Insulating a Dormer Crawlspace

BarraOG

I live in a dormer and am looking for advice on how to correctly insulate the crawlspace in the attic. I've included two pictures, the first is a plan of the upper level and the second is a photo of a portion of one of the crawl spaces. I've insulated the main attics with 100mm metac between the 100mm deep joists and I have a few rolls left over.

The crawlspaces are connected at both ends to the main attics but are tricky to access so I recently installed a small insulated door to access one of them. In the photo you can see that fiberglass insulation runs down between the rafters and the same piece is then tucked back between the 200mm deep joists. You can also see that there is no insulation in the knee-wall stud.

The rooms upstairs are cold and I've had to remove mould from the knee-wall where it is not insulated. Would I be mad to rip out the insulation in the crawl space and put 100mm metac between the joists and between the studs? What would you do instead?

The stud wall is 80cm high and crawl space is around 150cm from the stud to the soffit.

10-10-20

I for one am confused.

• You have insulation between the rafters and between the joists.
• You have two attics either end of the crawl space. Are these fully insulated between the rafters or joists?
• Does the rafter insulation continue above the two adjoining rooms and stairs?
• Is the insulation above the rooms the same spec as what we see in the rafters in the photo (200mm?)?
• Why is the rafter insulation hanging lose? Was it not tight fitted and does this not imply that it's gaped at the rafter?
• Are these rafters full-filled, or partially filled?
• How does ventilation work at the roof sarking/board/felt level?
• Why are the floor joists insulated in the crawl-space?

If you properly insulate the rafters above the two rooms and continue this insulation though the rafters of the crawl-space and terminate it by covering the top of the wall-plate, then you have created a complete insulation envelope. You would not normally then continue insulating the joists as this creates a hybrid situation where the crawl space won't receive sufficient heat from below and you'll encounter DP issues.

I think the answer here is to de-hybridise the situation and complete the rafter insulation sufficiently so that the wall-plates are covered, but maintaining air-flow into the rafters.

Wildly Boaring

I would also get rid of the "hybrid?"

But would do the it slightly differently

Joists, thev up the wall of upstairs and across joists over ceiling of upstairs.

"crawlspace" and other attics all to be ventilated

10-10-20

https://www.boards.ie/discussion/comment/118149712#Comment_118149712

That reduces the heated space alright and will improve the thermal response of the rooms, but the only problem there is that the pipes are outside of the envelope if you move the insulation down. Given the unpredictability of Winter these days, I'd keep my pipes under the rafter insulation where possible.

BarraOG

https://www.boards.ie/discussion/comment/118149661#Comment_118149661

Thanks for replying. I'm going to give some more information to better explain my situation and answer your questions. When I bought the house the surveyor put in his report that the insulation in the attics (Attic 1 and Attic 2 in my drawing) should be replaced, since it was covered in building debris and didn't cover the capping stones adjacent to the soffits. So I did this with 100mm metac, between the 100mm joists (coloured bright yellow in my drawing). There is nothing between the rafters now or before in these attics. I also insulated the two stud walls upstairs in Room 1 and Room 2 that are adjacent to Attic 1 and Attic 2 respectfully, as they weren't insulated at all. Again I put 100mm metac in these walls. As part of this process, I also removed the standard sized doors that were in these walls leading into the attics and installed small insulated doors instead. I haven't touched the crawlspaces yet, so what you see in the photo is what was done when the house was built in 2006. The crawlspaces are both connected to Attic 1 and Attic 2 and there is no door or insulation or anything separating them, so cold air is easily able to pass into the crawl spaces and into the upstairs rooms.

The ceilings in each upstairs room are insulated the same way as the crawlspaces are insulted, as best as I can tell, with fiberglass between the rafters. The fiberglass insulation between the rafters is definitely not 200mm, rather between 100mm and 150mm but its not a consistent thickness like the metac I used, so its hard to tell. Again all of this was done when the house was built in 2006. I'm not sure why some of the insulation is hanging down, maybe the electrician snagged on it. After I bought the house, I got retention for the rooms upstairs. As part of that process, I asked the surveyor if he though I should be worried about air not circulating between the rafters over the ceilings upstairs. He didn't think it was an issue since the rafters were 200mm and the insulation was likely less.

