going to take the time to do it. Gary

12-19-09, 07:53 AM	#5
Xringer Lex Parsimoniae Join Date: Feb 2009 Location: Woburn, MA Posts: 4,918 Thanks: 114 Thanked 250 Times in 230 Posts	If I was "getting a house built" in a cold climate, I would really worry about windows. Make sure they are the best you can get and limit the amount on the north side, or those that can't see much sun during the day. The north side of my home has an add-on Den. The sliding Anderson windows keep that room extra cold all winter! I am now considering adding another layer of glass over them. Storm Windows ? Rolling Storm Window by Harvey The sliding Anderson windows come packed with buyers remorse. They leak and double pane just isn't enough for shaded areas when it 10 deg outside..