Attic heat project designed from the bottom up

[Exeric]

I have been working on the attic air heating idea and have done most, but not all, of the construction on it. This is an offshoot from the Using Attic Heat for the House thread:
http://ecorenovator.org/forum/solar-...-house-17.html

It uses an open system after all, but where the air enters the vented roof through baffles at the eaves a radiant foil barrier comes down to meet it. This way a thin layer of pretty hot air moves up between the rafters without danger of insulation contamination. Also there is a readily accessible place to change the filter down in the occupied living area. I will be posting pictures soon.

This is a designed from the bottom up system so if if doesn't end up making financial sense here in a moderate climate - California - then it won't make sense anywhere. Having said that, the project is not yet completed, nor have I yet experienced a winter with the system complete, so only time will tell how effective it is. I promise to show you any problems with the system over time and won't keep you hanging if it turns out not to have have been a good idea.

[Daox]

Woohoo, can't wait to see this.

[Exeric]

Here are some pictures, hopefully in the sequence of construction, so you can get an idea of what's going on. This first picture is of the interior roof that I painted flat black. It shows the 8" roof vents that haven't yet been disabled.


The is a picture of the flashing and turbine used to house the new motorized damper. The lower part of the flashing is 16" wide to accomodate the motor that is on the 12" damper.


This is a picture of the roof damper itself before installation. It is used to close off all attic air from escaping in winter.


Here's the picture of the roof prepared for installing the new damper. You can see the key hole notched out for the motor assembly on the new damper.


Here's the roof damper installed. The remaining 8" vent is blocked off and the beginnings of the white painted boards that the radiant barrier will attach to is installed. This is a hip roof house to it was easy to decide to get all the hot air to come together at the top-middle.


We're finally getting somewhere in this picture. All the white painted boards are attached and you can get a better feeling for what is going on. The round duct in the center board is a backdraft damper. Gravity normally holds it shut. But I plan on installing a whole house attic fan to blow out any hot air that accumulated during summer evenings. So that damper should open when those fans are turned on.


This is the duct that pulls hot air down to the living space. It attaches to the remaining opening in the third white board. It's a 12" duct.


This is where that duct enters the living space. That orange thingy is the motor in a second 12" damper that controls/allows hot air into the living space. It runs right next to my propane fireplace and will be hidden behind a closet door, but still accessible.


This is the filter housing just below that damper, both in that closet area.


All ducts are wrapped in insulation and the reflective foil is installed from inlet soffit right upto and including the white boards at the top.


More later...

[Exeric]

A bit about the building philosophy behind this. From everything I've been reading most people install roof heating systems using the air to either heat some kind of reservoir, usually a crawlspace, or as an one input to the fresh air inlet on a forced air system. The reason is that this kind of attic heating doesn't usually supply a high enough temperature to reliably heat the house directly. Xringer, I think your idea of using attic heat to warm the basement fits into the first category.

I'm taking a bit of an educated gamble here. I have several things going in my favor. One of them is that it is extremely rare for the high temp of the day in winter where I live to be below about 45 degrees. It can get down to 20 degrees at night though. Here are the temperature readings for the house with the roof painted black but no radiant insulation barrier in place.

On the hottest day of the year so far you can see that it was 108F outside temperature in the shade. There was a 26 degree differential temp between that and the 134F temp (lower right in the picture) taken at the interior roof peak. A 26 degree boost ain't much to work with.

Here's a picture of the temps after installing the radiant barrier and insulating the attic ducts.

Here, on another hot day, it is 106F outside in the shade and it is up to 149.9F at the interior roof peak! That's a 43F temperature boost for the attic heat over ambient outside temps. I should add that all these temps were taken with the roof damper open and the living space damper closed. I'm betting that in winter conditions when the positions of those dampers are reversed there won't be much change in the differential temperature.

My reasoning is that the differentiall temps on a sunny day tracks consistently so far at 40 to 45 F, regardless of overall high temps. I been looking at the temps everyday for a month and a half. I also think it might be a reasonable guess that when the roof damper is closed and air is actually being taken into the living area that it won't cool the roof more than having an open roof vent would. Again, more of a hunch. We'll see. I've got tons more work to do on the house, most unrelated to this project.

At any rate, a 40 degree temperature rise on a sunny 40F day still gives me 80 degree air into the house for 2 or three hours a day. If its 50 degrees, overcast and I get only a 20 degree differential temperature then I'm still getting 70F air coming in. So overall I think I may just wiggle through with out having the complexity of having a reservoir system to take the relatively cool air from the attic.

[Daox]

Very cool, a 40F differential is pretty good! I like how you were able to boost it with the radiant barrier, very clever. Can't wait to see more.

[Exeric]

Daox, thanks for the kind words. I've been wondering how your remote located solar panel project is going. Any updates? I've been busy so I may not have kept up.

[Daox]

Sadly no, I've had no opportunity / time to work on it.

[MN Renovator]

Attic venting is to remove moisture from the attic and also keeps the attic cool to prevent ice dams once melted snow reaches the eaves. If you aren't in an area that has much snow, which you probably aren't since you said 20 degrees as an overnight low, that might not be a problem but the moisture issue likely is still an issue and could cause serious mold issues in the attic. How are you planning to handle that?

[Exeric]

I really don't think moisture is going to be a problem. For one thing, the radiant barriers come in two forms, perforated and unperforated. I'm using the perforated kind and that's what you should use if you want to let the moisture out.

Also, at night the roof will get just as cool as the outside weather because the sun no longer beats down. There's just a lag time before that happens. So there's no reason to keep the attic damper closed during those hours. That should clear out any moisture.

[Daox]

I think I get what is going on here, but where are your fan(s) going?