[Greenbuilding] Active Ventilation for Tiny Space

[Michael O'Brien]

Hi, Eli—

A very worthy project, I think your wife will really appreciate having a livable space. 

Here’s my 2 cents: thermal comfort requires maintaining 4 variables within a range the occupant prefers.

1. Air temperature—you already are on top of this. Raising air temperature alone will usually not compensate for items 3. and 4.

2. Air speed—you are taking measures to eliminate drafts by sealing the exterior components. There may still be (minor) drafts from cool air dropping down from the upper windows as they lose heat to outdoors.

3. Relative humidity—ideally should be 40-50% range. If it’s lower it will cause skin moisture to evaporate faster, which feels cool. Might need a humidifier in winter to maintain, though air sealing really helps.

4. Radiant surface temperature—if room surfaces are cooler than body surfaces, heat will radiate from the person to those room components. This, in my opinion, is the most important of the variables in determining comfort. Usually it’s large areas of glazing or uninsulated components, like that floor, that create cool surfaces. It might be worth getting even minimum insulation on the floor to raise its surface temperature.

Best wishes,

Mike
Sent from my iPhone

> On Nov 3, 2017, at 7:30 AM, conservation architect <elitalking at rockbridge.net> wrote:
> 
> I have returned to this great list for more good advice. 
>  
> I am thermally upgrading a small room that is a separate wing of the house to be tighter.  When I build new, I install continuous foam insulation to make the house tight, definitely requiring active ventilation.  In building that is conventional where more than enough passive ventilation is present, not so much need for active ventilation.  Where the decision becomes more difficult is when we incrementally improve the tightness, leaving portions of the thermal envelope with the classic flaws. 
>  
> In this case, I am on a steep hill.  The upper level has an open ventilated space under it.  This is a very small space.  (8’x12’) with cathedral ceiling.  Therefore, almost all surfaces are exterior.  I am upgrading the North Wall with 3” rigid foam.  I am installing 2” iso below the ceiling with new ceiling panel.  We are getting set to install mini split heat pumps in three zones.  My intention is to make a room that is super efficient so my wife who is always cold (poor circulation) can elevate the temperature to what ever she needs to achieve comfort (80F+) without requiring the whole house to be that hot.  I do not have the space to insulate the floor.  However, I was planning on putting building wrap under the carpet pad to create the continuous air barrier.  The South Wall has clearstory windows above lower roof.  This wall will remain with flaws. 
>  
> I am currently heating with wood.  However, we are wanting our photovoltaic array to meet the new electric loads of the mini split heat pump.  Though I claim net zero emissions from burning wood where I harvest from fallen trees on my land, producing heat with heat pump and home produced electricity, I am lowering my carbon footprint significantly.  Though the cycle for wood sequestration and emissions is quite short, if I leave the wood on the ground it will take a much longer time before decomposition eventually emits the  sequestered carbon. 
>  
> Thanks for reading my message. 
>  
> Eli
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