# Furnace Size with spray foam insulation



## markcrain (Oct 21, 2010)

I built a 660 sq.ft. cabin with 2x6 walls open cell foam filled, the roof is sprayed with 15" of foam and the crawl space is 3" of closed cell foam. I'm wondering what I'll need BTU wise to heat it. There are a couple calculators on line but they don't include a spray foam option. As I understand it with spay foam there are more benefits than just the R value. Anyone have experience with it?


----------



## Dixie Bee Acres (Jul 22, 2013)

Wow, don't know about furnace size, but a zippo lighter should do the trick.


----------



## HerseyMI (Jul 22, 2012)

How many rooms, not including the kitchen or bathroom? Insulated windows 2 pane or 3? Are you in a very cold zone subject to high winter winds or are you in a protected cove? Too many possibilities to give a flat out sure fire answer.


Honestly, btu costs are just about flat no matter what type of fuel you use these days... unless you burn wood but then you have hidden costs... chains, files, fuel to cut, fuel to transport... still less expensive but labor intensive and generally cannot be controlled as accurately as other heating systems.

Have you considered electric space heaters or electric baseboard heating? With that small of an area and that much insulation I would stay flexible and get small thermostatically controlled space heaters... one may even heat the entire place provided you do have electricity... Lol!

Sent from my MB855 using Homesteading Today mobile app


----------



## MushCreek (Jan 7, 2008)

R-value is R-value. There should be an R-per-inch value for the product you used, which can in turn be used for your calculations. The 2X6 studs will transfer heat/cold, though, unless you have foam over the outside for a thermal break. Figure a 25% reduction in R-value for that. The advantage with foam comes with the sealing value, but properly sealed, other forms of insulation should perform as well.


----------



## Snowfan (Nov 6, 2011)

Like MushCreek said, R-value is R- value. I think you would need more data to properly calculate your heating load or cooling load. Did you take into account your vapor barrier? What is your heat source? Most of those calculators are pretty accurate though. I'm afraid that, with the data you've given, it might be difficult to find a small enough furnace. Good luck.


----------



## markcrain (Oct 21, 2010)

HerseyMI said:


> How many rooms, not including the kitchen or bathroom? Insulated windows 2 pane or 3? Are you in a very cold zone subject to high winter winds or are you in a protected cove? Too many possibilities to give a flat out sure fire answer.
> 
> 
> Honestly, btu costs are just about flat no matter what type of fuel you use these days... unless you burn wood but then you have hidden costs... chains, files, fuel to cut, fuel to transport... still less expensive but labor intensive and generally cannot be controlled as accurately as other heating systems.
> ...


----------



## boxwoods (Oct 6, 2003)

I would use electric heat. a couple 1500 watt heaters should be fine most of time.
any burning flame will use up a lot of your oxygen in the air, being it's that air tight.


----------



## markcrain (Oct 21, 2010)

Electric is pricey and not always available. That's why I like the wood idea. I don't think there is a small enough stove for what I need. I thought about a rocket stove but I don't want the high weight needed sitting up high in my pole building cabin. Last night it went down to 30 degrees and a small space heater kept it warm in there without much trying. When I went out in the morning it was warm and the heater wasn't running and didn't feel like it had been recently.


----------



## MushCreek (Jan 7, 2008)

If you want to use wood, look at wood-burning boat stoves. They make some really small ones. The downside is that they need to be refueled more often.

We have the same problem with our new house- it's too well insulated for a conventional woodstove, and a tiny boat stove would look silly in a full-sized living room. We're probably going to put in a propane fireplace and just use it for atmosphere.


----------



## orionrising (Nov 18, 2012)

R value is R value, but not necessarily an accurate reflection of insulative ability. Spray foam, especially closed cell is does not pass air. 3-4 inches is generally 95% + effective. If you put any kind of wood stove or burning insulation be very careful with co2 and o2 levels as there will be miniscule amount of outside air exchange. You will have to be careful with moisture and mildew as well


----------



## jhambley (Nov 21, 2004)

We just built a new home using open cell foam insulation. The only way to determine the correct size of HVAC is to have a professional run a load calculation using the proper values for your entire house (including the foam). R value is R value but the load calculations also include factors like air infiltration. Properly installed foam insulation has very little infiltration. 

