[Strawbale] Forced air

Tue Jan 8 14:06:07 CST 2008

Yes, Rob, indoor humidity is lower here, but you exaggerate the effect 
by disregarding the principal sources of water vapour in a dwelling - 
not the external conditions but (i) human activities in the kitchen, 
(ii) human activities in the bathroom, and  (iii) people breathing.  In 
Britain (as per the building regs document I sent you, Rob) I used to 
mechanically extract from those areas during/after humidity-generation 
to reduce indoor r.h.  Here I don't bother unless there are nasty 
smells.  While there is a huge difference in outdoor humidity levels, 
the indoor difference is small - r.h. was consistently in the 50% to 70% 
range in Bristol, and is typically 50% to 60% here.  (One reason why the 
difference is so small is that this house is extremely well-sealed - 
would put most British builders to shame.)

So, yes, there will be an effect of the sort you indicate (especially, 
and quite tangibly, on those occasions when my Canadian wife insists on 
"airing the house out"), but most of the time not a big one.

mark

Robert Tom wrote:

>I saw Mark's message just before heading to bed last night (~3:30am) and  
>wanted to write a response but well, you know, and this morning, I saw  
>that Don beat me to it.
>
>As a sort of corollary to Don's comments about the *significant*  
>temperature difference in outside temperatures between the UK and SK, one  
>notes that in the significiantly warmer UK winters, the pea-soup-like  
>outside air will obviously still hold a significant amount of moisture ...  
>whereas in SK which is an arid climate at the best of times, minus 35 deg  
>outside air is going to have diddly squat moisture content.
>
>So two identical, not-particularly-air-tight houses in the two locales are  
>going to have vastly different interior humidity levels, even if no  
>heating were in place.
>
>However once you start heating the SK house, that tosses a humongous  
>O/S-I/S temperature differential into the equation as an additional  
>driving force behind Ma Nature's propensity to even things out, with the  
>result that the interior humidity levels of the SK house would be lowered  
>even more, and would probably be best described as "scratchy-throat/crusty  
>nose/constant-bad-hair-day/flaky-skin-ish" ... or "exceedingly dry" for  
>short.
>
>If the interior humidity levels are extremely low, and moisture is being  
>sucked out of one's body as a result, then it's essentially the same  
>natural cooling process that our bodies use is summer where excess body  
>heat is dumped into moisture which is transpired through our pores and  
>then carried away by air moving past our bodies to effect evaporative  
>cooling... except that in winter when Old Mark is trying to stay warm, he  
>doesn't really appreciate that involuntary evaporative cooling so he kicks  
>up the thermostat in an attempt to compensate.
>
>Perhaps what Mark really should be doing is running around the house  
>looking for and sealing up air leaks and perhaps considering  
>humidification.
>
>
>
>On Tue, 08 Jan 2008 00:58:48 -0500, Mark BP / Low Energy Design Ltd  
><mark at lowenergydesign.ca> wrote:
>
>
>  
>
>>I'm prepared to believe that forced air doesn't create any more of a
>>dust problem.
>>
>>Re energy consumption, though, the situation doesn't seem to me to be so
>>    
>>
>
>  
>
>>One not-particularly-scientific piece of evidence.  Back in Britain, I
>>used to be able to keep daytime thermostat settings (at least when my
>>wife wasn't in) down to 16 or 17 deg C.  Here in Saskatchewan, I
>>generally find that I need 18.5, 19 or even 20 deg C for my own comfort.
>>    
>>
>[snip]
>
>On Tue, 08 Jan 2008 09:49:06 -0500, <dfugler at cmhc-schl.gc.ca> wrote:
>[mostly <unsnipped> ]
>  
>
>>Mark,
>>
>>You are probably having to keep the temperature higher in Saskatchewan
>>than in England because the walls and windows are colder, not because of
>>the change from radiant to forced air heating. The difference between an
>>outside temperature of 5°C and - 35°C is substantial. You will have
>>increased radiant loss to an outside wall or window when the termperature
>>outside is far lower (we could discuss relative wall insulation levels  
>>but we won't).
>>
>>As far as the assertion that radiant heated houses have lower thermostat
>>settings, I had heard that frequently as well. So I hired a student in
>>Nova Scotia to check a sample of each type of house: radiant flooring and
>>forced air. Here is the link to the results:
>>http://www.cmhc-schl.gc.ca/odpub/pdf/62675.pdf
>>
>>Thermostat settings were the same for the forced air and radiant flooring
>>houses. The radiant heating industry didn't like the study. They said it
>>was not big enough, and that we did not measure comfort created in those
>>houses. I challenged them to do their own study, or help us create a
>>bigger survey. So far they have not come up with the money to verify  
>>their assertions.
>>    
>>
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