Insulation experiment

[guyerst]

So I recently finished putting up 2" polystyrene foam board between the roof joists in my garage. I only heat / cool the garage occasionally, so I wanted to keep this as cheap and simple as possible. I know there's a group that will say anything less than a ceailing with 40" blown in is a total waste of time, with the same passion as the diesel dually crowd with their belief in towing with anything less will lead to death and dismemberment on the highways. But for only 2-3 times a month, I'm going to put that much time and money into this.

Cost was under $300 (25 sheets at $11/ea)... relatively cheap.

Plan was to simply cut to fit tight between joistsand press up tight to the roof, turned out to be not so simple as the distance between joists was pretty inconsistent. Ended up being a bit of a pain in the *** to be honest. Turned out ok, but is no way close to air tight. Plenty of places where a portion sags a slight bit from the roof, and gaps between the foam and joist. Garage wasn't ventilated before, and isn't now. I'm not worried about shingle damage, but will be pulling sections here and there to check for mold... I think that's my biggest risk. I'm hoping the not airtight install will end up preventing mold.

Been a bit frosty the last couple mornings, and saw some interesting results on the roof. Where all the neighbors show the frost melting between the joists, my roof shows melting at the joists. Heater hasn't been turned on at all, so I'm thinking I'm retaining the daytime heat and the heat transfer through the joists is causing this. Hopefully this is a sign that this will make a pretty decent difference.

 

[Falcon67]

I'd say that's typical because of the exposed rafters inside the building. They will be at some different temp than the air space between them. With a ceiling and insulation over said ceiling, the whole roof structure would tend to be the same temp, especially if typically vented. Then the frost cover would be more uniform. Usually any insulation makes a difference and the more sealed the insulated area is to prevent air movement, the better.

When I put the OSB on the ceiling of my shop and then insulated it was "better". When I went back and caulked all the joints between the panels and the walls, it got "way better". You could feel the difference.

 

[Kaizen]

curious why you didn't just attach to the top of the ceiling joists? assume it was 4x8 sheets when you started would have been easier and less area to heat.

 

[yeldogt]

That's typical - you are seeing the differential. The studs in your case are allowing heat to flow to the roof sheathing.

Your neighbor clearly has less insulation in the garage or a leak that's allowing heat to bypass the ceiling insulation.

More insulation over the house ... most likely ceiling as well.

 

[guyerst]

curious why you didn't just attach to the top of the ceiling joists? assume it was 4x8 sheets when you started would have been easier and less area to heat.

Thought about it as it obviously would be easier and more effective. But, I store a fair amount up on the ceiling joists and wanted to keep that available if possible. I also have a lift up there to pull a dinghy up out of the way during the winter that would have complicated things a bit.

I was surprised that the thermal bridging by the roof joists was so pronounced when there wasn't all that much temperature difference. The garage couldn't have been more than 45 degrees inside.

 

[Blazinzuk]

I did foam board on my walls great stuff foam works well to seal the edges. Not too expensive either

 

[DenisG]

I've always been told that putting insulation directly against roof sheathing is a big no-no. You need an air gap and ventilation and without it you'll trap moisture and the sheathing will grow mold and rot over time. Who knows? Maybe things have changed.

 

[guyerst]

I am definitely concerned about mold, so will be curious to see what I find. I'll start pulling some of the panels in the spring to see what I find.

I debated putting in a ridge vent. If I do find some mold in the spring (or anytime in the future), I'll likely do that and bring the panels down to flush with the roof joists to create a gap.

If I was conditioning the garage all the time or even fairly frequently, it would be a no brainer...

 

[PassnThru]

I've always been told that putting insulation directly against roof sheathing is a big no-no. You need an air gap and ventilation and without it you'll trap moisture and the sheathing will grow mold and rot over time. Who knows? Maybe things have changed.

Yep - that's a pretty common warning. What if he put the foam against the decking so there was no air space? What if the foam is not permeable? The moisture can only come from the roof then.


Maybe I think too much.

 

[yeldogt]

Moisture is only a problem if it gets trapped -- cold roof deck --- moisture will condense.

This is the problem with open cell foam on a roof deck -- it needs to be quite thick for the permeability to drop to a point where moisture can't get all the way through. With typical roof construction -- you will get some drying to the outside ... this is dependent on climate. Obviously -- full ice shield and many other roof under-layments don't breath like tar paper -- so this is a danger with many roofs today.

In the OP's situation -- if the foam is tight again the studs and he has no air movement -- he should be fine .. he should check. Often this type of insulation is foamed along the edges or skinned.

Remember ZIP's new system is basically foam and OSB -- with full impervious roof sheet ..this could be a problem -- but ZIP seems unconcerned.

I did a Zinc roof on my recent project -- you don't ventilate a zinc roof as it does not like back moisture -- so I closed cell foamed and used an impenetrable barrier on the roof.

 

[Stuart in MN]

Some insulation is always better than none.

I've always been told that putting insulation directly against roof sheathing is a big no-no.

There are two schools of thought, a cold roof and a hot roof (you can google hot roof to get more information on that.) Both can work, it's in the details of how they're done.

 

[6768rogues]

Back when we gutted our our old farmhouse, I had access to large quantities of partial sheets of 3/4" Dow rigid styrofoam boards, and they were free. I put a layer between the studs in every stud cavity of the exterior walls, up against the sheathing boards. Then I caulked it to the studs. It meant cutting or bending a lot of nails, and it was very tedious, but I was young and had nothing else to do with my time when I got home from work. The old studs were almost 4", so I still had room left for traditional fiberglass batts. That was 30+ years ago and it was worth it. The house is very easy to heat and cool, and is almost sound proof with no drafts. Sometimes in the winter, the frigid wind is blasting outside and I don't hear it or notice it until I happen to look out a window.

 

[ddawg16]

A lot depends on where you live.

Here in S California, it's ok. You don't need the air gap. Not enough temp swings....and it doesn't get that humid....