Foam and Blown-in at the same time?

[Schumi]

Just built a 35'X40' garage with 11' ceilings. I used pre-engineered trusses for the hip roof. I have a 4" soffit, non-vented in anticipation of foaming the roof. The roof pitch is 14/12 on two sides and 8/12 on the other two. This made a really large roof area. I purchased a LG 18,000 24 SEER minisplit (models LAU180HYV1 and LAN180HYV1).

My question is this: if I spray foam the entire outside envelope (walls and roof), can I then put in blown-in fiberglass on top of my drywall ceiling so I am not heating/cooling the huge attic? I'm concerned my minisplit won't do the whole thing if I don't insulate the ceiling to keep the heat/cool in the garage.

Alternatively, if I don't spray foam the roof area and do just the blown-in fiberglass and put in regular square-type roof vents very low on the 2 backsides of the roof that you can't see from the house, will that be enough venting if I have a ridge vent? (I don't have soffit vents)

 

[mrramsey]

My understanding of spray in foam is that the areas that are sprayed need to be conditioned spaces (heated / AC). If you were spraying the bottom of the roof you would leave the ceiling off. If you want to fiberglass blow in the attic then don't spray the roof. You would need the proper venting though either way otherwise you have moisture issues. Thus the need to condition the space with foam sprayed insulation.

Spray foam is great but it does have special requirements. If you are having a ceiling, spray foam that instead of the roof then leave the attic space unconditioned.

 

[Schumi]

How much venting will be required? I think I screwed up by not using vented soffit material. I don't have much soffit to begin with and it is Hardie board.

Will the natural gaps in the Hardie board and fascia be enough with a ridge vent?

Also, do I gain much by spraying the top of the ceiling over just blowing in fiberglass?

 

[NotOrganized]

We did a similar thing in our home. Built in 2004. Foam the entire outer envelope. We ran duct work in the attic and no insulation on the sheetrock ceilings. No venting as you only need vents if you have moist air trapped in the attic.

 

[yeldogt]

venting the attic is to remove moisture that's going to leak from the living space.

Foaming the whole building w/ result in less BTU load vs fiberglass on the ceiling.

You will not regret foaming the whole building

 

[theoldwizard1]

Will the natural gaps in the Hardie board and fascia be enough with a ridge vent?

The point of spray foam is to seal any gaps. It is the only insulation product that does this quickly and easily.

Use the whatever insulation you want in the attic. Just add gable vents. Worst case, add solar powered roof vent/fans.

 

[rsanter]

Yes you can
I think the best way to insulate is all the walls and the bottom of the roof.
If you want you can also do blow in on the ceiling but you don’t want to put much.
Just a little to slow the movement of heat.
You really don’t need to but you can

Bob

 

[terabitdan]

To avoid condensation issues you must have a minimum percentage of spray foam or rigid board insulation to fiberglass or cellulose and it varies by zone. For complete information, read this article https://buildingscience.com/documen...ail&utm_term=0_194890bc8c-d8164f923c-63891105. For you thats 30% or R-15 or greator. The more you put on the underside of the roof the better.

You can also use insulation netting under the spray foam and blow in your choice against the rafters or batts.

How much spray foam were you planning under the roof?


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[dreamingmuscle]

Why not hang the dry wall ceiling first than spray the back side of that? You can plan for a large gable vent for attic access.

 

[Schumi]

Thanks for all the suggestions!

I don't have any gable ends as it is a hip roof.

We had 2" of closed cell foam bid for the whole thing - walls and roof. I am surprised to hear that the BTU load would be less with the whole thing spray foamed vs. fiberglass above the ceiling.

I've attached a photo of the garage to show the volume of space I am looking at conditioning with the mini split.

 

[yeldogt]

Fiberglass leaks -- spray will not. My guess is 2" of foam will not get you the R code. Is 2" enough for no condensation in your area.

Every building leaks -- with a drywall ceiling you will still get air circulation into the closed attic. There are so many variables its hard to generalize and give you specifics for your building.

When you spray the whole building -- it's done forever. Whats the price difference to foam the ceiling vs the roof deck. When ever I price it -- the difference is so small i don't want to feel with making sure the attic is OK

 

[theoldwizard1]

I don't have any gable ends as it is a hip roof.

Yeah, I just re-read the original !

You need a ridge vent and you might need some (solar powered ?) vents on the back. Looks good from the front so you don't want to mess that up !

We had 2" of closed cell foam bid for the whole thing - walls and roof. I am surprised to hear that the BTU load would be less with the whole thing spray foamed vs. fiberglass above the ceiling.

