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Can I heat-harvest my cathedral ceiling internally?

I have an unventilated cathedral ceiling, insulated with 7 cms of rigid foam. I have tested it and found a 1" airspace between the top of the foam and the backs of the slates. Would it be OK to blow air in at the ridge and force it out at the (internal) eaves, to circulate any entrapped heat ?

I have a good mechanical ventilation system, but I still don't want to risk introducing humidity into the ceiling !

I would then install a siphon near the ridge, and pump the hot air down to the basement.

Grateful for any professional wiews on this !

Asked by green villager
Posted Feb 9, 2013 2:48 PM ET


4 Answers

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Green Villager,
I'm going to guess (because of your references to centimeters and roof slates) that you live in Britain.

You have raised many questions. I'll take them one at a time.

You are implying that your house suffers from temperature stratification -- that heat is collecting near your ceiling, and that your floors are cold. These symptoms mean that your house has too much air leakage. If you can plug the air leaks, you will greatly reduce the stratification problem. The best way to find and address air leaks is by following a method called blower-door-directed air sealing. For more information on blower-door-directed air sealing, see Blower Door Basics.

The second issue raised in your question concerns the insulation level in your roof. You now have 7 cm. (2.8 inches) of rigid foam -- in other words, between R-11 and R-14. Here in the U.S., this very low level of insulation would be illegal, everywhere from Florida to Minnesota. It's hard to imagine a climate, even in Britain, where such a low level of insulation would be appropriate.

You need to install more insulation in your roof assembly, if at all possible. For more information on this topic, see How to Build an Insulated Cathedral Ceiling.

Finally, concerning your "heat harvesting" idea: I can't tell whether you propose to harvest the heat from between your insulation and your slates, or from the area on the interior side of your roof insulation. But in either case, the plan is misguided. What you need to do is address the deficiencies of your house -- not move air from one area to another.

Answered by Martin Holladay
Posted Feb 10, 2013 6:35 AM ET


Green Advisor
Hello, Martin,

I am very grateful for such a prompt and comprehensive answer !
You are almost right - I live in Northern France !
You are also almost right in saying "I have stratification because my floors are cold"

I apologise first off, for not giving more details, however I guess the upside is, you can & should be proud of giving such a precise analysis on such scanty information !

In fact the house (built 1985) is fitted with in-floor electric heating. Since we installed a double-glazed portico, 3 yrs ago, we have actually *not used* the in-slab heating, as the house stays warm ! In winter (October to May is the "normal heating season" here... we use electric oil-filled radiators as occasional heating. Otherwise it's our large south-facing windows that keeps the house warm.(160 sq.ft) Our utility bills have actually maintained a continuous descending trend, since the portico went in.

So where is my problem, you might say...

Well, I want to reduce electric usage even more by removing need for occasional radiator use. Currently, heating is 60% of the bill, and DHW is 6%. There’s four of us live her, and our electric consumption was 7700 kWh for six months July to December. 5% is for lighting and 10% is administrative standing charge. 12% is other ‘other’ such as TV, video, PCs, power tools etc…). Then washing drying cooking etc. around 10%. Lighting is BIG, because… we … have a cathedral ceiling (not a joke)!

Therefore, as I ought to have mentioned, I am looking at solar additions.

I am not interested in PV, and I wanted to avoid a roof-mounted air collector on my slope (which would require planning permission that I would probably not get…). I have a 12 x 8-meter south facing roof slope, that is 1000 sq. foot of slate. I also have a vertical slate façade of 10 meters, that I have tested, the temps are very interesting !
Rather than remove these slates, to install a solar water radiator then replace them, I came up with my internal ceiling harvest approach. However, I shall install Solar Water behind that 100ft slate façade, one day soon. I am already working on a prototype demonstration rig to check out the theory.

Therefore, in my plea for help, I was talking about air & space heating, and evacuating heat from the one-inch gap between the slates and the top of the insulation. I guess this gap is not entirely ‘sealed’ as the slates are nailed to lathes, however in my opinion, the roof is sealed because of the foam (therefore the ceiling does not leak, due to continuous chipboard shathing).

