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-   -   Attic heat project designed from the bottom up (https://ecorenovator.org/forum/showthread.php?t=3141)

Daox 11-18-13 10:24 AM

Very nice work. Its really cool to see you work on this and continue to improve the design!

Exeric 11-20-13 06:41 PM

Thanks Daox. It rained all yesterday and the temp rise got to an amazing 4 degrees in the attic! Nothing like rain to keep down the roof temps. Today it was totally overcast and threatening until late afternoon. Yet the attic temp still got to 24 degrees over ambient. No pictures.

I finally closed the motorized roof damper. I bought an optional switch that mounts on the damper so I could monitor the damper position without physically inspecting it. My ohmmeter on the switch contact wiring told me it had closed but since it was the first time operating it I didn't quite trust my reading. So I went up, removed the center insulation and peeked through the backdraft damper. It did indeed close.

So we'll see just how much difference a closed roof vent makes on my system. Here's hoping it's significant.

Daox 11-21-13 07:38 AM

I would imagine that that will make a pretty big difference. I look forward to hearing the results.

Exeric 11-23-13 07:00 PM

3 Attachment(s)
It seems the world is never quite as simple as one anticipates. It's almost like the more I assume something without actually having prior experience the more likely I am to be wrong. You might be guessing about now that closing the roof damper didn't have the dramatically positive results I anticipated. You'd be right.

Here's some results:
11/20/2013 - high roof temp 96 degrees F, temp rise 33 F, very windy day
11/21/2013 - high roof temp 104F, temp rise 32F, windy day
11/22/2013 - high roof temp 101F, temp rise 28F, calm day

I'm slightly embarrassed to admit I was hoping the temp diffs would get into the high 30s with the roof vent closed. Not even close. The biggest positive difference is that when it is very windy the roof temperatures no longer plummets, like they used to. On Sept 20 a 96 degrees F high was a good result because it was so windy that a big rig truck actually blew over on a local highway. Also a 33F temp rise was very good under those circumstances. I'm sure with the vent open I previously would have been lucky for that temp rise to get to 25F from past experience of windy conditions.

What I did not take into consideration is that a closed roof vent works not only on the front end but also the back end. In summer one is into the air conditioning season. The sun is still up greater than 12 hours. So a closed vent in that season will mostly restrict the exit of hot air percolating under the roof.

In winter it can be just the opposite. I've been monitoring the times at which the roof temperatures cools back down to ambient outside air temps. Usually its between 7 and 8 PM. It doesn't require a open vent for this to happen. The roof just radiates heat it has accumulated to the cold night air, regardless. If I had a monitoring system like Xringer does I could show you. Gotta get one one of these days.

For instance the roof got cold soaked on 11/22/2013. At 7 A.M the internal roof temperature was 10 degrees colder than the outside air temperature. Because the roof vent was closed the warmer ambient air temperature could not warm up the roof and evacuate the cold air that accumulated inside during the night. At 9 A.M. even as the roof was warming the air was warming just as fast. So the roof was still 10 degrees colder than outside air temp. Because the days are so short this time of year there really isn't time for that beginning temperature deficit to be overcome as the day heats up. Soooo, the temperature rise in winter with the vent closed at night and early morning will be lower than it otherwise would be, except on a windy day.

Anyway, that's my excuse for now, and I'm sticking to it.:p

Exeric 11-26-13 07:53 PM

Here's the plan
 
1 Attachment(s)
I've been thinking about how this is all going to work without heat storage. Despite our reputation here it gets cold at night and the unconditioned uninsulated part of the house has already gotten down to 40F degrees in the middle of the night, while the outside temperature is 30F. And that part of the house rarely gets to 60F at any time. So what seems to be happening is that there is about a 20 degree variation in the uninsulated part of the house. On warmer days it goes from about 45F to a high of 65F.

I'm going to be insulating it with dense packed cellulose due to SF and Daox's good advice. It will be equivalent to about 2x6 framing, but without most of the thermal bridging that goes on there. I put 2x2 staggered against the existing 2x4s. If I can get that temperature variation down to 6 degrees F from warm to cold without any heating input during an 18 hour period then this will work. I really do not have the experience to know if the level of insulation I'm putting in will accomplish that. In this climate my house will be hugely better insulated than my neighbors, but nowhere near to Passivhaus standards. If anyone has direct experience with temperature variations in an unconditioned house with my level of insulation (R60 in ceiling, R30 in floor, and spray foam to R10 on the rim joists) I'd be thankful.

