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# Heat Loss Calculation on high R value wall

Member Posts: 5
Hi all, I'm in the process of planning a renovation on my 1950's house and I'm having problems figuring out my heat loss so I can size a new heating system. The exterior walls will have R18 and the wood frame R13 which gives me effective R31. Tools like slant fin don't seem to be handle that much. Could anyone direct me to something that could do that?

• Member Posts: 4,692
I don’t understand R-18 and R-13 in the same wall?
• Member Posts: 5
Sorry early morning post before the coffee kicked in. I'll be adding 4" of foam with an r value of 4.7" outside the sheeting. That's where the R18 comes from.
• Member Posts: 3,230
Hi, If the app doesn’t go high enough in R value, you could put in fewer square feet of wall area to compensate. For example if your R value is twice what the app gives you, then cut the square footage in half.

Yours, Larry
• Member Posts: 22,910
edited January 2021
A true Manual J calculation will handle it, but I'm not familiar with a plug and chug application which will go that high. You can finagle it, though with some hand calculation and the Slant/Fin results, as I recall. If you use say R10 for the walls, and get the partial heat loss for the walls and windows and infiltration and all separately, then cut the partial loss for the walls (just the walls, not the windows!) by 3 (that would be R30) and then add them all back together, you'll be well within the margin of error.

A word of caution when working with very high R value insulating systems, however: Either take thermal bridging into account in your calculations, or arrange your construction to avoid it (which is much easier said than done). Thermal bridges, such as studs or other framing, or even through bolts, become important in the performance of a wall even at R20
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 2,766

A true Manual J calculation will handle it, but I'm not familiar with a plug and chug application which will go that high. You can finagle it, though with some hand calculation and the Slant/Fin results, as I recall. If you use say R10 for the walls, and get the partial heat loss for the walls and windows and infiltration and all separately, then cut the partial loss for the walls (just the walls, not the windows!) by 3 (that would be R30) and then add them all back together, you'll be well within the margin of error.

A word of caution when working with very high R value insulating systems, however: Either take thermal bridging into account in your calculations, or arrange your construction to avoid it (which is much easier said than done). Thermal bridges, such as studs or other framing, or even through bolts, become important in the performance of a wall even at R20

Exterior foam insulation virtually eliminates thermal bridging . How will your air sealing be handled ? ACH ( air changes per hour) does much more tyo affect your load than any R Value increase above R 15 .
You didn't get what you didn't pay for and it will never be what you thought it would .
Langans Plumbing & Heating LLC
732-751-1560
Serving most of New Jersey, Eastern Pa .
Consultation, Design & Installation anywhere
Rich McGrath 732-581-3833
• Member Posts: 22,910
@Rich_49 mentions air changes. Which is a very good question. How do you propose to get adequate air changes per hour? You need at least two to maintain anything like livable indoor air quality. Sensible heat heat recovery unit?
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 527

Hi all, I'm in the process of planning a renovation on my 1950's house and I'm having problems figuring out my heat loss so I can size a new heating system. The exterior walls will have R18 and the wood frame R13 which gives me effective R31. Tools like slant fin don't seem to be handle that much. Could anyone direct me to something that could do that?

If all of your walls will have the same insulation value, you can pick the closest match in your heat loss software. The result will be that our radiation will be a little oversized, which will allow you to use lower water temperatures.
• Member Posts: 5
Those are great comments. As well as adding the extra insulation I'll be working on improving the air sealing in the house (right now it's 11.93ACH!). I'm planning on installing a HRV from VanEE as I'm in Canada. For the heating I'm leaning to a gas Combi Boiler and want to make sure it's sized right to avoid short cycling.
• Member Posts: 2,766
I hope you mean 11.93 ACH50 .
You didn't get what you didn't pay for and it will never be what you thought it would .
Langans Plumbing & Heating LLC
732-751-1560
Serving most of New Jersey, Eastern Pa .
Consultation, Design & Installation anywhere
Rich McGrath 732-581-3833
• Member Posts: 5
Hi Rich, yes the house is just bit leaky right now.
• Member Posts: 2,766
Did someone perform a blower door test depressurizing the building to -50 pascale to determine that number ? Do you or someone counciling you have a target blower door number that you will reach ultimately ? Of not sizing a system properly is 100% impossible and a waste of your time and effort .
You didn't get what you didn't pay for and it will never be what you thought it would .
Langans Plumbing & Heating LLC
732-751-1560
Serving most of New Jersey, Eastern Pa .
Consultation, Design & Installation anywhere
Rich McGrath 732-581-3833
• Member Posts: 5
Yes I had the initial blower door test done to get that first reading. I'm aiming for around 3.0 or better after I'm done the renos.