An Invisible Solar Attic Space Heating Collector

Randy describes his prototype of a system that makes use of attic heat for space heating The system is simple and  effective, and provides free heat from the attic on sunny days.

Invisible Solar Attic Collector
No visible signs of the collector from the outside.

Randy also goes into some innovative future plans for the system.

Thanks very much to Randy for sending this in!!  

More projects that use attic heat...


I have been up in my attic and noticed the heat produced. After putting my hand on the underside of the roof I found the heat was coming from the sunny side of the roof. This heated my attic warmer than my house inside even on cool days. I am located in Grants Pass, Oregon.

For my system, I wanted to circulate air from the living space, through the collector, and back to the living space.  I did not want to pull the attic air into the living space because of the potential problems from the fiberglass insulation particles.  

I painted the collector area and I decided to bridge the gap between the 2x4 truss bays. The trusses make a great base for this system. Their design blends well with my plans.

As a test, I used furniture cardboard to enclose the truss bays and also to make the supply and return plenums.  The supply and return plenums are at the bottom and top of the bays, and are supported by the trusses. I used overlapping seams, sealed with caulking, taped with masking tape and then stapled to keep it tight.

So, the air from the living space is ducted to the inlet plenum.  The air then flows up the now enclosed truss bay spaces and is heated by the underside of the roof surface.  The now heated air then flows into the return plenum where a duct and fan take the now heated air back to the living space.

Attic solar collector
The lower plenum with intake duct.
attic solar collector
The exhaust plenum. 
The 2 by 4's between the trusses were
added and foamed.
attic solar collector
The whole collector is supported
by the roof trusses.

The total length of the collector is 18ft., Height is 12 ft (216 sqft). 

Twelve inch insulated ducts are used to convey the air between the collector and my living room. The intake duct run of 10 ft. long, is to a 16inch. sq. filter grill.  The exhaust duct is 20ft. long and is powered by an ridge vent attic fan with the thermostat set to come on at 93deg. I am sure that some heat is lost through the cardboard, however, it was free, abundant, and a good way to build the prototype.

The ridge vent fan with thermostat was purchased at Grovers Electrical and Plumbing.  It was opened no box and discounted to $20. It has a 12 inch. housing, so the 12inch ducting slides right over it.

Solar Colector Inlet Vent
The inlet vent for the attic collector.
Solar Collector Outlet Vent
The outlet vent for the attic collector.


The system really works. I have benefited from it for 6 months. The back of my home faces south-south west. This causes my roof to miss morning sun/heat. It comes on around 12 noon, runs till 6pm. because of stored up heat. My daily increase in temperature was around 5 deg. Enough to keep the house heater off many but not every day.

Outside temperature
Outside temperature
collector outlet temperature
Collector out temperature.

Note that these temperatures were achieved in spite of their being some clouds.

We get a lot of clouds here, when there is no sun the system produces little or no heat.

What I have learned and Improvements

Some things learned from the prototype and some future improvement ideas:

- The cardboard can be improved on. Maybe 4x8 polystyrene board, 1 inch thick.

- Cold air comes in at night. A one way valve could fix this. I have not figured out how to do this well yet. For now the vents must be closed at night.  I did try one of the light poly film dampers, but found that it fluttered and leaked some air.  Anyone have any good ideas for a damper?

- The cardboard smells a little at first.

- Installing a larger fan does not mean more heat, just more air.

- The intake and exhaust ducts should be installed at opposite ends of the plenums to balance the air flow in the bays.

- Moisture needs to be vented after rain to keep it out of the home.   Moisture produced after system not used while raining. Will clear up in about 5 minutes. There are no droplets of condensed water under my roof.

- The system doesn't add outside air if sealed properly. When it leaks air you can smell the attic air.

- The unused hot air in summer will soon be used to run through my dryer. Unused, the attic heater heats up to 150 plus deg. This should dry clothes quickly. To test this a 110 volt gas dryer is being converted be me now for this. It will be installed in the garage below and ducted to the heater. Depending how this works I might convert my other dryer in the house to run on the attic heater permanently, with 220 back-up. The air setting could be used.

- The attic heater was installed above the garage. This was a convenient location since there was a floor above the ceiling already to be used for storage.

- On days when only a few degrees of heat is needed a switch or thermostat should be installed to limit overheating.

- My roof is black. At first I wished it was lighter in color. I had to double the vents because of the excess heat. Now it's my friend.

- If I had the money and expertise I would add solar hot water as one unit with my attic heater. Think of this... An attic heater that heats water, the home, supplies all the dryer heat and cools the home at night, all through the central forced air heater.

- I am thinking of reducing the air bays between the 2x4 trusses to a 1-2 inch. gap to concentrate the heat. The gap is now 3 1/2 inch.

- Tape does not hold up well to high heat and time.  After trying many different tapes. I found that the cheap- lower cost, thinner masking tape from Wal-Mart superior here.

- This solar design can not be seen from outside the home. No one will know it is there. HOA rules may not apply.

Randy   July 2, 2011

Some thoughts on Randy's system

Just a few thoughts on Randy's simple and effective system:

- As Randy points out, the cardboard is a good way to get a prototype built, but is probably not a good long term choice.  I think that the high temperature polyisocyanurate insulation board that many lumber yards sell would be a good choice.  The "Great Stuff" polyurethane foam in a can is a good way to bond the insulation board parts together and seal them.  I would not use polystyrene insulation board as it does not have a high enough temperature capability for this kind of application.

- Bear in mind that the performance of unglazed collectors like this one will vary a lot with climate.  In some climates, this type of system may not be effective.  More on making use of attic heat here...

- Based on Randy's experience with a little moisture after rains, it would be good to monitor the humidity levels in the collector area for a while just to make sure there is not a problem.

- It will be great to see how the use of the attic heat for the dryer comes out.  While this has been discussed, to my knowledge, Randy will be first to actually give it a try.  Four KWH per dryer load is a very worthwhile saving to go after.


Gary July 2, 2011

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