Infiltration & Below Grade Slab Heat Loss

JohnG_3

I have a 20' x 30' x 7' basement, 5' 6" below grade and 1' 6" above grade.  I live close to Boston.  My contractor modeled the floor heat loss as slab on grade and got a slab loss of 5,184 BTU/hr at 70F inside and -10F outside.   Using guidelines in Modern Hydronic Heating by Siegenthaler for a below grade slab, I get a slab heat loss of 1152 BTU/h (Siegenthaler's equation is 0.024 x slab area x delta T).



My contractor also calculated the infiltration loss using 1.5 air changes per hour, giving a loss of 9,072 BTU/hr.  Siegenthaler classifies 1.5 air changes per hour as worse (leakier) than average, with 0.4 being best, 1.2 being average and 2.2 being poor.  I am remodeling the space with 2 1/2" of closed cell Icynene below grade, 6 1/2" above grade and 7" open cell Icynene in the ceiling (below heated space), five new hopper windows, a tightly sealed bulkhead door, a door to a boiler room (that room is not well sealed but has only one non-sealed combustion appliance, a gas dryer), and a door to the first floor at the top of the basement stairs.



My questions are:

1. What is a good rule of thumb for estimating heat loss for a below grade slab near Boston?

2. What is a good number to use for air changes per hour for the described remodeling in which the envelope is significantly tightened?

3. At what point should I consider getting a heat recovery ventilator?



Thanks!

Brad White

The Bunker -To your questions

Slab on grade heat loss is a high value for a below-grade basement. Your delta-T is between your living space and ground temperature, which in Boston is about 52 degrees below frost line. (Trivia fact- the below frost line temperature in most locations very closely mimics the average annual temperature of the location.) So if your space is 70F, the delta-T is 18 degrees. Without getting into the middle of the discussion, given your insulation levels and general approach, I would be surprised if the heat of lights, appliance, the wine cooler and lava lamp do not heat the space or overheat it.



One note of concern- Combustion Air for your boiler and water heater. See to it that you meet our gas code 248 CMR (I live outside Boston, too.) . Our gas code is based on NFPA-54/ANSI Z223.1, but is pretty straight forward. You may need a fan in a can or other mechanical solution. Please see to that!



To your questions:



1. What is a good rule of thumb for estimating heat loss for a below grade slab near Boston?



A. If insulated below the slab to at least R-5 and with the space at 70F, about 3 BTUH per SF. If not insulated, about 18 "on paper". But keep in mind that your basement is a submerged heater as far as the earth is concerned and the soil temperatures below and around your house will eventually warm up quite a bit. So the 18 is fine "day one" but after a few months, I would cut that number in half at least.



The walls I usually figure in 1/3 stripes. The bottom third figure the soil is 52F, the next third, (at or around frost line) at 32F and the upper third assume above grade at ambient air temperature, (say 6F). With your insulation system, especially if external, heck, awesome. Practically body heat will do it.





2. What is a good number to use for air changes per hour for the

described remodeling in which the envelope is significantly tightened?



A. I can guess as well as anybody. Could be 0.35, 0.50, 0.75, 1.0. It all depends. The only way to know is to test it but remember, your older portions can be air sealed to the nth degree and this will have twice to three times the leakage of a modern concrete, SIP or ICF structure. So your aggregate value will be somewhere in between.



I am not shilling for work here, but as an FYI, I do blower door testing and IR thermography as a sideline to my regular job. So I have a pretty good handle on the range of what I have found. One can test individual parts of your home (to be dealt with individually), or test the aggregate, how the house normally is open to each area. You can imagine how this varies.







3. At what point should I consider getting a heat recovery ventilator?



A blower door test can measure and reveal this, what we call Depressurization Tightness Limits. Basically how your house performs with dryers, kitchen hood fans and bathroom exhaust fans in operation yet still to allow your combustion appliances sufficient combustion air to have effective draft on their flues. Imagine the tighter houses have much lower tolerance.



So one can always put in an HRV and many do, especially in newer basements where there are fewer air leaks than an old field stone foundation. Often they are not needed in older homes, even with improvements. I have never seen a renovated older home get to that point unless it was subject to a deep energy retrofit, (external insulation cladding). I tell people that having a house that tight, that needs an HRV, is a nice problem to have.



But rather than guess, we can measure. Again not seeking business here, just here to help and advise.

JohnG_3

Combustion Air

Thanks for your reply.  To your concern about combustion air, the boiler is a wall hung Viessmann unit with sealed combustion using a coaxial pipe running through the side of the house, and the water heater will become an indirect heater running off the Viessmann with this remodel.  That leaves the gas dryer as the only appliance needing combustion air and we think there is plenty of make up air for that.  All work is fully permitted which means we will seek the plumbing inspector's approval when we have the work inspected.

Brad White

All good

I had a Viessmann which I installed in my last house and know it well. You made a good choice.

You have the situation under control, but I had to ask. Your sealing efforts had my antennae up.