A soffit runs around the entire house and the vents are around 2m apart. As I understand it, putting in more than 100mm of insulation would require increasing this ventilation.

The crawlspaces are very awkward to access from the attics so I recently installed an insulted door in the knee-wall of one of the rooms. Its marked red in my drawing. U value is 0.6.

So my thinking is that the insulation envelope should consist of the walls of the upstairs rooms that are adjacent to attics or crawlspaces, as well as between the joists in the attics and crawlspaces. And that the insulation between the rafters in the upstairs ceilings would also form the envelope. To achieve this, I would need to remove the insulation in the crawlspaces and put 100mm metac between the joists (dark yellow areas in my picture), covering the cap stones but leaving the soffits free and then also insulate the knee-wall studs in the crawlspace, again with metac. Re-insulating the ceilings upstairs, ensuring proper ventilation, sounds like a big job so I'd prefer to leave that for the future since we currently use the room as an office. Does my thinking make sense? Am I leaving my options open should we want to increase the insulation in future?

Wildly Boaring

https://www.boards.ie/discussion/comment/118150611#Comment_118150611

Appreciate that. Lag them or move them.

The issue with insulating the rafters is vapour forming at the interface with the roof.

Would need to leave a vented gap between insulation and roof or vapour barrier (but still getting moisture at rafter/roof interface)

BarraOG

https://www.boards.ie/discussion/comment/118149712#Comment_118149712

Thanks for replying. I've added more information to 10-10-20's reply, but it sounds like your solution is the same as mine. Do you think I should continue as I did in the rest of the house using 100mm metac between joists and studs?

Wildly Boaring

Don't think the 100mm is near enough.

My maths might be mad here.

R value is 0.1 divided by the 0.034 = 2.94

U value is 1/R = 0.34

So you need over just 200mm to get the usual 0.16 u value

Biker1

Put 200mm Loft Roll between the joists and another 150mm across the top. No need to go for expensive Metac. Remove the insulation between the rafters as is of no use and may be blocking the required ventilation. Knee walls will be more difficult but try and get a minimum of 125mm PIR between the uprights. The sloped sections room-side will be losing a lot of heat as well but it is a major job to sort that effectively.

BarraOG

https://www.boards.ie/discussion/comment/118151659#Comment_118151659

https://www.boards.ie/discussion/comment/118151312#Comment_118151312

Appreciate all your answers! The joists are only 100mm in the two main attics hence the fact that I used 100mm insulation there. The joists are 200mm in the crawlspace, but do I not need to increase the ventilation in the soffit if I go for more than 100mm there? I think I read before that you can put 150mm on top of 100mm insulation but you would need to increase the ventilation. Additionally, the pipes are also located in the main attics so the risk of freezing is still an issue. Would you recommend better cladding than that visible in the photo? I need to get more cladding anyway since its missing in the odd spot.

Wildly Boaring

If you do as biker1 says.

Then you can run the 150mm thr opposite direct. ie 90 degrees to joists.

You can get most of your pipes under it too.

Then lag anything else.

Probably worth marking the pipe locations to avoid damaging after

BarraOG

https://www.boards.ie/discussion/comment/118152217#Comment_118152217

Yeah that makes perfect sense. Would I need to add more ventilation to the soffit? The 100mm insulation I put down stops just short of the soffit. If you put another 250mm on top of that, I presume a reasonable gap must be left so that air can blow up into the attic. Are the gaps created by the 200mm rafters adequate?

Wildly Boaring

The gap provided by the rafters will be enough

Just make sure that you don't block the gaps between the rafter and the wall plate.

In fact make sure the insulation isn't touching the roof anywhere, just touching rafters.

Up top it'll be hard to keep the gap to the top attic. You may need to trim the insulation locally