Because it seals up all the little cracks/gaps.

 

[terabitdan]

Typical spray foam is about R6.8 per inch or R13.6 for your 2 inches. That probably meets code for the walls, but not for the attic. Of course for the garage there might not be any code requirement to meet. The majority, like 90%+ of cost savings is also in that R-13.

That said, you really need to do a heat loss calc. I have a spreadsheet to calculate the heat loss for my renovations and garage, so I put your garage into that and even with R21 In the walls and R34 in the ceiling the 18k mini-split won’t keep the garage above freezing all the time. It will be able to maintain about 22deg above outside.

Maybe you can live with that. If you want to maintain 50, with dips below for the really cold nights you’ll need around 30k BTU/hr at 2” of foam or 25k at R21 walls and R38 attic.

If you have natural gas available that’s probably your best option depends on your gas and electric rates, and what you want.

Remember, I’m just some guy with a spreadsheet, based on the websites with manual J instructions. YMMV.




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[bobbyjean]

don't know about spray foam in attic...my thought is do the walls with foam..and rock the ceiling with 5/8 drywall ...airseal very well..alot of the prep work will be done before the sheetrock goes up...then at least r-38 in attic...raftermate installed even though you don't have the soffit vents at this point-they can always be done later.
as far as the lg unit-it will put a dent in the heat/cool load....but you may need to add something else down the line... 42k to 56k...if strictly a garage and not a shop go to low side(42k) .

 

[Schumi]

as far as the lg unit-it will put a dent in the heat/cool load....but you may need to add something else down the line... 42k to 56k...if strictly a garage and not a shop go to low side(42k) .

If I understand, you don't think the unit I have is going to keep the shop/garage at a comfortable level - say 60 degrees in winter and 78 in summer?

Of the 35X40 interior space, I have taken a 13X22 space out that will be walled off, insulated and not part of the conditioned space. Does this help?

 

[d300]

You need to verify the local needs for the minimum foam thickness to control the condensation. 2" usually does the job. Then, as mentioned, decide how much 'R' you want/need to help determine the size of the heat/cool source. If you already have the 'unit' then work backward through the load calc's to see what is needed for insulation.

You will be surprised at how little is needed to heat your giant 'picnic cooler'.

 

[Schumi]

Thanks again for all the help!

I am now planning on doing one of the following:

Option 1:
3.5" of closed cell foam in the walls = R-21
18-20" of blown-in fiberglass in attic = R-60
I do plan to gasket/air-seal the drywall connection to the attic to insure air-tight seal.

Option 2:
3.5" of closed cell foam in the walls = R-21
2" of closed cell foam on the entire interior attic/roof area = R12

Which one is the best?

 

[yeldogt]

Thanks again for all the help!

I am now planning on doing one of the following:

Option 1:
3.5" of closed cell foam in the walls = R-21
18-20" of blown-in fiberglass in attic = R-60
I do plan to gasket/air-seal the drywall connection to the attic to insure air-tight seal.

Option 2:
3.5" of closed cell foam in the walls = R-21
2" of closed cell foam on the entire interior attic/roof area = R12

Which one is the best?

You will never seal the building with fiberglass -- no matter what you do. I would save some cash and do the 2" in the walls and fill the rest with batt ... spend the savings on as much foam as possible on the roof. I did a flash and batt job -- worked very well.

 

[Schumi]

Yeldogt -

So you suggest doing a flash-n-batt on the walls, then filling the cavity on the roof to get R-22?

If I do that isn't the most R-value I can get in the walls going to be more like R-15?

 

[Denwood]

The standard method here (particularly with 2x4 wall retrofit) is to foam the walls, then flash the attic ceiling from above. Then blow in cellulose to meet/exceed code. In my case, we were looking for R60-R70 in the attic. Blow in cellulose is much cheaper than foam.

The walls benefit from the higher R value/inch of foam. In the attic, (keeping in mind we rarely run HVAC in attic) 1.5 to 2” closed cell gets you the vapor seal, the blow in cellulose gets you to target R-value.

 

[bobbyjean]

If I understand, you don't think the unit I have is going to keep the shop/garage at a comfortable level - say 60 degrees in winter and 78 in summer?

Of the 35X40 interior space, I have taken a 13X22 space out that will be walled off, insulated and not part of the conditioned space. Does this help?

yes...i see you will be insulating really well...so..you already have unit..just go with it and see how it does..but if possible plan a location for a back up..run wire if electric...or you could prep for another ductless...