Insulation Overkill????

[Ed-G]

Hi All,

Designing up an 1800sf garage/shop. I am in the U.P. of Michigan and plan on heating the full space in the winter to 65-68 degrees with natural gas radiant slab.
1. Radiant slab w/2" XPS.
2. Foundation/stem wall w/2" XPS on the inside with the slab decoupled from the stem walls.
3. Ceiling with about R-50 blown in insulation
4. Walls will be 2x6 w/R21 cavity insulation.

My heatload analysis comes out to about 27000 btu's. If I add 3" of XPS foam to the outside of the walls the heatload drops to 23000 btu's. I was all fired up about doing the foam until I did the heatload and now I'm wondering is the 4000 btu's worth it? At my current gas price I will save $60 per heating season. I am guessing the insulation with labor will cost $3500. Most of me says its overkill even though its a really efficient wall!

What say you all!

Thanks,
Ed

 

[brewchief]

I would spend the money on other things, buy a more efficent boiler or better doors.

Sent from my SPH-D600 using Tapatalk 2

 

[Blk88GT]

Ed, can you tell me where/how you are calculating the heat load?


Thanks.

 

[37ford4dr]

if your numbers are correct its overkill

at current heating cost pay back is 58 yrs

at 2x current heating cost 29 yrs

at 4x current heating cost 14 yrs

i would look to see where better efficiencies could be purchased for that $3500

 

[Ed-G]

Ed, can you tell me where/how you are calculating the heat load?

I'm using Loopcad 2012.
Using wall r-value of 14 (whole wall value), 50 in the ceiling.
Planning on 3 8x10 foot Raynor doors-R18 but using R-8 in the program.
A couple of man doors using r8 for those.
I forget if I had 2 or 3 2x4 double glazed windows.

-Ed

 

[Mr onetwo]

yep, it's overkill. Just look at the payback period honestly and you will see that it is overkill.

 

[37ford4dr]

I bet for $3500 you could get some solar panels on the roof for some sort of water based recirculation system to aid in sunny winter day heating. maybe even going into the radiant floor system

 

[jvitez]

Around here a few builders will put 1 1/2" foam on the outside of a stick frame building. It allows them to build out windows & doorframes with a 2x4 on the flat. 3" is pretty thick. Re-do your heat load calc with 1.5" foam. It will stop thermal bridging across each stud, which makes up more of the benefit than simply R-value. It should make for a warmer feeling wall, which isn't just about the pure cost savings but also comfort.

 

[Ed-G]

Around here a few builders will put 1 1/2" foam on the outside of a stick frame building. It allows them to build out windows & doorframes with a 2x4 on the flat. 3" is pretty thick. Re-do your heat load calc with 1.5" foam. It will stop thermal bridging across each stud, which makes up more of the benefit than simply R-value. It should make for a warmer feeling wall, which isn't just about the pure cost savings but also comfort.

Big issue of using foam is the wall must dry to the inside. The thicker the foam the less moisture condenses in the wall during the cold months. Think temperature gradiante from 0 outside to 70 inside and were the dew point is in the physical wall. The more foam the less fiberglass is subjected to temps below the dew point and the result is less moisture that needs to evaporate out in the warmer months. For my climate, 3 inches is the recommended amount. I would love to be able to put 1 or 1.5" on but I am concerned about the wall not drying and mold forming. I'm going to revisit my required thichness and make sure I don't have that screwed up in my head.

Back to the original issue of "overkill", return is way to slow, so no 3 inches of foam.

Thanks,
Ed

 

[ForceFed70]

Definately overkill. That foam is expensive!

27,000 BTU for an 1800sqft shop does not sound right! Even for a well insulated shop such as what you describe. I strongly reccomend you double check that calculation.

Did you factor in the windows and doors? They are lossy.

I would expect the heat load for the building you describe to be at least 75,000 BTU.

 

[WVBrady]

Big issue of using foam is the wall must dry to the inside. The thicker the foam the less moisture condenses in the wall during the cold months. Think temperature gradiante from 0 outside to 70 inside and were the dew point is in the physical wall. The more foam the less fiberglass is subjected to temps below the dew point and the result is less moisture that needs to evaporate out in the warmer months. For my climate, 3 inches is the recommended amount. I would love to be able to put 1 or 1.5" on but I am concerned about the wall not drying and mold forming. I'm going to revisit my required thichness and make sure I don't have that screwed up in my head.

