Internal insulation - 1st floor, single skin bay wall

Adrian1999

UPDATED since original post.

After fully opening the bay area, the way I am going to tackle the bay has completely changed. The exterior bay wall is not bricked. It is a layer of external render, internal rigid bitumen membrane DPC layered from top to bottom, with the inner most layer at the top, and whatever intermediate layer to hold these together (probably rendering) - so imagine however thick that is (centimetres). See images.

New Plan

1. Inspection DPC membrane - check for nicks or open layers - patch with bitumen

2. Bottom of Bay "lip" or cantilevered shelf (see image below, silver triangles at bottom of beams) - infill with PIR triangles sort of toblerone between the studs and bed them with insulation foam.

- Question - If I cut like this, should the silver foil go against the DPC or the timber?

3. PVC top of ground floor windows/doors - Fill with insulation foam, then trim flush to the plater board.

4. Stud nogging - install across the bottom of pre-existing timer verticals, behind PIR triangles.

5. PIR over ceiling plasterboard

5. 1st layer of Rockwool - Fill between verticals up to the DPC, down to the bottom nogging

- correction - beams are 90 mm x 50 mm, 1st layer will be 90 mm thick 

6. 2nd layer of rockwool

- due to the nature of the frame and building a second frame - thoughts on a thin layer between the entire face of the original timber and rock wool and the second frame of rock wool?

- Disconnects the verticals and achieves a continuous thickness at points: 1st 90 mm, 2nd whatever is available (40 mm or 50 mm), 3rd another 90 mm.

7. Infill Void with wool up to floor - this will be sealed with a VLC, either taped OSB between joists or airtight membrane as shown here (if latter, I will bring the membrane to overlap over the frame and side walls. 

8. VLC seal timber frame insulation - end goal will be to joint and tape, as I do not have the time, experience or money to pay for plastering (my thought too, 1/3 of the nations houses are joint and tape'd these days!).

Original post

Hi all, a few questions and thoughts.

Planning to insulate under a bay window area on the 2nd floor. I have done a rough mock-up of the current bay window area, and with an non-insulated internal wall. The bay protrudes in-front of the house, not covering the cavity space (see image below), so when I open the bay space I intend to cap the cavity wall (foil facing into the space).

Plan:

1. Full height insulated frame of the bay wall (See image below) using 100 mm PIR
2. Insulating Void space - with rock roll up to the downstairs ceiling OR into the 1st floor, flooring space? - The surface of the void space is spot that accumulates significant visible condensation.
3. Alternative void insulation - is it best practice to open the dining ceiling, put in PIR boards? - I would rather not do this, as I want to limit the work to the one room and finish it as quickly as possible. However if this is the genuinely best option then I will have to consider it.
4. Breathable membrane - would a membrane as shown here be suitable at any point in this? For either or both the frame and the rock wool.
5. Radiator to be installed at a future date - I will run piping to the bay and allow suitable access for ease of install.  

What common mistakes are made and anything that I may have not considered?
- Do I need to think about a DPC against the single brick wall? OR use a breathable membrane around the insulation?

Is there any additional things or curve balls I could do with the brick wall itself, or any other thoughts?  Really open to ideas.

Thank you in advance!

FreeBear

You want to take the insulation down below the floor level. Ideally, all the way down to the door/bay downstairs. make sure that any potential air gaps are sealed at the junction between downstairs door/bay and insulation.

If you are using foil faced PIR/PUR sheets, no real need for a vapour control membrane. The foil facing will do the job for you. Just make sure to fill any gaps with expanding foam and tape the joints with aluminium foil tape.

Tip. Use a sharp serrated kitchen knife to cut the boards rather than a hand saw. Much cleaner cut with less waste.

You may also be better off using two layers of (say 50mm) and staggering any joints. Gives you multiple layers of foil, and thinner sheets are easier to cut square(ish).

Adrian1999

FreeBear said:

You want to take the insulation down below the floor level. Ideally, all the way down to the door/bay downstairs. make sure that any potential air gaps are sealed at the junction between downstairs door/bay and insulation.

If you are using foil faced PIR/PUR sheets, no real need for a vapour control membrane. The foil facing will do the job for you. Just make sure to fill any gaps with expanding foam and tape the joints with aluminium foil tape.

Tip. Use a sharp serrated kitchen knife to cut the boards rather than a hand saw. Much cleaner cut with less waste.

You may also be better off using two layers of (say 50mm) and staggering any joints. Gives you multiple layers of foil, and thinner sheets are easier to cut square(ish).

Hi FreeBear, thanks for getting back to me! Unfortunately I have to really overhaul my original plan due to the nature of the bay wall and supporting timber frame.