The problem is, as implied, technical: I am not at all sure about the wisdom of introducing air into a sealed cathedral ceiling (please forgive me, I am an absolute layman). Also, I would be incapable of doing the required heat calculations, and airflow calculations (what size of pump, load loss across the roof, possibility of blowing off a slate etc.).

Finding any information in this respect is pretty hard – nobody seems interested in the possibility of heat harvesting under a slate roof, yet I cannot have invented the concept ! There are commercial products for new build, involving plug-toghether slate cassettes comprising copper pipe etc, but they cost a fortuna nd I am not counting on reding my roof. AS it stands, the power in slate must be amazing – a thousand square foot of 3 mill slate, wow ! On a 15°C day here, last week (midday temp), my vertical slate surface was at 25°C and at 30°C behind the slates! Also that core temperature decays slowly at the end of the day.

I found just one reference on the web, to a house renovation in England, where they used a water-collection system under slate, and it worked a dream, they are dumping heat into concrete in the basement. The guy said that as a rule of thumb, one-third of the total south-side of a slate roof is enough energy to power the house (heating and DHW) year-round… That is what has been tempting (= INSPIRING) me !

So again, apologies for being so brief in describing my problem. I did not want to go on and on (but now, I just HAD to anyhow !).
Well done again for a very perspicacious analysis of my problem !

Also, thanks for pointing me at a blower-door test – I just read it all and found it most interesting ! Guess I would previously have classed myself as one of these unguided individuals who says, ‘my house does not leak !’. Well, of course, it MUST leak ! So I just learned something ! I’d love to do a blower door test now !

The thing is, my house can ALMOST get by with NO HEATING ! despite only 7 inches of rigid foam. I feel bad, because I did not give you all the info – I know my house operates as a system and that over here, a cathedral ceiling goes with a reversible ceiling fan, no roof-lights, plus low-temp in-floor heating, and large south-facing double-glazed windows. I was just trying to think out of the box !

Best wishes from FRANCE
Green Villager

Answered by green villager
Posted Feb 10, 2013 7:42 AM ET


Villageois Vert,
Salut et bienvenue.

Many people have experimented with collecting warm air from behind roofing. The bottom line is simple:
- the air temperatures of the air gathered by such systems are relatively low;
- once you begin pulling warm air from this location, the temperature of the air quickly drops to the outdoor temperature; and
- the cost of the equipment (fans, ductwork, and thermal storage equipment) required for this scheme cost far more than the value of the harvested heat.

In the 1970s and 1980s, many solar warm-air systems were installed, using warm-air collectors (glazed, insulated boxes painted black), ductwork, blowers, and bins of hot rocks for storage. The cost of the equipment and electricity to run the blowers was far beyond any energy savings.

These days, even the best methods for collecting solar thermal energy (solar thermal collectors that circulate fluid) aren't very cost-effective. (See my article on the topic, Solar Thermal is Dead.)

Your suggestion that you might pull air from behind south-facing slates would cost more money to execute, and yield far fewer BTUs, than a solar hot water system. Abandon the idea unless you are an inveterate tinkerer (bricoleur) who doesn't care about payback. (As soon as you try to depressurize the space behind the slates, outdoor air will rush into the cracks between the slates, and the air temperature will drop to the outdoor temperature.)

If you want to gather energy from sunlight, buy some photovoltaic equipment. PV systems have a fairly quick payback. In Europe, the cost of PV systems has dropped below the cost in the U.S.

Bonne chance!

Answered by Martin Holladay
Posted Feb 11, 2013 9:54 AM ET
Edited Feb 11, 2013 9:55 AM ET.


Excellent response again, thanks, Martin !

A most comprehensive reply ; I reckon this puts paid to my idea !
I must admit, I did not relish the thought of tinkering with my beautiful ceiling !
(neither did my wife !)
I am now going off to read your Article "Solar Thermal is Dead !"

Merci encore pour vos précieux conseils !
Would-Be Green Villager (-:

Answered by green villager
Posted Feb 11, 2013 10:39 AM ET

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