Back to the original issue of "overkill", return is way to slow, so no 3 inches of foam...

I think I followed this up until the last sentence. Are you going to put less than 3 inches of foam and accept the amount of moisture in the fiberglass?
The reason I am interested is because I have seen old houses in WVa being retrofitted with foil covered foam insulation on the outside of the wall. I thought that this would result in moisture being trapped in the wall, but if there is no other insulation, the dew point would be inside and foam and should not cause a problem. This is assuming that the foam does not allow moisture to penetrate. Comment?

 

[jack stand]

Sounds like a well insulated bldg.! Just remember the overhead doors, I've never seen one with a good seal around the edges (weatherstripping) or one that is over a r-11/12 insul. value. This is a weak point in my shop that really helps to defeat some of my "above and beyond" insulation efforts. I have 2 12'X14' and one 10'X12' OHD's. I'd spend the bucks on them but I haven't seen anything other than the rubber flap type that will loose it's spring in the cold and allow mucho air in, that's 114' of less than perfect air seal, prolly equal to a window being open 3" or so. Good point on the dew point and moisture in the wall!

 

[jvitez]

Big issue of using foam is the wall must dry to the inside. The thicker the foam the less moisture condenses in the wall during the cold months. Think temperature gradiante from 0 outside to 70 inside and were the dew point is in the physical wall. The more foam the less fiberglass is subjected to temps below the dew point and the result is less moisture that needs to evaporate out in the warmer months. For my climate, 3 inches is the recommended amount. I would love to be able to put 1 or 1.5" on but I am concerned about the wall not drying and mold forming. I'm going to revisit my required thichness and make sure I don't have that screwed up in my head.

Back to the original issue of "overkill", return is way to slow, so no 3 inches of foam.

Thanks,
Ed

Oh, right! "Slap myself on forehead." I forgot most places in the US don't require a complete air/vapour barrier. We always use 6 mil poly (special vapour barrier grade) completely sealed with acoustical sealant/sheathing tape on the "warm in winter" side of an insulated stud wall. This means no real issue with 1.5" foam, as no water should be condensing within the wall cavity.

I agree, if it's 3" of foam or nothing, "nothing" wins with your numbers.

 

[brewchief]

Definately overkill. That foam is expensive!

27,000 BTU for an 1800sqft shop does not sound right! Even for a well insulated shop such as what you describe. I strongly reccomend you double check that calculation.

Did you factor in the windows and doors? They are lossy.

I would expect the heat load for the building you describe to be at least 75,000 BTU.

I did a bunch of guessing and threw some numbers for a 36x50 shop(1800sf) with 12' tall walls working off the info I had and came up with around 29k BTU, with better info I could get closer but 27k isn't impossible by any means in my book.

 

[pseudorealityx]

I did a bunch of guessing and threw some numbers for a 36x50 shop(1800sf) with 12' tall walls working off the info I had and came up with around 29k BTU, with better info I could get closer but 27k isn't impossible by any means in my book.

Depends a lot on location... UP of Michigan gets cold. Down here, our design heating day is like 17 degrees. I think the UP Michigan guys are wearing T-shirts and shorts if it's 17.

 

[Ed-G]

Thanks all who commented. I am skipping the foam all together. The R-Value of foam is dependant on climate zone, I'm in zone 7, the building code requires r-15 (3" XPS). I could probable get away with 2" of Polyiso (r-13) as we are warmer than most of climated zone 7 but it still makes no economic sense.

WVBrady- If it is an uninsulated stud wall, foam on the outside is fine. If its a stud wall with insulation and you are putting foam on the outside the climate zones come into play. The wall will not dry to the outside so any moisture in the stud insulation has to dry to the inside. Thats why the colder it gets the more insulation you need to prevent to much moisture condensing in the stud insulation. At least thats how I understand it!

-Ed

 

[WVBrady]

...WVBrady- If it is an uninsulated stud wall, foam on the outside is fine. If its a stud wall with insulation and you are putting foam on the outside the climate zones come into play. The wall will not dry to the outside so any moisture in the stud insulation has to dry to the inside. Thats why the colder it gets the more insulation you need to prevent to much moisture condensing in the stud insulation. At least thats how I understand it!

-Ed

Thanks for the reply. I thought you were going to put a lesser amount of foam and still have the fiberglass, but if you are going to skip the foam altogether, I understand. I assume you will then have a vapor barrier to the interior side.