We had a horrible time getting a HVAC contractor who had experience with foam insulated homes. Most want to oversize your air conditioning and then the unit doesn't run long enough (short cycles) to properly dehumidify your home. The new high SEER units make this even worse because they are "tweaked" to achieve that high SEER rating and don't dehumidify as well as the older units. 

As far as wood stoves in a very tight homes, you need to supply outside air directly to the wood stove. Most all major stove manufactures provide a Outside Air Kit for just this purpose. 

You'll also need "makeup air" for your range vent hood etc. Simple put, it's easy to build negative pressure in a very tightly sealed foam insulated home.

Most builders of very tight homes install a HRV or ERV to provide enough fresh air to prevent indoor pollution.

I'll most likely install an Ultra-Aire dehumidifier with fresh air intake next Spring to help with high levels of humidity on cooler days as well as fresh air exchange. Here's a link to one of their PDFs on foam insulated homes.

http://www.ultra-aire.com/pdf/Ultra-Aire-Spray-Foam-Case-Study.pdf


----------



## spud (Feb 3, 2007)

I have 2400 sq ft home in nw ohio and I have a 80,000 btu furnacee that is 80% eff(only 64,000 btu). Perfert for my house and have only a two ton AC. I used spray foam insulation and it was worth every penny. Spray foam is so much better with air inflitration and is much more efficient. 

You could use hot water from a hot water heater possibly, might take a little convincing but would likely be enough btu for your low needs and could use propane if you don't have access to natural gas. 

There are also some small rocket stoves you could copy that are on youtube, one made the author tryin2hard is very high tech and you could make a tall thermal mass so it wouldn't take so much floor space. You can always crack a window or run a ground tube for air intake if needed. 

[ame]http://www.youtube.com/watch?v=hzc9rUq3Flc[/ame]


----------



## markcrain (Oct 21, 2010)

spud said:


> I have 2400 sq ft home in nw ohio and I have a 80,000 btu furnacee that is 80% eff(only 64,000 btu). Perfert for my house and have only a two ton AC. I used spray foam insulation and it was worth every penny. Spray foam is so much better with air inflitration and is much more efficient.
> 
> You could use hot water from a hot water heater possibly, might take a little convincing but would likely be enough btu for your low needs and could use propane if you don't have access to natural gas.
> 
> ...


WOW! I was just looking at rocket stoves this morning. My cabin sits on a hillside and has a crawl space the goes from 8' to about 2'. I though maybe put the stove at the bottom and the exhaust at the top. Run it through a thermal mass and let the heat rise up though the floor. My crawl space walls are insulated and the floor is not. I've decided to go with a propane direct vent wall heater in the crawl space as the other source of heat. I'll check out your video. Thanks.


----------



## lamoncha lover (Mar 1, 2009)

we have 1800 sq feet metal pole style home 10 inch sidewalls with 2 inches of closed cell foam on the walls. and 8 "open cell on the interior of roof line. I plan on adding 3" pink fiberglass to the interior walls before next summer. We have had some 20 degree nights and the wood stove we have cooks us about out even closed way down. We are also toasty all night. Love it. Of all the things we have put in to this place..foam insulation is the BEST investment by far.It costs a bit..but thrilled with it. Also invested in a quite costly efficient wood stive,


----------



## arbutus (Jun 8, 2006)

markcrain said:


> I built a 660 sq.ft. cabin with 2x6 walls open cell foam filled, the roof is sprayed with 15" of foam and the crawl space is 3" of closed cell foam. I'm wondering what I'll need BTU wise to heat it. There are a couple calculators on line but they don't include a spray foam option. As I understand it with spay foam there are more benefits than just the R value. Anyone have experience with it?


