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beno
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Posts:33
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| 08/22/2007 9:22 AM |
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Hi there,
We design a 2 storey ICF house, 1800 sq.ft. on each level, located near Ottawa, Canada.
The slab on grade will have hydronic radiant heating. No basement.
The question is how much insulation to put under the slab? The designer suggested 2" rigid foam but this doesn't seem enough to me, so I told him to double it, to 4" rigid foam. I am looking for a trade-off between insulation cost and energy savings.
Based on my research on the internet, the standard size of insulation is 2" like the designer said, but for colder climates like the one we are living in they suggest you could put more.
I even saw a super-insulated house having 14" foam under the slab.
What do you think?
Thanks,
Beno |
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PanelCrafters
Registered Users
Posts:1330
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| 08/22/2007 4:03 PM |
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Posted By beno on 08/22/2007 9:22 AM
The question is how much insulation to put under the slab? The designer suggested 2" rigid foam but this doesn't seem enough to me, so I told him to double it, to 4" rigid foam. I am looking for a trade-off between insulation cost and energy savings
Beno,
Maybe by doing the calculations you'll get a better idea of what your 'Trade Offs' will provide.
Let's assume that your ground temperature is 50° and that you want the indoor temperature to be 70°. That gives you a Delta T of 20°. The formula is: 1/R-Value x Delta T x Area. So here are your Heat Losses:
1" Styrofoam(R-5) 1/5 x 20° x 1,800 sqft = 7,200 BTU/hr
2" Styrofoam(R-10) 1/10 x 20° x 1,800 sqft = 3,600 BTU/hr
4" Styrofoam(R-20) 1/20 x 20° x 1,800 sqft = 1,800 BTU/hr
That should give you an idea of what your choices are.
Good Luck! |
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....jc
If you're not building with OSB SIPS(or ICF's), why are you building? |
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beno
Registered Users
Posts:33
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| 08/22/2007 4:28 PM |
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Thanks for the examples. I wonder though if 4" (R20) is enough? The ICF walls offer (in my area) an effective R35. And it seems to me that the insulation under a radiant floor is even more important than the walls, because the floor is heated. This is also a passive solar house.
Is the 1,800 BTU/hr heat loss through slab a small loss? I will heat the water with an oil boiler with 97,000 - 174,000 BTU/hr, so I guess it will have enough power for the 1800 BTU/hr loss (and of course for the heat loss from windows, doors, walls and ceiling).
Also, I am not sure that the ground temperature is 50°, because I will not bring fill around the slab on grade perimeter, the house will look like a 2 storey without a basement. |
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PanelCrafters
Registered Users
Posts:1330
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| 08/22/2007 7:21 PM |
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Posted By beno on 08/22/2007 4:28 PM
The ICF walls offer (in my area) an effective R35.
Be careful out there! 'Effective' is really meaningless. Once the heat escapes the interior layer of foam, it's trying to heat the concrete core. That core is usually fed by earth temperatures. What is going to 'win'? My guess is the earth. Many interesting scenarios can then be discussed.
And it seems to me that the insulation under a radiant floor is even more important than the walls, because the floor is heated.
Well, maybe. My current house has no insulation below the radiant slab(last minute & budget issues). Am I losing efficiency? Absolutely! But what is happening is that the ground below the slab is being heated. So, by force, I'm creating a bit of more of a Heat Sink. That fact may keep the area warm a bit longer, and it's the most comfortable area in the house(upper floors don't have radiant).
This is also a passive solar house.
Is the 1,800 BTU/hr heat loss through slab a small loss?
Yes. But remember it's constant due to the fact that the earth temperature is constant.
I will heat the water with an oil boiler with 97,000 - 174,000 BTU/hr, so I guess it will have enough power for the 1800 BTU/hr loss (and of course for the heat loss from windows, doors, walls and ceiling).
Have a heat loss calculation performed. That boiler sounds to me to be too large.
Also, I am not sure that the ground temperature is 50°, because I will not bring fill around the slab on grade perimeter, the house will look like a 2 storey without a basement.
You should insulate along the edges of the slab. That will help.
Good Luck! |
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....jc
If you're not building with OSB SIPS(or ICF's), why are you building? |
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beno
Registered Users
Posts:33
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| 08/23/2007 12:27 AM |
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Thanks again for your comments. I am insulating along the edges of the slab because I use ICF with footings on the bedrock. The slab will be between the ICF walls.
I will go with 4" foam all over under the slab, I have enough thermal mass in slab and I prefer not to warm the earth.
Is there a formula to help me decide the size of the boiler? Let's say that the total heat loss of the house is 10000 BTU/h. And the house has 5 baths for 6 people. Will a 100000 BTU/h boiler be too much?
Thanks again,
Beno |
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PanelCrafters
Registered Users
Posts:1330
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| 08/23/2007 12:05 PM |
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Posted By beno on 08/23/2007 12:27 AM
Is there a formula to help me decide the size of the boiler? Let's say that the total heat loss of the house is 10000 BTU/h. And the house has 5 baths for 6 people. Will a 100000 BTU/h boiler be too much?
Yes! NRT.Rob might be able to give you a more exact answer, but most people I know size the boiler for the Heat Loss and maybe plus a little. 90k is a lot more than a little. I would think that for a 10k Heat Loss a boiler of 12k would do the job. Please remember that your Heat Loss is for the coldest temperature expected at the site. So, it's only running at that capacity a fraction of the year!
And, by all means, purchase a modulating condensing boiler. You'll save even more $$$.
Good Luck! |
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....jc
If you're not building with OSB SIPS(or ICF's), why are you building? |
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