In the meantime I'm planning on a 6 degree variation. Here's a picture of the hopeful, but not neccessarily accurate predicted outcome.
http://ecorenovator.org/forum/attach...1&d=1385516661

Right now on average to good solar days the roof temperature reaches 75F at noon and does not dip below it until 6PM. On those days it is over 90F in the attic for 2 or 3 hours. On cold days it doesn't do as well. For instance today it only reached 84F and the outside high temp was 57F. The coldest day so far. (I know, its not cold) So that's what I'm basing it on. Obviously this is a crude estimate but for now I think it's in the ballpark. I feel very fortunate that I have the attic temperature that I have to work with.

Exeric 11-26-13 11:02 PM

2 Attachment(s)
I don't know what I was thinking when I said a few comments back that the attic and outside temps converged between 7 and 8 PM. They don't. The attic and "unconditioned space temps" converge at that time. The attic temperatures and outside temps usually never converge and the outside temps remain colder than the attic. I was so flummoxed by the lack of a big change with the attic vent being closed I think I was reaching for an explanation. That one day when the temp rise was only 27F seems to have been an aberation.

The day before yesterday I got an attic high of 97 and a outdoor high of 61. A new temp difference record. Here's a pic:
http://ecorenovator.org/forum/attach...1&d=1385527823

The vent being closed seems to be working on average. You just can't pinpoint when its contributing and when it's not. The long and short of it is that I was getting lost in the weeds while the average temps have improved. Overall the temps and temp rises for the last week have been quite consistantly over 90F and 30F respectively. And even when they fell short on the temp rise a 100F attic high temp is great! What was I worried about?

Today wasn't so good but then we're only 25 days or so from the winter solstice. Here's a pic:
http://ecorenovator.org/forum/attach...1&d=1385527823

The attic temps are still useable even on a bad day like this. (Rainy weather, forget it). So far things are on track for this plan to work, with or without mild disappointments. Even today it was over 75 degrees F in the attic from 1:00 PM to 5:00 PM. Again, what was I worried about?

jeff5may 11-27-13 06:59 AM

If you insulate the unconditioned room to the levels you posted, the room will hold heat very well. As long as it is not open to the outside, it will in fact float to an average charted temperature like your graph. How much variance from average will depend more on the thermal mass in the room and how heat is supplied to the space. Unless there is lots of solar gain from skylights or windows, outdoor temperature will play a much smaller role in changing the temps from your average.

Exeric 11-27-13 08:21 AM

That's really good to hear. There's not going to be windows or thermal mass used to acquire heat. I'm just using the roof to supply heat. It's beginning to feel like I can quit obsessing about the plan itself and just concentrate now on getting the rest of the house in shape. It could take awhile.:rolleyes:

Exeric 12-06-13 05:37 PM

In the interest of full disclosure
 
3 Attachment(s)
I probably ought to disclose that I have a significant area of roof for roof heating that is not used as habitable space. This is always an advantage for collecting the rays. I built a large 'L' shaped porch addition to my house in 2011. Here's some pics:

http://ecorenovator.org/forum/attach...1&d=1386372859

http://ecorenovator.org/forum/attach...1&d=1386372032

The first is the East side and the second is the South side. I'm thinking of adding some solar PV on the South side at a later date. That will cut into the heating effect somewhat. I have a plan though. I have an attached garage that is disconnected from the rest of the house from the ceiling to the roof. If I go ahead with the solar PV I will build a thermal channel from the garage roof to the house roof above the ceilings. Here's the plan. The top is North. Everything within the dotted line is habitable space and will remain so.

http://ecorenovator.org/forum/attach...1&d=1386372859

It would give me a little over 3000 watts, which would I think would be plenty since I don't need it for heating. No car charging though.

Exeric 12-06-13 08:54 PM

It's not really authorized by code to have combined attic spaces of homes with attached garages because of the danger of automobile fumes reaching the living space. If I decide I need the combined space with the garage it means any carbon monoxide from a running engine would automatically be pumped into the living space. Soooo, I would only do it because the garage is used for storage only, as most one car garages are around here. Once I convert that attic space then no running internal combustion engines would ever be allowed in there. As is often said, kids, don't do this at home.:eek:

PS. I would only do it if it was found that the removal of roof space for solar electric required it. It might not be needed.


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