 
My inlaws built a cottage that is a little larger - 28x28 with an uninsulated slab floor in Michigan, and 2x6 walls with spray foam insulation. I bought them a 28 or 30k btu vent free blue flame heater from TSC and it heats up comfortably. I didn't see your location. My mother in law's house is 950 sq ft and spray foam insulated, and her furnace is 35 k btu *I think*. My brother in law heats his 1500 sq ft house that is spray foam insulated with a decorative natural gas vent free heater that is about 30k btu. He has a 45k btu forced air furnace, but it doesn't run when the vent free heater is running, and he says it short cycles at night. These are all in mid to southern Michigan, with a wide open floorplan.

Cheap and easy = electric, otherwise a vent free natural gas heater is what I would use.


----------



## Jolly (Jan 8, 2004)

MushCreek said:


> If you want to use wood, look at wood-burning boat stoves. They make some really small ones. The downside is that they need to be refueled more often.
> 
> We have the same problem with our new house- it's too well insulated for a conventional woodstove, and a tiny boat stove would look silly in a full-sized living room. We're probably going to put in a propane fireplace and just use it for atmosphere.


 I've got an idea for your stove...I've had a Hearthstone for 25 years. They make high quality heaters. And they make a very pretty, small heater:


http://www.hearthstonestoves.com/store/wood-products/wood-stoves/tribute-soapstone


----------



## AR Aaron (May 26, 2010)

Not sure where you are at with this if you went with vent free gas, but I have been looking into build something on the smaller scale also in the future. I want wood as a secondary and off grid heat source if needed. Two stoves that I have been looking at online that have caught my fancy as of recent are the Jotul 602, and the Vermont Bun Baker Stover. Price tag is what has me on both of them. 

http://www.woodlanddirect.com/Wood-Stove-and-Accessories/Cook-Stoves/Vermont-Bun-Baker-Wood-Stove
http://jotul.com/us/products/stoves/jotul-f-602

Price tag is rather high, but my smaller house will most likely not have a Central Heating System or Forced Air Furnace. So when you compare these prices to duct work and furnace ect, the price is really not all that high. 

I love the rocket stove mass heater concept, but have come to the realization it would probably never make it to completion if I attempted it, so have put that concept on the back burner for now. Maybe for my workshop or greenhouse after I retire from my hectic job. Too much time required to feed and tend it to make it a viable concept I can use right now in my current situation.


----------



## markcrain (Oct 21, 2010)

I'm still looking for a wood stove and they all look to big. I saw one called a sardine that would be right on but special order, takes months to get, and big bucks. I have however bought a 15K btu direct vent wall furnace and I think it will be plenty big. An 1800 watt space heater kept it 60 degrees when in went down to 17 the other week. I paid the extra for direct vent so I don't eat up the O2.


----------



## PorkChopsMmm (Aug 16, 2010)

I would buy a small box wood stove, think Vogelzang cheap model at your local hardware store, and use an outside air kit. That way you are pulling in cold air from the outside and not using up the air in your air-tight environment and it will be small. Place some large stones/brick around the stove and it should retain some heat. That way you can run the stove periodically to heat the space.


----------



## markcrain (Oct 21, 2010)

PorkChopsMmm said:


> I would buy a small box wood stove, think Vogelzang cheap model at your local hardware store, and use an outside air kit. That way you are pulling in cold air from the outside and not using up the air in your air-tight environment and it will be small. Place some large stones/brick around the stove and it should retain some heat. That way you can run the stove periodically to heat the space.


I didn't know they had an outside air kit for those. I'll have to look into that. Thank you. Only draw back to those box stoves (other than outside air) is not airtight and no ash dump but the price is surely right.


----------



## PorkChopsMmm (Aug 16, 2010)

You may have to make the kit work but I don't think it would be too hard. I think your biggest problem would be re-starting fires after you let them die down, but some fatwood and a small container of kindling near the woodstove will make it easy.

FWIW this is coming from someone who lives offgrid and heats with wood in an EPA air-tight stove -- but our place is ~1200 sqft and not as well insulated as yours.


----------