OT- In Calif. I once had a roomate from the U.P.; Ishpeming, I think. He didn't talk about the cold so much as the amount of snow. Took a swim in the ocean on New Years Day just to tell his folks and caught a cold.

 

[trbomax]

The only thing I can say about this is that I have 1.5" of foam board on each side ,and Ive been into the walls,and they are not wet.The house is that way too,been that way for 12 yrs.3.5" foam board and 20" of celulose on top of that in the cieling .I heat the clean area of the shop,about 900 sq ft with a 40k 93% goodman,and last winter I used a little over $200 of propane,keeping it at 60 to 70 ,7 days a week. Call it overkill if you want,I'm just sayin.................

 

[Ed-G]

The only thing I can say about this is that I have 1.5" of foam board on each side ,and Ive been into the walls,and they are not wet.The house is that way too,been that way for 12 yrs.3.5" foam board and 20" of celulose on top of that in the cieling .I heat the clean area of the shop,about 900 sq ft with a 40k 93% goodman,and last winter I used a little over $200 of propane,keeping it at 60 to 70 ,7 days a week. Call it overkill if you want,I'm just sayin.................

Hello fellow Michigander Trbomax,

With foam on the inside you prevent moisture from entering your inner cavity. Insulation on the outside does the same and both add R-Value to your wall. The issue is; how does the inner insulation in your stud wall dry should there be a leak? THis is the reason your method is not recommended. Not saying you are going to have a problem, just saying the reason its not done. The term "overkill" may not be the best. As pointed out by others and my own heatload analysis shows very little savings and a very long payback period beyond my basic 2x6 wall. I did some additional calculation and 2" of polyisocyanurate will work based on my local average temperatures. Still would require extra effort in furing strips and window install and the saving in fuel are very, very small.

2x6 studwall it is. Now I just need to decide the type of cavity insulaton. I'm leaning to "damp sprayed" cellulose.

-Ed

 

[trbomax]

If there is a "leak" and the core insulation gets wet,its not gonna matter if its covered with foam board or drywall,the only way to fix it is to tear off the inside wall covering and replace the fg.Once its wet its trash. All that being said,Ive built 6 homes and shops this way since 1974 and never had any issues. Still own the first one built in74,my son lives in it.

 

[JakeKohl]

Hello fellow Michigander Trbomax,

With foam on the inside you prevent moisture from entering your inner cavity. Insulation on the outside does the same and both add R-Value to your wall. The issue is; how does the inner insulation in your stud wall dry should there be a leak? THis is the reason your method is not recommended. Not saying you are going to have a problem, just saying the reason its not done. The term "overkill" may not be the best. As pointed out by others and my own heatload analysis shows very little savings and a very long payback period beyond my basic 2x6 wall. I did some additional calculation and 2" of polyisocyanurate will work based on my local average temperatures. Still would require extra effort in furing strips and window install and the saving in fuel are very, very small.

2x6 studwall it is. Now I just need to decide the type of cavity insulaton. I'm leaning to "damp sprayed" cellulose.

-Ed

I studied a lot of different types of insulation for my garage (1700 sq ft in 2 stories) and a good builder friend of mine who builds multi-million dollar homes highly recommended cellulose. It really does seem like the most efficient and cost effective manner of insulating. Foam was going to cost me $10,000. I had fiberglass quotes as low as $1,800. Cellulose was $2,400. The wet-blown corn starched cellulose is actually pretty close to the air flow resistance and R-value of foam.

 

[walrus]

I put 2 inches of foam on the outside of my shop, overkill, probably but its one comfortable shop in the winter.The older I get the more I appreciate comfort. The foam makes air infiltration a function of my overhead door and thats about it, very tight other than the door. My main heat is a homebuilt 10 by 16 solar panel, circulated thru 1100feet of pex in the floor. I have a woodstove also but it rarely gets used. So far the lowest temp I've seen in the shop is 46 and its generally in the 50s.
Here is a link to my build if interested
http://www.garagejournal.com/forum/showthread.php?t=40482&highlight=walrus&page=4

 

[BadgerBoilerMN]

A vapor barrier is required in every model code in N.America.

The heat load is not far off...75mbtuh...I don't think so.

Foam under slab 2" XPS, the first four feet of the perimeter can be bumped up (the frost line) with reasonable return on investment.

The vapor barrier is meant to keep moisture from traveling from the inside out generally. Foil on the outside is for solar gain and must include an air space to be effective.

If anyone know where I can get a solar panel system to help heat my shop for $3500.00 please call ASAP!