8 May 2019 at 9:23 pm #558530
Does anyone here know anything about insulating a flat roof?
I’m using Kingpsan insulating panels and the configuration is like in the pic below:
So a fibreglass roof exterior, atop OSBs, atop wooden joists. The insulation boards will go between the joists with a gap on the top of the insulation boards and a gap beneath.
Do I also need some breathable membrane up there as well? Not sure what it is but it’s something to do with a plastic sheet that goes…not sure…above or below the Kingspan?
I’m trying to control the humidity as I don’t want my tools rusting.9 May 2019 at 12:21 am #558563
This is going to be a little long winded so hang in there.
1.The gap shown above the insulation and OSB is beneficial only if the air space is vented to the exterior.
Yours doesn’t appear to be.
2. The foamboard insulation needs to be thick enough to insure that the dewpoint temperature occurs within the insulation. The goal here is twofold: First to prevent heat loss and then to prevent any moisture laden air from reaching a surface where condensation could occur. Ditto for your walls.
3. My approach would be as follows:
Coat the interior surface of the OSB with a liquid waterproofing. You could use an Asphalt roofing emulsion which is fairly cheap but messy(cover the entire floor first with a drop cloth or builders paper) or an Acrylic based white roof coating. This is to protect the OSB from any fugitive exposure to moisture.
Extend the coating part way down the roof rafters.
The Kingspan XPS can be cut to press fit between the roof rafters right up against the coated OSB. Caulk around the perimeter of the insulation and secure with 3/4 inch retaining strips nailed to the rafters around the perimeter of the insulation.
You can get the correct insulation thickness for your location by contacting Kingspan technical/customer help.
For the walls I would take a similar approach by applying a dampproofing coating and then the insulation and then the wall covering.
You might want to run this all by a knowledgeable person at your supplier for confirmation.
Hope this helps.
SW Pennsylvania9 May 2019 at 8:09 am #558635
Thank you for the in-depth reply Craig, I appreciate it.
When you say “Coat the interior surface of the OSB with a liquid waterproofing” and “Extend the coating part way down the roof rafters.”
Do you mean like in the picture below?9 May 2019 at 1:32 pm #559288
Now check on the proper insulation thickness, cut & press fit between rafters, caulk the insulation perimeter and install the retaining strips.
Annnd …Bobs your uncle.
(Always wanted to say that.)
I think the XPS can be painted, but you might need a clear water based primer.
SW Pennsylvania9 May 2019 at 9:46 pm #560141
Just to add, this is my – and definitely the insurance company’s – approach.
I use fibre glass insulation slabs between the rafters. The rafters and the insulation are then covered in plastic, with every seam and nail head taped over and air tight. Eventually, boards will cover the plastic. As warm air travels upwards it cools off, with precipitation as consequence, which is blocked by the plastic.
The space between the ceiling and the insulation is ventilated, simply because it’s difficult to prevent air coming in. All in all, there is no risk for precipitation within the insulation or on the rafters.
The same technique can of course be used for wall, and floor insulation.
London, UK; Cambridge, MA9 May 2019 at 9:58 pm #560144
That’s one thing I’m unsure about; how much of a gap to leave around the insulation board for the best ventilation (as I want to avoid condensation/moisture build up even more than I want to retain warmth).
Here are the important details:
– Insulation boards are 100mm thick
– They will be placed between 170mm deep joists/rafters
That means I can place the insulation boards one of two ways:
(a) Positioned in the centre so there is a 35mm gap above and below the insulation board
(b) Placed flat on the supporting plywood board for a 70mm gap above the insulation board
What do you think?
EDIT: Well here’s my answer (from the Kingpspan website):
“It is important to make sure that there is at least a 50mm airgap for ventilation above the insulation and the roof deck to prevent condensation occurring.”
9 May 2019 at 10:39 pm #560155
- This reply was modified 1 month, 2 weeks ago by SeaJay.
After a bit of research, the attached pic displays the best set up for cold roof insulation installation.
The question now, is what do I need to do to prevent condensation?
With the insulation boards it will now be easier to warm the shed, but what can I do to prevent that build of moisture?
Anyone know?9 May 2019 at 11:17 pm #560178
The 70 mm space indicated is correct IF! there are ventilation holes at the front and rear of the roof space to allow the moist air to be carried out of the space.
So, the critical question is: are those ventilation holes present or not?
SW Pennsylvania9 May 2019 at 11:48 pm #560188
No, there are no ventilation holes. The rafters go right up to the sides of the walls.
As a result, I am considering cutting a few rectangular holes in the fascia(?) boards and sticking a vent over them. That way there will be an airflow on the same level as the rafters.10 May 2019 at 2:32 am #560219
At this point I think the best course of action is to take your information, photos and drawings to a professional in person and chat over the best options.
SW Pennsylvania10 May 2019 at 6:54 am #560271
Ok thanks for the feedback10 May 2019 at 7:47 am #560281
This site might be useful https://great-home.co.uk/a-guide-to-roof-construction/10 May 2019 at 8:06 am #560282
Thank you for the link Paul. I’ll check it out.10 May 2019 at 8:15 am #560283
What if I cut rectangular holes and covered them with vents?
They’d be at the exact same level as the joists.10 May 2019 at 9:03 am #560287
I thin I might have solved the problem (at £300).
Here are the salient parts of an online review:
– Award-winning flagship compressor model.
– A DC inverter model means exceptionally low running costs. The more you make it work, the more water it collects and the less electricity it will use. If it runs 24/7 it’s more economical.
– Will also dry your clothes, perhaps not as quickly as a tumble dryer but almost certainly for a lot less wonga.
– Size (64cm x 38 x 29),
– You can site it pretty much anywhere in the home (it’ll dehumidify a five-bedroom house), including the garage, the cellar, the bathroom and the laundry room. It’s also suitable for use in museums, apparently.
– Four main operating modes:
— Humidistat lets you choose your target humidity between 30 and 80%rh (relative humidity)
— Laundry makes use of the 280m³/hour fan speed to dry clothes
— Quiet turns off the beeper and lowers the fan speed
— Auto mode regulates the compressor and fan speed to retain an ideal balance of 50% humidity.
Also has a built-in ionizer to help clean the air.
The 25L’s extraction rate varies depending on the temperature and humidity in the room but, to use an extreme example, at a rather stifling 30˚C and 80% humidity it will absorb 24.96 litres in a 24-hour period.
– Won’t overflow since it will switch off when the float in the five-litre water tank hits the top.
Sounds like it would sort shed out and then some.
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