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The old ‘keeping the room above the garage warm’ conundrum

I have a 7 yr old 2 story colonial in Zone 4/5 (just outside of NYC)

The room above the garage is cold, I opened a 1ft sq up and found R38 but it hadnt been attached the the floor joists above so it was resting compressed on the garage ceiling, so rather than tear the ceiling down I blew in cellulose to fill in the floor joists.

So result floor warmer but still cold.

Question, if I screwed and taped 2" foil faced poly iso sheets to the garage ceiling would that be enough of a thermal break to make it any warmer,
The builder put a thermostat in the room that covers it and 2 other bedrooms so if its cold in that room and if guests stay over and crank up the heat, the other two bdr are too hot.

Im sick of hearing its too cold from the mother in law every time she visits (luckily not regularly)

Any advice would be appreciated

Asked by Darren Finch
Posted Thu, 02/20/2014 - 12:38
Edited Thu, 02/20/2014 - 13:40

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6 Answers

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1.
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Darren,
First of all, you may be interested in reading this article: How to Insulate a Cold Floor.

Here are two other points to consider:

  • The fact that a single thermostat controls the heat to three bedrooms is part of the problem. The existing heating system sounds like it is poorly designed. If the heating system were designed properly, and delivered heat to each bedroom based on its actual heat loss, then there wouldn't be much of a temperature spread from bedroom to bedroom. The solution to this issue is to either (a) improve the delivery of heat to the coldest bedroom, or (b) put the cold bedroom on its own heating zone with its own thermostat.
  • This type of room loses heat due to conduction (a phenomenon that can be slowed by adding more insulation to the floor assembly, wall assemblies, and ceiling assembly), but also due to air leakage. The solution to the air leakage problem is to seal air leaks.

Answered by Martin Holladay, GBA Advisor
Posted Thu, 02/20/2014 - 13:38

2.
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Martin
Ok looked through the article you sent, I have
Blocked the floor joists with rigid foam and can foam to seal any air leaks into the house, re C02 leakage
The cellulose I dense packed on top of the existing batts as much as I could to fill the void between the batt and subfloor above.
Airsealed the kneewall when the garage roof meets the outside wall with rigid foam and can foam to air seal it.
The outside wall is cold as well, so I feel this spring/summer take the siding above the roof line off and see whats going on underneath, as I have a feeling that there is a gap that is leaking air.
My question originally is that with the poly iso as a thermal break at the garage side, would that make the batt/cellulose combo work as it should?
The only other thing I can see to do is pull the ceiling and start again, which Id prefer not to do, if the poly iso thermal break will make it far more livable
As for the thermostat, if the room wasnt as cold it wouldnt create an issue, but because the builder doesnt subscribe to GBA, I have had to go back and air seal the house the best I can.

Answered by Darren Finch
Posted Thu, 02/20/2014 - 14:09

3.
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Darren,
Installing polyisocyanurate on your garage ceiling is a fine idea, but it may not solve your problem. (By the way, most building codes will require the rigid foam to be covered with 5/8-inch-thick gypsum drwall on the garage side.)

It's possible that this cold bedroom already has adequate insulation and is adequately air-sealed. Perhaps the only problem is that it isn't getting enough heat.

If it is served by a forced-air heating system, perhaps the room needs to be served by a larger duct, or by an additional duct. It's also possible that your HVAC contractor forgot to install a return-air duct.

If the room is served by a hydronic heating system, it's possible that the room requires more feet of fin-tube baseboard.

Answered by Martin Holladay, GBA Advisor
Posted Thu, 02/20/2014 - 14:21

4.
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Ok the room is heated by baseboard heat which runs the entire length (17') of the room 12x17 room
It is the first room on that zone, which then goes to the other 2 bds
I knew about the foam needing to be covered, so I think I have no choice but to pull the ceiling
Again I thank you for all your help and keep up your great work
Regards
Darren

Answered by Darren Finch
Posted Thu, 02/20/2014 - 14:35

5.
Helpful? 0

Darren,
1. You can put this room on its own zone by adding a circulator controlled by a thermostat on the bedroom wall. Then the room will get heat, regardless of whether the other two bedrooms are calling for heat.

2. If the amount of fin-tube baseboard is really insufficient to meet the room's heat loss at the design temperature -- and I doubt that it is -- you can add a kickspace heater or a wall-mounted fan-coil unit to provide more heat from your hydronic distribution system.

Answered by Martin Holladay, GBA Advisor
Posted Thu, 02/20/2014 - 15:24

6.
Helpful? 0

Unless there's a truly substantial amount of window area (with some of the windows left open) 17' of baseboard should be enough to handle the peak loads of ANY ~200' room, even with 140F water, but it needs to be operated as it's own zone, since it's heat loss characteristics are pretty different from other rooms in the house. Typical baseboard output at 180F average water temp delivers on the order of 600 BTU/hr per foot, so you're looking at about 10,000 BTU/hr, which would be a ratio of about 50BTU per square foot, whereas the actual design heat load for the room is likely to be well under half that. At 140F AWT 16-17' of fin-tube baseboard delivers about 5500BTU/hr which would be north of 25 BTU/foot of conditioned space, which is a ratio that you'd see at your likely 10-15F outside design temp in a house that had single pane double-hungs and no wall insulation, maybe a bit of attic insulation.

The problem isn't under-radiation, but a balance issue- the difference in heat loss characteristics with the rest of the zone. If you moved the zone thermostat to that room that room would be fine, but every other room on the zone would have comfort issues like spiky temperature overshoots and occasional undershoots.

But 17' of fin-tube set up as a single zone is a guaranteed short-cycle zone since it's heat emittance even at 180F is well under the minimum fire output of a modulating condensing boiler. If you're going to micro-zone it, replace the fin-tube with low-temp panel radiators with an "equivalent baseboard length" of at least 50', which will have sufficient heat emittance and at least some thermal mass in the water volume to limit the amount of short-cycling, and smooth out the room temperature swings.

Short cycling the boiler is hard on the equipment and can cut severely into it's lifespan, so even though breaking it into another zone without upgrading the radiation can seem cheap in the short term, it has other costs, and isn't really the "right" way to go about it. It's best if each individual zone has sufficient mass to handle the output of the boiler, and if you're cutting out 17' of fin-tube, you've shortened the res of that zone by that amount, and it may now short cycle on either/both of those zones.

But you can figure out the short-cycling risk with napkin-math , given enough information. What boiler(make & model #) is driving this system, and how many feet of baseboard is there total on the system (broken down by zone)?

Answered by Dana Dorsett
Posted Thu, 02/20/2014 - 16:57

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